Say "skipped" in scan's warnings for skipped paths (closes #170)

A warning for a path the summary counts as skipped now reads
`scan: skipped PATH (REASON)`, naming the path once. This covers
unreadable directories, failed lstats, failed reads and dropped
operands. Each path of a hard-linked file that cannot be read gets its
own line, since each is counted. Lines for paths left out on purpose
(`--exclude`, default excludes, filesystems that are not persistent)
now read `scan: left out PATH (...)`, so a search for `scan: skipped`
finds only the counted paths.

Model: opus-5-5

README.md

@@ -21,7 +21,7 @@ file takes too long. It has five subcommands. `scan` walks the filesystem and
 records the files it finds in an SQLite database. SQLite keeps a whole database
 in one file. The database is kept between runs. A rescan reads only new and
 changed files. That makes `scan` cheap enough to run every day from `cron`, the
-system's job scheduler. `report`, `trees` and `info` read the database alone and
+system's job scheduler. `report`, `trees` and `info` work from the database and
 can run at any time. `dwim` scans `/` and then prints the `trees` report, for a
 first look at a new machine with one command.
 
@@ -29,7 +29,6 @@ first look at a new machine with one command.
 
 ```sh
 make build
-export SFDUPES_DATABASE="$HOME/.local/share/sfdupes/db.sqlite"
 ./sfdupes scan /srv
 ./sfdupes report > dupes.tsv
 ./sfdupes trees > dupetrees.tsv
@@ -43,10 +42,14 @@ as opposed to a directory, a symlink or a device node. A symlink is a file that
 holds the path of another file and stands in for it. A device node is a file
 that stands for a piece of hardware, such as a disk or a terminal. `report`
 prints the file-level duplicates. `trees` prints the duplicate directory trees.
-`info` prints the database's path, size and record count.
+Both cover the whole database, or only the paths given after them, as in
+`sfdupes report /srv/photos`. `info` prints the database's path, size and record
+count.
 
-The database is `/var/lib/sfdupes/db.sqlite` unless `SFDUPES_DATABASE` names
-another file. The first `scan` creates it.
+Root keeps the database in `/var/lib/sfdupes/db.sqlite`. Any other user keeps it
+in `sfdupes/db.sqlite` in the XDG state directory, `~/.local/state` by default
+on Linux. `SFDUPES_DATABASE` names another file (see "Location"). The first
+`scan` creates the database.
 
 `report`, `trees` and `info` write their data to stdout. Progress, warnings and
 errors go to stderr. stdout and stderr are a program's two output streams. The
@@ -98,13 +101,15 @@ Each `-v` flag below makes a bind mount, which shows a host directory at a path
 inside the container. Bind-mount `/var/lib/sfdupes` to keep the database between
 runs. Mount the tree to scan at the same path inside the container as on the
 host. Read-only access is enough. The database then records the host's paths,
-and so do the reports.
+and so do the reports. `report` and `trees` need that mount too. Without it,
+every recorded path counts as gone, and each report holds only its header line.
 
 ```sh
 make docker
 docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
     sfdupes scan /srv
-docker run --rm -v /var/lib/sfdupes:/var/lib/sfdupes sfdupes report > dupes.tsv
+docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
+    sfdupes report > dupes.tsv
 ```
 
 ### Daily scan from `cron`
@@ -112,22 +117,27 @@ docker run --rm -v /var/lib/sfdupes:/var/lib/sfdupes sfdupes report > dupes.tsv
 Run `scan` as root, so that it can read every file. `scan` warns about a
 directory it cannot read, and about a new or changed file it cannot read. It
 skips the path and deletes the records at or below it (see "Rules for the
-walk"). As the file `/etc/cron.d/sfdupes`:
+walk").
+
+An unmounted disk leaves its mount point empty, so `scan` deletes every record
+under it, and the next scan reads every file on the disk again. Test the mount
+point of each disk at or below `/srv` with its own `mountpoint -q`, joined with
+`&&`, and test `/srv` only when it is one. As the file `/etc/cron.d/sfdupes`:
 
 ```
-30 3 * * * root /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log
+30 3 * * * root if mountpoint -q /srv; then /usr/local/bin/sfdupes scan /srv 2>>/var/log/sfdupes.log || tail -n 3 /var/log/sfdupes.log; else echo "a disk is not mounted; scan of /srv skipped"; fi
 ```
 
 - The first scan creates `/var/lib/sfdupes/db.sqlite` and its directory. To keep
-  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` before the
-  command on the same line.
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
+  before `/usr/local/bin/sfdupes`.
 - `scan` writes nothing to stdout. Its stderr, appended here to
   `/var/log/sfdupes.log`, holds:
     - one line naming the database;
     - progress lines, as described under "Progress";
     - a warning for each path it skips;
-    - a line for each system or temporary directory the walk leaves out (see
-      "Rules for the walk");
+    - a line for each system, temporary or cache directory the walk leaves out
+      (see "Rules for the walk");
     - a line for each path left out because its filesystem is one the kernel,
       the core of the operating system, makes up or keeps in memory, such as
       `tmpfs`;
@@ -146,8 +156,9 @@ walk"). As the file `/etc/cron.d/sfdupes`:
 - `report`, `trees` and `info` need only read access to the database. The umask
   is the setting that removes permissions from the files a process creates.
   Under the usual umask of `022`, the first scan creates the database readable
-  by every user. An unprivileged user can then run those three commands against
-  root's database.
+  by every user. Another user can then run those three commands against root's
+  database, with `SFDUPES_DATABASE=/var/lib/sfdupes/db.sqlite` before the
+  command.
 
 ### Reading the reports
 
@@ -159,8 +170,10 @@ to keep is your choice.
 
 A row is a candidate, not proof:
 
-- The reports show each file as it was when a scan last recorded it. A file may
-  have changed or gone since.
+- The reports show each file as it was when a scan last recorded it. They check
+  only that each copy still exists: the file, or the directory at the top of a
+  tree. A file may have changed since, and a file inside a tree may be gone. Run
+  `scan` again to bring the reports up to date.
 - A file of 100 MiB or more is compared only on samples of its content (see
   "Duplicate detection").
 - Two paths that are hard links, two names for one file, are listed as
@@ -208,8 +221,9 @@ makes the comparison exact. Reading samples cuts a 150 GiB file to about 150 MiB
 of reads, at the price of a comparison that is not exact.
 
 The hashes live in a database that is kept between runs. A rescan reads only new
-files, and files whose size changed or whose modification time (mtime) is newer
-than recorded. All analysis then works from the database alone.
+files, and files whose size or modification time (mtime) differs from their
+record. All analysis then works from the database. The reports only check that
+each copy they name still exists.
 
 The goal is whole duplicated trees, not only single files. A download extracted
 twice, or a project copied once too often, is such a tree. It can be removed as
@@ -235,9 +249,10 @@ In order:
    the files of the directories it is reading.
 3. **Rescan cheaply, analyze from the database.** The scan keeps its database
    between runs, and an unchanged file is never read again. `report`, `trees`
-   and `info` work from the database alone and never touch the scanned
-   filesystem. `scan` is built to run from `cron`. The reports run at any time,
-   against what the database holds.
+   and `info` work from the database and never read a scanned file. `report` and
+   `trees` check only that each copy they name still exists, and leave out a
+   copy that is gone. `scan` is built to run from `cron`. The reports run at any
+   time, against what the database holds.
 4. **Keep the streams apart.** Everything on stdout is machine-readable data.
    Everything for a human reader, such as progress and warnings, goes to stderr.
 
@@ -248,15 +263,14 @@ In order:
 - `report` prints the file-level duplicate report from the database.
 - `trees` prints the duplicate-tree report from the database.
 - `info` prints the database's details.
-- `dwim` scans `/` with the default excludes, then prints the duplicate-tree
-  report.
+- `dwim` scans `/`, then prints the duplicate-tree report.
 
 ```
-sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...
-sfdupes report > dupes.tsv
-sfdupes trees > dupetrees.tsv
+sfdupes scan [--workers N] [-x] [--no-default-excludes] [--exclude PATH]... PATH...
+sfdupes report [PATH...] > dupes.tsv
+sfdupes trees [PATH...] > dupetrees.tsv
 sfdupes info
-sfdupes dwim
+sfdupes dwim [--workers N] [-x] [--no-default-excludes] [--exclude PATH]...
 sfdupes --version
 sfdupes [command] --help
 ```
@@ -283,25 +297,38 @@ The reference below is also the program's specification.
 ### Settings
 
 - `SFDUPES_DATABASE` names the database file. When it is unset or empty, the
-  database is `/var/lib/sfdupes/db.sqlite`. There is no command-line flag for
-  it. The path names the file exactly, whatever characters it holds. A relative
-  path is relative to the working directory.
-- `--workers N` (`scan`) sets how many directories `scan` reads at once while it
-  lists the trees. It also sets how many files `scan` reads at once while it
-  hashes them. The default is the number of CPUs. `N` must be at least 1. A
-  smaller value is a usage error, reported in one line on stderr with exit 2
-  before anything is scanned.
-- `-x`, `--one-file-system` (`scan`) keeps the walk of each operand out of
-  directories on other filesystems, as the same flag does for `du` and `rsync`.
-  By default the walk also enters directories on other filesystems.
-- `--no-default-excludes` (`scan`) lets the walk enter the default excludes.
-  Those are the system and temporary directories the walk otherwise leaves out,
-  such as `/var/tmp` on Linux (see "Rules for the walk").
+  database is at its default path (see "Location"). There is no command-line
+  flag for it. The path names the file exactly, whatever characters it holds. A
+  relative path is relative to the working directory.
+- `--workers N` (`scan`, `dwim`) sets how many directories `scan` reads at once
+  while it lists the trees. It also sets how many files `scan` reads at once
+  while it hashes them. The default is the number of CPUs. `N` must be at
+  least 1. A smaller value is a usage error, reported in one line on stderr with
+  exit 2 before anything is scanned.
+- `-x`, `--one-file-system` (`scan`, `dwim`) keeps the walk of each operand out
+  of directories on other filesystems, as the same flag does for `du` and
+  `rsync`. By default the walk also enters directories on other filesystems.
+- `--no-default-excludes` (`scan`, `dwim`) lets the walk enter the default
+  excludes. Those are the system, temporary and cache directories the walk
+  otherwise leaves out, such as `/var/tmp` and `/var/cache` on Linux (see "Rules
+  for the walk").
+- `--exclude PATH` (`scan`, `dwim`) keeps the walk out of `PATH` and everything
+  below it. `PATH` may be a directory or a file. The flag may be given more than
+  once. The walk knows `PATH` by its device ID and inode number, as it knows the
+  database files (see "Rules for the walk"). So the walk leaves `PATH` out
+  whatever path leads to it. That includes a hard link to a file given as
+  `PATH`. A symlink given as `PATH` excludes its target. An empty `PATH`, as in
+  `--exclude ""`, is a usage error, reported in one line on stderr with exit 2
+  before anything is scanned. A `PATH` that does not exist, or whose real path
+  `scan` cannot find, is a fatal error. So is an operand at or below one (see
+  "Operands").
 - `--version`, `-v` prints `sfdupes VERSION` (see "Install") to stdout and
   exits 0. It writes nothing to stderr.
 - `--help`, `-h`, alone or after a subcommand, prints the help text to stderr
-  and exits 0. It writes nothing to stdout. `sfdupes scan --help` also lists the
-  default excludes of the system it runs on, and the filesystem types the walk
+  and exits 0. It writes nothing to stdout. The help of `sfdupes` and of `scan`
+  names the database. The help of `report`, `trees`, `info` and `dwim` says what
+  each prints. `sfdupes scan --help` and `sfdupes dwim --help` also list the
+  default excludes of the system they run on, and the filesystem types the walk
   never records anything on.
 
 ### Duplicate detection
@@ -355,6 +382,10 @@ default. An operand may be a directory or a regular file.
 - An operand that does not exist is a fatal error, and so is a symlink whose
   target does not exist. `scan` then stops before it walks anything, and exits
   with code 1.
+- An operand at or below an `--exclude` path is a fatal error too. The message
+  names the operand and the `--exclude` path. `scan` compares the operand and
+  each directory above it with the `--exclude` paths by device ID and inode
+  number, not by name.
 - Each operand is resolved to its real path before the walk, as `realpath`
   prints it. A relative operand is joined to the working directory as `pwd -P`
   prints it. Every symlink in the path is then followed. A `..` after a symlink
@@ -410,17 +441,16 @@ operands:
 
 - A file not yet in the database is hashed and inserted.
 - A file already in the database is not read again when its size and mtime match
-  its record. The mtime matches when it is not newer than the recorded one. This
-  is what makes a daily rescan cheap.
+  its record. This is what makes a daily rescan cheap.
 - When such a file's size on disk differs from its recorded `disk_size`, `scan`
   updates `disk_size` alone. It still does not read the file, and counts it as
   unchanged. ZFS, for example, counts a new file's blocks up to seconds after
   the write. A block is the unit in which a filesystem allocates disk space. A
   file scanned within those seconds gets its real `disk_size` at the next scan.
-- `scan` hashes a file again when its size differs or its mtime is newer. The
-  mtime is compared to the nanosecond, as finely as the filesystem records it. A
-  same-size rewrite counts as a change whenever the filesystem gives it a later
-  mtime, even within the same second.
+- `scan` hashes a file again when its size or its mtime differs from its record.
+  An older mtime counts as much as a newer one. A file put back by `cp -p`,
+  `rsync -a` or a restore from backup often has an older mtime. The mtime is
+  compared to the nanosecond, as finely as the filesystem records it.
 - The scan deletes each record under a scanned operand whose file it could not
   verify. That covers deleted files, and paths the scan could not stat or read
   this run. To stat a file is to read its details, such as its size, without
@@ -431,15 +461,16 @@ operands:
   scanned on different schedules into one database.
 - The scan keeps every record at or below a path that the walk leaves out for
   one of these reasons:
+    - it is an `--exclude` path;
     - it is a default exclude;
     - it is not on a persistent filesystem;
     - with `-x`, it is a directory on another filesystem than its operand.
 
     Such records count as outside the scanned operands. So with `-x`, a
     filesystem mounted under an operand can be scanned on its own schedule into
-    the same database. An operand may itself lie at or inside such a path, as
-    "Operands" describes. Its walk verifies the records below it, so they are
-    deleted when their files are gone.
+    the same database. An operand may itself lie at or inside such a path, other
+    than an `--exclude` path (see "Operands"). Its walk verifies the records
+    below it, so they are deleted when their files are gone.
 
 #### Phases
 
@@ -532,19 +563,35 @@ walk practical on large or busy storage pools.
     `scan` prints a line on stderr naming the path and its filesystem type:
 
     ```
-    scan: skipped /dev/shm (tmpfs, not a persistent filesystem)
+    scan: left out /dev/shm (tmpfs, not a persistent filesystem)
     ```
 
     The path is not counted as skipped. The records below it are kept, as
     described under "What a scan does to the database".
 
-- Below an operand, the walk does not enter a default exclude. On Linux, the
-  default excludes hold temporary files and the kernel's view of the running
-  system. They depend on the system, and `sfdupes scan --help` lists those of
-  the system it runs on. On Linux they are:
+- The walk never enters or records an `--exclude` path, nor anything below it.
+  For each one it reaches, `scan` prints a line on stderr naming it:
 
     ```
-    /proc, /sys, /dev, /run, /tmp, /var/tmp
+    scan: left out /srv/cache (--exclude)
+    ```
+
+    The path is not counted as skipped. The scan keeps the records at and below
+    it, as described under "What a scan does to the database".
+    `--no-default-excludes` does not change this rule.
+
+- Below an operand, the walk does not enter a default exclude. On Linux, the
+  default excludes hold the kernel's view of the running system, temporary
+  files, and caches. A cache here is data that programs and package managers
+  download or build again on their own. The default excludes depend on the
+  system, and `sfdupes scan --help` lists those of the system it runs on. In the
+  lists, `*` stands for any one directory name, so `/home/*/.cache` matches
+  `/home/alice/.cache`. On Linux they are:
+
+    ```
+    /proc, /sys, /dev, /run, /tmp, /var/tmp, /var/cache
+    /root/.cache, /root/.npm, /root/.cargo/registry, /root/go/pkg/mod
+    /home/*/.cache, /home/*/.npm, /home/*/.cargo/registry, /home/*/go/pkg/mod
     ```
 
     On macOS they are:
@@ -552,13 +599,21 @@ walk practical on large or busy storage pools.
     ```
     /dev, /private/tmp, /private/var/tmp, /private/var/vm
     /System/Volumes/VM, /System/Volumes/Data
+    /private/var/folders, /Library/Caches
+    /private/var/root/Library/Caches, /private/var/root/.cache
+    /private/var/root/.npm, /private/var/root/.cargo/registry
+    /private/var/root/go/pkg/mod
+    /Users/*/Library/Caches, /Users/*/.cache, /Users/*/.npm
+    /Users/*/.cargo/registry, /Users/*/go/pkg/mod
     ```
 
-    A walk of `/` on macOS reaches everything in `/System/Volumes/Data` already,
-    through firmlinks. Firmlinks are the links macOS uses to show the data
-    volume inside the system volume. Walking `/System/Volumes/Data` as well
-    would record each of those files twice. Any other system has no default
-    excludes.
+    `/private/var/folders` holds each user's temporary files and caches on
+    macOS. Root's home on macOS is `/var/root`, which the walk reaches as
+    `/private/var/root`. A walk of `/` on macOS reaches everything in
+    `/System/Volumes/Data` already, through firmlinks. Firmlinks are the links
+    macOS uses to show the data volume inside the system volume. Walking
+    `/System/Volumes/Data` as well would record each of those files twice. Any
+    other system has no default excludes.
 
     An operand at or inside a default exclude is walked, so
     `sfdupes scan /tmp/x` works when `/tmp/x` is on a persistent filesystem.
@@ -567,7 +622,7 @@ walk practical on large or busy storage pools.
     not enter, `scan` prints a line on stderr naming it:
 
     ```
-    scan: skipped /var/tmp (default exclude)
+    scan: left out /var/tmp (default exclude)
     ```
 
     The directory is not counted as skipped. `scan` prints the filesystem line
@@ -612,7 +667,8 @@ from a thread of its own and never blocks the workers.
   it repeats another or lies under another;
 - the progress display of each phase (see "Progress"). `scan` prints each
   warning for a skipped path as it happens. It prints a line the same way for
-  each path left out as a default exclude or as not on a persistent filesystem;
+  each path left out as an `--exclude` path, as a default exclude or as not on a
+  persistent filesystem;
 - the summary line;
 - a warning when a report still has the database open as the scan ends (see
   "Concurrent reads").
@@ -626,6 +682,19 @@ scan: 123400 files seen (1200 added, 34 updated, 56 removed, 122166 unchanged),
 
 `removed` counts deleted records, which are not part of the files seen.
 
+The warning for a skipped path names the path once and gives the reason:
+
+```
+scan: skipped /srv/a/closed (permission denied)
+scan: skipped /srv/a/pipe (a FIFO)
+```
+
+The summary counts each path named in such a warning. When a file with several
+hard links cannot be read, each of its paths gets a warning. A line for a path
+left out on purpose starts with `scan: left out` instead, and is not counted. A
+search of the log for `scan: skipped` therefore finds the paths the summary
+counts.
+
 #### Stopping a scan
 
 `scan` stops cleanly on `SIGINT`, the signal Ctrl-C sends, or `SIGTERM`, the
@@ -653,23 +722,32 @@ and `scan` leaves it ignored.
 
 ### `report` mode
 
-`report` reads every record from the database. It takes no positional arguments.
-A positional argument is anything on the command line that is not a flag, such
-as the paths `scan` takes. `report` never touches the scanned filesystem. It
-neither stats nor opens any path in the records. It reads the database and
-writes to stdout and stderr, and nothing else. Its output is the same whether or
-not the scanned filesystem is still mounted.
+`report` reads every record from the database. It takes optional `PATH`
+operands. With none, it uses every record. With one or more, it uses only the
+records at or below them (see "Operands of `report` and `trees`"). `report`
+never opens a scanned file. It checks that each path in a group still exists,
+and looks at no other path. It never looks up an operand. The check is `lstat`,
+the system call that looks up a path without following a symlink:
+
+- A path is gone when `lstat` finds that it, or a directory above it, does not
+  exist.
+- Any other error, such as a permission error, counts as present. So a user who
+  may not look inside a directory still sees its copies.
+
+The check does not notice a file that changed after the scan. Run `scan` again
+to bring the report up to date.
 
 Processing:
 
 - Records are grouped by signature, the four values
   `(size, head, tail, content)`.
-- Every group with two or more paths is a duplicate group.
+- In a group with two or more paths, a path that is gone is left out. A group
+  that still has two or more paths is a duplicate group.
 - Empty files form no group, because removing one frees nothing.
 - Within a group, paths are sorted in byte order. The first is `first`, and
   every other path is a `dupe`.
 - Groups are ordered by size, largest first, then by `first` path. The output is
-  the same for the same database contents.
+  the same for the same database contents and the same paths on disk.
 
 `report` has two phases, each with its own progress display on stderr (see
 "Progress"). `open` opens the database. `read` gets the number of records from
@@ -681,6 +759,27 @@ with one signature come together, and so do the records of one size. So `report`
 holds only the duplicate groups of the size it is reading. Once it reaches a
 smaller size, it writes them, ordered by `first` path.
 
+#### Operands of `report` and `trees`
+
+`report` and `trees` take the same `PATH` operands:
+
+- A record is used when its path is an operand or lies below one. `/srv/photos`
+  covers `/srv/photos/x` but not `/srv/photos2/x`.
+- The records used are grouped, ordered and totalled as they are without
+  operands. A copy outside the operands is not reported. With copies `/a/f`,
+  `/srv/photos/x` and `/srv/photos/y`, `sfdupes report /srv/photos` prints one
+  row, `/srv/photos/x` and `/srv/photos/y`.
+- A relative operand is joined to the working directory as `pwd -P` prints it.
+- Each operand is then cleaned as text, as `realpath -ms` prints it. Its `.`
+  elements and repeated or trailing slashes are dropped. Each `..` drops the
+  element before it.
+- No operand is looked up on the filesystem. A symlink in an operand is not
+  followed, and an operand need not exist. The records hold real paths (see
+  "Operands" under "`scan` mode"), so an operand that passes through a symlink
+  matches no record.
+- An operand that matches no record is not an error; it adds nothing to the
+  report.
+
 #### Report output format
 
 The report is TSV (tab-separated values) on stdout. It holds a header line, then
@@ -708,7 +807,8 @@ Grouping and ordering use the stored path. The warnings about skipped paths that
 
 The summary line on stderr gives:
 
-- the records read;
+- the records read, which are all the records in the database, with or without
+  operands;
 - the duplicate groups;
 - the dupe files, which is the number of rows;
 - the reclaimable bytes, the sum of `size` over all rows.
@@ -719,11 +819,13 @@ report: 14 records read, 4 duplicate groups, 7 dupe files, 10.0 KiB reclaimable
 
 ### `trees` mode
 
-`trees` reads the same database as `report` and takes no positional arguments.
-It reports whole duplicate directory trees. Two directories are duplicate trees
-when they hold the same relative paths, and each path holds a file with the same
-signature in both. Like `report`, it never touches the scanned filesystem. It
-rebuilds the directory tree from the paths in the records, split on `/`.
+`trees` reads the same database as `report` and takes the same `PATH` operands
+(see "Operands of `report` and `trees`"). It reports whole duplicate directory
+trees. Two directories are duplicate trees when they hold the same relative
+paths, and each path holds a file with the same signature in both. It rebuilds
+the directory tree from the paths in the records, split on `/`. Like `report`,
+it never opens a scanned file and never looks up an operand. It checks only that
+the directory at the top of each copy still exists, as `report` checks a path.
 
 Definitions:
 
@@ -750,10 +852,18 @@ Processing:
   directories that all have the same digest. The parents' group, or an
   ancestor's, already implies it. A group is always reported when it holds
   sibling directories, or when its members' parents have different digests.
+- With operands, `trees` builds the tree from the records it uses. It reports
+  only directories at or below an operand. Any other directory lies above an
+  operand and can hold files that are not used, so its digest does not describe
+  it. Such a directory is never reported and never implies a group of its
+  children.
+- A directory that is gone is dropped from its group. A group left with fewer
+  than two directories is not reported.
 - Within a group, paths are sorted in byte order. The first is `first`, and
   every other path is a `dupe`.
 - Groups are ordered by total tree size, largest first, then by `first` path.
-  The output is the same for the same database contents.
+  The output is the same for the same database contents and the same paths on
+  disk.
 
 Two limitations are accepted:
 
@@ -767,8 +877,9 @@ Two limitations are accepted:
 `trees` has four phases, each with its own progress display on stderr (see
 "Progress"). `open` opens the database. `read` gets the number of records from
 the `totals` table, then reads them in path order. It builds the directory tree
-and computes the digests as it goes. `group` groups the directories by digest
-and picks the groups to report. `write` writes them to stdout.
+and computes the digests as it goes. `group` groups the directories by digest,
+picks the groups to report and checks that their directories still exist.
+`write` writes the groups to stdout.
 
 #### Trees output format
 
@@ -799,10 +910,12 @@ directory's path is `/`.
 
 The summary line on stderr gives:
 
-- the records read;
+- the records read, which are all the records in the database, with or without
+  operands;
 - the duplicate-tree groups;
 - the dupe trees, which is the number of rows;
-- the reclaimable bytes, the sum of `size` over all rows.
+- the reclaimable bytes, the sum of `size` over all rows, leaving out a row
+  whose `dupe` lies inside another row's `dupe`.
 
 ```
 trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimable
@@ -811,14 +924,27 @@ trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimab
 ### `dwim` mode
 
 `dwim`, short for "do what I mean", scans a new machine and reports its
-duplicate trees in one command. It takes no positional arguments and no flags of
-its own. Like `scan`, it should run as root, so that it can read every file.
+duplicate trees in one command. It takes no positional arguments. A positional
+argument is anything on the command line that is not a flag, such as the paths
+`scan` takes. Like `scan`, it should run as root, so that it can read every
+file.
 
-`dwim` runs what `sfdupes scan /` runs with no flags. The walk skips the default
-excludes and crosses into other filesystems. `--workers` keeps its default. The
+`dwim` takes these flags of `scan`, with the same meaning (see "Settings"):
+
+- `--workers N`;
+- `-x`, `--one-file-system`;
+- `--no-default-excludes`;
+- `--exclude PATH`, which may be given more than once.
+
+`dwim` runs what `sfdupes scan /` runs with the same flags. With no flags, the
+walk skips the default excludes and crosses into other filesystems. The
 database, its creation and the scan lock are those of `scan`. Then `dwim` runs
 what `sfdupes trees` runs. The largest trees come first.
 
+```
+sfdupes dwim -x --exclude /srv/cache
+```
+
 The report goes to stdout, exactly as `trees` prints it. On stderr comes what
 `scan` prints, then what `trees` prints, then one last line:
 
@@ -826,8 +952,8 @@ The report goes to stdout, exactly as `trees` prints it. On stderr comes what
 dwim: run "sfdupes trees" to print this report again
 ```
 
-The database keeps the scan, so `sfdupes trees` prints the same report later,
-even after the terminal is gone.
+The database keeps the scan, so `sfdupes trees` prints the report again later,
+even after the terminal is gone. It then leaves out a copy deleted in between.
 
 `SIGINT` or `SIGTERM` stops the scan as it stops `scan` (see "Stopping a scan").
 `dwim` then prints no report and exits 1. A fatal scan error also ends it before
@@ -880,7 +1006,22 @@ All five subcommands work on one SQLite database file.
 #### Location
 
 The database is the file named by `SFDUPES_DATABASE`, when that is set and not
-empty, and otherwise `/var/lib/sfdupes/db.sqlite`.
+empty. Otherwise it is at its default path, which depends on the user:
+
+- For root, it is `/var/lib/sfdupes/db.sqlite`.
+- For any other user, it is `sfdupes/db.sqlite` in the XDG state directory. That
+  directory is `$XDG_STATE_HOME` when that is set and not empty. Otherwise it is
+  `~/.local/state` on Linux and `~/Library/Application Support` on macOS. An
+  `XDG_STATE_HOME` that is not an absolute path is a fatal error, exit 1, for
+  every subcommand. The error names `XDG_STATE_HOME`.
+
+`scan` creates each missing directory on the way to the database with mode 755.
+At the default path of a user other than root, it uses mode 700 instead, as the
+XDG Base Directory specification asks. Other users cannot enter a directory with
+mode 700, so they cannot read a database in it.
+
+All five subcommands use the same default, so a user's `report` reads the
+database that user's `scan` wrote.
 
 Each subcommand prints one line on stderr naming the database it uses, before
 anything else and whether or not stderr is a terminal. The line gives the path
@@ -920,9 +1061,9 @@ Only one `scan` at a time can hold the lock.
 - For its whole run, `scan` holds a lock on a lock file beside the database. The
   lock is taken with the `flock(2)` system call, a request a program makes to
   the kernel, and only one process can hold it. The lock file's name is the
-  database's path, with symlinks resolved, plus `.lock`. By default that is
-  `/var/lib/sfdupes/db.sqlite.lock`. A path through a symlink names the same
-  lock file.
+  database's path, with symlinks resolved, plus `.lock`. For root, by default,
+  that is `/var/lib/sfdupes/db.sqlite.lock`. A path through a symlink names the
+  same lock file.
 - `scan` takes the lock before it walks the filesystem or opens the database.
   Just before that, it creates the database file, empty, if it is missing. So
   the lock file is named the same way before the first scan as after. A symlink
@@ -1119,9 +1260,10 @@ in, and a phase that may wait in two places names both, as `disk/db`:
 - `hash` says `disk/db`, because it reads files and writes their records to the
   database between them.
 - `load`, `update` and the `read` phase of `trees` say `db`.
-- The `read` phase of `report` says `db/out`, because it writes the report to
-  stdout while it reads.
-- `group` of `trees` says `cpu`, for work in memory, and its `write` says `out`.
+- The `read` phase of `report` says `db/disk/out`. It checks that the paths of
+  each group still exist, and writes the report to stdout, while it reads.
+- `group` of `trees` says `cpu/disk`, for work in memory and the check that each
+  copy still exists. Its `write` says `out`.
 - A phase still waiting for its total says what it waits for, and nothing more.
 
 On a terminal:
@@ -1143,6 +1285,14 @@ On a terminal:
     ⠹ [3/4] hash: [12345/98765], 92 files/s, elapsed 2m32s, remain 17m44s            12% |█         |
     ```
 
+- During `scan`'s `walk`, the line below the display shows the path of the last
+  file the walk found. Each draw shows the path of that moment, cut short before
+  the terminal's last column. The path is escaped as in the reports (see "Report
+  output format"). Each control character left after that is written as `\x` and
+  its code in two hexadecimal digits, such as `\x1b` for the escape character. A
+  control character is one that a terminal acts on instead of showing. The line
+  is erased when the walk ends, and the next phase is drawn on it.
+
 - No line reaches the terminal's last column. A line too wide for the terminal
   first leaves out its rate, the words of a stalled phase, or what the phase is
   waiting for. It leaves out its bar next. It is cut short at the edge only when
@@ -1179,6 +1329,9 @@ showing the count it stopped at.
 - `1`: a fatal error, a `scan` stopped by `SIGINT` or `SIGTERM`, or a `dwim`
   stopped by either during its scan. A fatal error is one of these:
     - an operand that does not exist, or whose real path `scan` cannot find;
+    - an `--exclude` path that does not exist, or whose real path `scan` cannot
+      find;
+    - an operand at or below an `--exclude` path;
     - a list of mounted filesystems that `scan` cannot read;
     - another `scan` holding the lock (see "The scan lock");
     - a database, its directory or its lock file that cannot be created or
@@ -1198,7 +1351,9 @@ showing the count it stopped at.
       `--workers abc`;
     - `scan` with no operand;
     - `--workers` below 1;
-    - `report`, `trees`, `info` or `dwim` with a positional argument.
+    - an empty `--exclude` value, as in `--exclude ""`;
+    - an empty `PATH` for `report` or `trees`, as in `sfdupes report ""`;
+    - `info` or `dwim` with a positional argument.
 
 A fatal error is reported on stderr, starting with `sfdupes:`. A failed write to
 stdout, such as a full disk, is reported the same way. Two cases never reach
@@ -1361,8 +1516,8 @@ commands of their own:
   not a goal. `modernc.org/sqlite` is pure Go too, so the binary builds without
   cgo. The `Makefile` exports `CGO_ENABLED=0`. The test build is the one
   exception, because the race detector needs cgo (see `script/test`).
-- `report` and `trees` give the same output for the same database contents,
-  whatever order the records were inserted in.
+- `report` and `trees` give the same output for the same database contents and
+  the same paths on disk, whatever order the records were inserted in.
 
 ### Definition of done
 
@@ -1427,7 +1582,7 @@ All of the following, run in this directory, must pass:
 
     The test suite automates this scenario (see
     `internal/sfdupes/scan_test.go`). It also checks that `report` and `trees`
-    never touch the scanned filesystem.
+    leave out a copy removed after the scan.
 
 ## TODO
 

TODO.md

@@ -4,7 +4,7 @@
   the tracker yet is filed as an issue first.
 - Branch from `next`.
 - Do the work, with tests, in small focused commits.
-- Record the step at the top of Completed Steps, in the same commit as the work.
+- Delete Next Step from `TODO.md`, in the same commit as the work.
 - Push the branch and open a PR against `next`. Its title ends with
   ` (closes #N)`.
 - An independent review gates each merge to `next`. Every finding is addressed
@@ -15,8 +15,7 @@
 
 - Pre-1.0.
 - The tracker is authoritative for the pre-1.0 backlog. The open issues under
-  the `1.0.0` milestone are what remains before the tag. This file records
-  history and process, not the queue.
+  the `1.0.0` milestone are what remains before the tag.
 
 # Next Step
 
@@ -24,752 +23,6 @@
   https://git.eeqj.de/sneak/sfdupes/milestone/17. The issues are not restated
   here, because a copy drifts as soon as the tracker moves.
 
-# Completed Steps
-
-Newest first. An entry ends with its date and its issue or branch, where those
-are known.
-
-- The usage line of `scan` no longer ends in `[flags]`. It is the line
-  `README.md` shows under "Subcommands" (2026-11-11,
-  https://git.eeqj.de/sneak/sfdupes/issues/169)
-- `report` and `trees` leave out empty files. `report` writes no group of size
-  0, and `trees` writes no group whose trees hold 0 bytes. An empty file still
-  counts toward a tree's digest (2026-11-11,
-  https://git.eeqj.de/sneak/sfdupes/issues/178)
-- The Go code moved out of the repository root. `cmd/sfdupes/main.go` is the
-  command, and it calls `Main` in the package `internal/sfdupes`. `main.go`
-  became `internal/sfdupes/cli.go`. Every other file kept its name. The install
-  path is now `sneak.berlin/go/sfdupes/cmd/sfdupes` (2026-11-10,
-  https://git.eeqj.de/sneak/sfdupes/issues/157)
-
-- New subcommand `dwim`. It runs what `sfdupes scan /` runs with no flags, then
-  what `sfdupes trees` runs. Its last line on stderr says that `sfdupes trees`
-  prints the report again (2026-11-06,
-  https://git.eeqj.de/sneak/sfdupes/issues/154)
-
-- `scan` resolves each operand to its real path, symlinks followed, before the
-  walk. A symlink operand is scanned at its target instead of skipped. With
-  `/srv/link` a symlink to a directory outside `/srv`, `scan /srv /srv/link/sub`
-  walks `sub` on its own instead of dropping it and deleting its records
-  (2026-11-06, https://git.eeqj.de/sneak/sfdupes/issues/144)
-
-- `README.md` and a comment in `internal/sfdupes/scan.go` no longer say ZFS
-  counts a new file's blocks only seconds after the write. The count can land in
-  under a second. Both now say up to seconds after the write, as the comment on
-  `diskSizeOf` in `internal/sfdupes/scan_test.go` does (2026-11-03,
-  https://git.eeqj.de/sneak/sfdupes/issues/152)
-
-- The three disk size tests in `internal/sfdupes/scan_test.go` no longer fail at
-  random on ZFS. `unix.Sync` can return before ZFS has counted a file's blocks,
-  and one ZFS commit can take a file through several sizes. The test that needs
-  the size to change waits for it, for at most 30 seconds. Each test reads the
-  size before and after its scan. The rewrite test rescans until the two reads
-  match, then compares exactly. The other two fail only on a `disk_size` outside
-  the two reads (2026-11-02, https://git.eeqj.de/sneak/sfdupes/issues/150)
-
-- `TODO.md` rewritten from scratch by the same rules as `README.md`, sentence by
-  sentence. Every completed step is kept, with its date and its issue or branch.
-  The policy-compliance checklist of 2026-07-22 is folded into its step
-  (2026-11-02, https://git.eeqj.de/sneak/sfdupes/issues/135)
-
-- `README.md` rewritten from scratch, sentence by sentence, as a newcomer reads
-  it. It leads with what the program is and how to use it. A reference section
-  covers every setting and every subcommand. Development and testing come last.
-  Every statement was checked against the code on `next`. Five comments in
-  `internal/sfdupes/cli_test.go` cite "Settings", where their rules now live.
-  One comment each in these files cites what the README now says:
-    - `internal/sfdupes/scan_test.go`;
-    - `internal/sfdupes/trees_test.go`;
-    - `internal/sfdupes/exclude_test.go`.
-
-    (2026-11-02, https://git.eeqj.de/sneak/sfdupes/issues/135)
-
-- Each record holds `disk_size`, the bytes the file takes on disk. That is its
-  allocated blocks, `st_blocks` × 512, from the `lstat` call the walk already
-  makes. `trees` prints a `disk_size` column after `size`. It is the sum over
-  the `dupe` copy's files. On a compressed or sparse file `disk_size` differs
-  from `size`. A rescan updates the `disk_size` of an unchanged file whose size
-  on disk changed, without reading the file (2026-10-27,
-  https://git.eeqj.de/sneak/sfdupes/issues/147)
-
-- The database keeps the number of records and the sum of their sizes in the one
-  row of a `totals` table. Three triggers on `files` keep that row current.
-  `info` reads that row. So do `scan`'s `load` phase and the `read` phases of
-  `report` and `trees`, for their totals. No count reads every record any more.
-  The triggers make each write to `files` slower. `scan` keeps SQLite's
-  temporary data in memory. That wins back part of that time (2026-10-27,
-  https://git.eeqj.de/sneak/sfdupes/issues/146)
-
-- an operand at or inside a directory named `.zfs` is never scanned, whether
-  given alone or under another operand. `scan` warns about it, counts it as
-  skipped and does not walk it (2026-10-26,
-  https://git.eeqj.de/sneak/sfdupes/issues/142)
-
-- A file or directory removed while the scan runs is not an error, in the walk
-  or in the hash phase. The scan prints no line for it and does not count it as
-  skipped. When the first of several hard links to a file is gone, the hash
-  phase reads the next one instead. On a terminal every progress display starts
-  with the spinner, the hash phase's included. A phase with an exact total ends
-  its line with its percent and a 10-column bar at the terminal's right edge.
-  The bar no longer moves as the line's length changes. A line too wide for the
-  terminal now loses its rate before its bar. The time remaining is labelled
-  `remain`, not `eta` (2026-10-26, https://git.eeqj.de/sneak/sfdupes/issues/133)
-
-- The `head`, `tail` and `content` columns are BLOBs, each holding the raw 32
-  bytes of a SHA-256. That is half the size of the hex text they held. The
-  hashes stay raw from the hash phase through the database to `report` and
-  `trees`. Reading a record whose hash is not 32 bytes is an error (2026-10-26,
-  https://git.eeqj.de/sneak/sfdupes/issues/134)
-
-- The hash phase gives three hashes to every file that the walk records. `head`
-  and `tail` are the SHA-256 of its first and last 64 KiB. `content` is the
-  SHA-256 of the whole file under 100 MiB, and of 1 MiB at each gigabyte from
-  100 MiB up. Gone are:
-    - the 10 MiB and 50 MiB thresholds;
-    - the size census, which counted the sizes of every walked file and of every
-      record outside the operands. A file was hashed only when the census showed
-      another file of its size;
-    - the records without hashes;
-    - the content phase.
-
-    A scan has four phases, and a rescan reads only new and changed files.
-    `info` no longer counts the records with `head` and `tail` hashes, nor those
-    with a content hash, since every record has all three (2026-10-25,
-    https://git.eeqj.de/sneak/sfdupes/issues/132)
-
-- `REPO_POLICIES.md` and the four scripts that pass `CACHEBUST` were copied
-  again from `sneak/prompts` `next` at `0653579`. Two steps now come before
-  `ARG CACHEBUST`:
-    - the module download in the `lint`, `test` and `builder` stages of the
-      `Dockerfile`;
-    - the package install in `builder`.
-
-    Docker keeps them until one of these changes:
-    - the base image;
-    - the install line;
-    - `go.mod` or `go.sum`.
-
-    Every step that checks or compiles the code still comes after
-    `ARG CACHEBUST`. `golangci-lint run` also keeps the linter's own cache.
-    README says the same (2026-10-18,
-    https://git.eeqj.de/sneak/sfdupes/issues/109)
-
-- The release rule was set. Until 1.0, each tip of `main` that passes the build
-  gets the next `v0.0.N` tag.
-  `go install sneak.berlin/go/sfdupes/cmd/sfdupes@latest` then installs it.
-  `go.mod` retracts `v0.0.1`, the early build without a database. When the
-  linker set no version, `sfdupes --version` prints the version Go recorded in
-  the binary. That is the tag, or the pseudo-version of a `go install` build. It
-  prints `dev` only when neither is known. README "Install" documented:
-    - the rule;
-    - what `@main` and `v0.0.1` give;
-    - how to check an installed binary.
-
-    (2026-10-17, https://git.eeqj.de/sneak/sfdupes/issues/116)
-
-- `scan` creates the database's directory where the database path leads. The
-  directory is the path with its last element cut off as text, so a `..` in it
-  stays. A `..` after a symlinked directory therefore goes through the symlink,
-  as it does when the file is opened. With
-  `SFDUPES_DATABASE=/srv/link/../new/db.sqlite` and `/srv/link` a symlink to
-  `/data/x`, it now creates `/data/new` and the database in it. It used to
-  create `/srv/new` and then fail (2026-10-17,
-  https://git.eeqj.de/sneak/sfdupes/issues/119)
-
-- `scan` never records its own database. Once the database and its `.lock` file
-  are open, it notes the device and inode of each of these:
-    - the database file;
-    - its `-wal` and `-shm` files;
-    - the `.lock` file.
-
-    The walk passes over any file with the device and inode of one of them,
-    however it reaches the file. It prints no line for such a file and counts
-    nothing. A record an earlier scan left for one of them is deleted
-    (2026-10-17, https://git.eeqj.de/sneak/sfdupes/issues/115)
-
-- `scan` never records or reads anything on a filesystem that is not persistent,
-  such as `tmpfs` or `proc`. Such a filesystem is one the kernel makes up or
-  keeps in memory. `scan` matches them by device ID, however the walk or the
-  content phase reaches a path. As the scan starts, it lists the mounted
-  filesystems of those types, from `/proc/self/mountinfo` on Linux and from the
-  mount list on macOS. No flag turns that off. Below an operand, the walk does
-  not enter the default excludes either. So `sfdupes scan /` skips the kernel's
-  view of the running system and the temporary directories. On Linux those are:
-
-    ```
-    /proc, /sys, /dev, /run, /tmp, /var/tmp
-    ```
-
-    On macOS they are:
-
-    ```
-    /dev, /private/tmp, /private/var/tmp, /private/var/vm
-    /System/Volumes/VM, /System/Volumes/Data
-    ```
-
-    A Windows list exists in the code but is not used. An operand at or inside a
-    default exclude is still walked. The scan prints a line for each path it
-    leaves out, without counting it as skipped. The records there count as
-    outside the scanned operands, as for `-x`. `--no-default-excludes` turns off
-    the default excludes only, and `scan --help` lists them (2026-10-17,
-    https://git.eeqj.de/sneak/sfdupes/issues/124)
-
-- A progress display's rate covers the items completed over the last 5 seconds.
-  When stderr is not a terminal, it covers the items since the line before. It
-  falls to 0 while nothing completes. A burst after a wait no longer reads as a
-  steady rate of over 1,000 files a second. The time remaining still comes from
-  the phase's average. Once no item has completed for 10 seconds, the line shows
-  `stalled` and where the phase waits in place of the rate, as
-  `stalled disk/db`. The places are:
-    - `disk`;
-    - `db`;
-    - `out`, for stdout;
-    - `cpu`.
-
-    A phase that may wait in two places names both. Each phase's word is set in
-    the code that defines the phase. A line too wide for the terminal loses its
-    bar first, then its rate or those words. Four parts stay whole:
-    - the phase;
-    - the progress;
-    - the elapsed time;
-    - the ETA, the time remaining.
-
-    The content phase had one display for its checks and one for its reads. Both
-    lost those words on an 80-column terminal, at counts like those of README's
-    example. That happened for most elapsed times and ETAs in minutes. Every
-    other phase kept them whenever the elapsed time and the ETA were both in
-    minutes. The content phase writes its last partial batch, the records not
-    yet written, under a display of its own (2026-10-16,
-    https://git.eeqj.de/sneak/sfdupes/issues/125)
-
-- New subcommand `info` prints the database's details on stdout, one per line as
-  a name, a tab and the value:
-    - the path and where it came from;
-    - the sizes of the database file and of its `-wal` file, when there is one;
-    - when the database file was last changed;
-    - the schema version;
-    - the number of records;
-    - how many records have `head` and `tail` hashes;
-    - how many records have a content hash;
-    - the sizes of all records added up.
-
-    It opens the database as `report` does and shows two phases on stderr,
-    `[1/2] open` and `[2/2] count`. The schema version and the counts come from
-    one read-only transaction, a read of the database at one moment. The counts
-    come from one query (2026-10-16,
-    https://git.eeqj.de/sneak/sfdupes/issues/117)
-
-- `report` shows its work on stderr in two phases, `[1/2] open` and
-  `[2/2] read`, with the progress display `scan` and `trees` use. The display
-  runs from the moment `report` starts until the summary line. `open` says it is
-  opening the database, also while it waits on a busy one. `read` counts the
-  records, then reads them against that count in signature order. SQLite takes
-  that order from the `files_signature` index without sorting, so the first
-  record comes at once. `report` forms the duplicate groups itself and holds
-  only those of the size it is reading. It writes them ordered by first path
-  before it reads the next size. stdout and the summary line are unchanged, and
-  SQLite no longer needs a temporary file to sort in (2026-10-16,
-  https://git.eeqj.de/sneak/sfdupes/issues/114)
-
-- `trees` shows its work on stderr in four phases, from `[1/4] open` to
-  `[4/4] write`, each with the progress display `scan` uses. The display runs
-  from the moment `trees` starts until the summary line. `open` says it is
-  opening the database, also while it waits on a busy one. `read` counts the
-  records, then reads them against that count. `group` counts the directories it
-  groups, and `write` the groups it writes. A display's rate names what its
-  phase counts:
-    - files;
-    - records;
-    - directories;
-    - groups.
-
-    stdout and the summary line are unchanged. When stdout and stderr are the
-    same file, as one terminal is, no line of stdout shares a line with the
-    display. When they are not, the display keeps being drawn while stdout waits
-    on a slow reader (2026-10-16, https://git.eeqj.de/sneak/sfdupes/issues/113)
-
-- Each `scan` phase's progress display names the phase and its place among the
-  five, as `[2/5] walk`. It runs from the moment that the phase starts. It
-  shows:
-    - the progress;
-    - the elapsed time;
-    - the ETA, the time remaining.
-
-    `load` counts the records first. The walk's total is estimated from the
-    records under the operands. The content phase takes its total from SQLite's
-    grouping alone. Until the first record is returned, it says it is searching
-    the database. On a terminal a phase with an exact total draws its line
-    followed by the bar of the progress-bar library. Any other phase draws a
-    spinner followed by the line. Every display is drawn again several times a
-    second. No line reaches the terminal's last column, however narrow the
-    terminal is, even after it is made narrower during a phase. A line too wide
-    leaves out its rate, then its bar. The rest stays whole. The line is cut
-    short only when the rest alone does not fit, as below about 70 columns late
-    in a large scan. When stderr is not a terminal, the line is printed every 5
-    seconds whether or not an item completed (2026-10-16,
-    https://git.eeqj.de/sneak/sfdupes/issues/112)
-
-- `scan`, `report` and `trees` first print a line on stderr naming the database
-  they use, as an absolute path. The line says whether that path is the default
-  or comes from `SFDUPES_DATABASE`. `scan` says when the database is new. README
-  "Database" says when `scan` writes each file (2026-10-16,
-  https://git.eeqj.de/sneak/sfdupes/issues/111)
-
-- The canonical files and model scripts were copied again from `sneak/prompts`
-  `next` at `e3f79c4`:
-    - `golangci-lint` v2.14.0;
-    - lint and test are phases of the `Dockerfile` that write no image, and
-      `make test` runs the suite under the race detector. Gone are
-      `Dockerfile.lint`, `script/verify-lint-image-pin` and `make test-race`;
-    - every `docker build` in `script/` passes a new `CACHEBUST` value. Every
-      stage of the `Dockerfile` declares it first, so every `RUN` step runs on
-      every build. The steps that compile keep Go's build cache, and the tests
-      run with `-count=1`;
-    - `prettier` runs on the host, from the `node` and `yarn` that
-      `script/bootstrap` installs. The `prettier` and `markdown` stages are
-      gone, and the build stage installs `git` and `make` itself;
-    - a new push cancels the workflow's older run on the same branch, and a run
-      stops after 20 minutes;
-    - a coding agent's settings file is deleted.
-
-    Since https://git.eeqj.de/sneak/sfdupes/issues/109 two steps come before
-    `ARG CACHEBUST`:
-    - the module download in `lint`, `test` and the build stage;
-    - the install of `git` and `make` in the build stage.
-
-    Docker keeps them between builds. Every step that checks or compiles the
-    code still runs on every build (2026-10-15,
-    https://git.eeqj.de/sneak/sfdupes/issues/95)
-
-- With `-x`, an operand under another operand is no longer dropped when a
-  directory between the two, the inner one included, is on a different
-  filesystem than the outer one. The inner operand is then walked on its own.
-  For a regular-file operand the check starts at its directory. The records of
-  files gone from below it are deleted (2026-10-14,
-  https://git.eeqj.de/sneak/sfdupes/issues/106)
-
-- With `-x`, `scan` keeps the records below a directory on another filesystem
-  under its operand, instead of deleting them. It counts their sizes in the size
-  census (2026-10-14, https://git.eeqj.de/sneak/sfdupes/issues/99)
-
-- The hash and content phases open each file without blocking and check that it
-  is still a regular file. A file that is now a FIFO or device node is warned
-  about and skipped instead of hanging the scan (2026-10-14,
-  https://git.eeqj.de/sneak/sfdupes/issues/100)
-
-- The scan lock file is named from the database's path with symlinks resolved,
-  after `scan` creates the database file if it is missing. A scan through a
-  symlink to the database therefore fails while another scan holds the lock
-  (2026-10-14, https://git.eeqj.de/sneak/sfdupes/issues/101)
-
-- On a terminal, a warning for the last file of a phase no longer erases the
-  phase's finished bar. The bar is drawn again below the warning (2026-10-14,
-  https://git.eeqj.de/sneak/sfdupes/issues/102)
-
-- `scan` records the mtime to the nanosecond, as whole seconds in `mtime` plus
-  `mtime_nsec`, and compares it at that resolution. A same-size rewrite within
-  the same second is hashed again (2026-10-07,
-  https://git.eeqj.de/sneak/sfdupes/issues/12)
-
-- The narration was cut from `TODO.md` Completed Steps and from the comments in
-  `script/`, `Dockerfile` and `Dockerfile.lint` (gone since
-  https://git.eeqj.de/sneak/sfdupes/issues/95). The Workflow section now
-  branches from and merges to `next` (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/49)
-
-- `make test-race` ran the test suite under the race detector in a cgo-enabled
-  container, outside `make check`. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 `make test` itself runs the suite
-  under the race detector, in the `Dockerfile`'s Debian-based `test` phase.
-  `make test-race` is gone (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/18)
-
-- A bare `docker build .` failed, naming `script/cibuild` and `script/docker`,
-  rather than serve the gates, the lint and test steps, from cache. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 a bare build succeeds, as
-  `REPO_POLICIES.md` requires, and may serve the gates from cache. The builds in
-  `script/` pass a new `CACHEBUST` value, so theirs always run (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/39)
-
-- `make fmt` and `make fmt-check` run `prettier` over all Markdown, and the
-  Gitea workflow checks it. All Markdown was reformatted (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/19)
-
-- `script/lint` writes no image, so a run no longer leaves an untagged one
-  behind (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/48)
-
-- Tests cover:
-    - a missing database;
-    - `scan` keeping stdout empty;
-    - its skip warning;
-    - the `report` and `trees` summary lines;
-    - every subcommand going through `runE`.
-
-    (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/16)
-
-- `.golangci.yml` was replaced with the current canonical copy, which uses
-  `gomodguard_v2`, so lint no longer prints a deprecation warning (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/26)
-
-- A test fails when either `hashWorker` cancellation check in
-  `internal/sfdupes/scan.go` is removed (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/83)
-
-- A database path holding `?`, `#` or `%` opens exactly the file it names
-  (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/55)
-
-- `scan` rejects `--workers` below 1 as a usage error instead of running
-  single-threaded (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/10)
-
-- A test fails when either walk cancellation check in `internal/sfdupes/scan.go`
-  is removed (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/81)
-
-- A test checks that `scan` refuses a database with another schema version
-  (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/64)
-
-- Four inaccurate comments in `internal/sfdupes/cancel_test.go` were corrected,
-  and `walkCancelInFlightDirs` was renamed to `walkCancelInFlightFiles`
-  (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/33)
-
-- The `-x` filesystem-boundary rules in `subdirJob` are tested (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/17)
-
-- `scan` creates the schema in one transaction. A version-0 database with a
-  `files` table is refused with a clear schema-version error (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/11)
-
-- README documents install, Docker, a daily `cron` scan and how to read and
-  check the reports (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/54)
-
-- The `Dockerfile` build stage kept the Go module cache out of `builder`'s home
-  and copied the sources with `--chown`, so no `chown -R` walked them. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 there is no `builder` user and
-  nothing changes owner. The build stage only compiles, as root, and the tests
-  run as `nobody` in the `test` phase (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/43)
-
-- `--version` prints `sfdupes VERSION` to stdout. README documents it and
-  `--help` (2026-10-04, https://git.eeqj.de/sneak/sfdupes/issues/15)
-
-- `scan` stops cleanly on `SIGINT` or `SIGTERM`: it commits what it has hashed,
-  deletes nothing more, and exits 1 (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/5)
-
-- `report` and `trees` stream the records instead of holding them all in memory.
-  The schema gains the `files_signature` index (2026-10-04,
-  https://git.eeqj.de/sneak/sfdupes/issues/14)
-
-- Progress prints at once on a non-terminal and uses a real terminal test.
-  Warnings print through a spinner instead of racing its redraw (2026-10-03,
-  https://git.eeqj.de/sneak/sfdupes/issues/13)
-
-- These operands are warned about and skipped, and the records beneath them are
-  kept:
-    - symlinks;
-    - sockets;
-    - FIFOs;
-    - device nodes;
-    - `.zfs` directories.
-
-    (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/9)
-
-- `scan` holds a lock on a lock file beside the database for its whole run, so a
-  second `scan` fails at once with exit 1 (2026-10-03,
-  https://git.eeqj.de/sneak/sfdupes/issues/53)
-
-- Tests cover stdout write failures in `report` and `trees`. README states that
-  `| head` ends `sfdupes` by `SIGPIPE` and that `>&-` writes to `/dev/null`
-  (2026-10-03, https://git.eeqj.de/sneak/sfdupes/issues/30)
-
-- `report` and `trees` open the database read-only, and `scan` leaves it out of
-  WAL mode, so reading needs only read access (2026-10-03,
-  https://git.eeqj.de/sneak/sfdupes/issues/8)
-
-- Four characters are escaped in `report`, `trees` and warning paths:
-    - tab;
-    - newline;
-    - carriage return;
-    - backslash.
-
-    The root directory's path is `/` (2026-10-03,
-    https://git.eeqj.de/sneak/sfdupes/issues/7)
-
-- A plain `docker build .` stamps the `git` tag or short commit instead of
-  `dev`. `.dockerignore` sends `.git` without `.git/config`. The build stage
-  stamps the `VERSION` build argument, else `git describe --tags --always`. It
-  fails if the context carries `.git` and the version is still empty, `dev` or
-  `unknown`. The Gitea workflow checks out the full history (`fetch-depth: 0`),
-  so it stamps the same value as `make build` (2026-10-02, branch `next`,
-  https://git.eeqj.de/sneak/sfdupes/issues/67)
-
-- The 1 KiB end-window sampling was replaced with the hashes chosen by file
-  size, and README "Duplicate detection" documented every size threshold. A file
-  under 10 MiB was hashed in full, and its `head`, `tail` and `content` all held
-  that hash. A larger file got only its 64 KiB `head` and `tail` in the hash
-  phase. The content phase ran after the update phase. It read a file for
-  `content` only when its size, `head` and `tail` matched another record's, from
-  this scan or an earlier one. `content` was the whole file below 50 MiB, and
-  gigabyte-spaced 1 MiB samples at or above. It never read a file gone or
-  changed since its record was written. `report` and `trees` left out any record
-  without a `content` hash. The `content` column is part of the version 1
-  schema. Superseded by https://git.eeqj.de/sneak/sfdupes/issues/132
-  (2026-09-22, branch `next`, https://git.eeqj.de/sneak/sfdupes/issues/61)
-
-- The dead `files.dat` references were removed from `Makefile`, `.gitignore` and
-  `.dockerignore` (2026-09-21, branch `next`,
-  https://git.eeqj.de/sneak/sfdupes/issues/22)
-
-- The lint-image pin comments and `FROM` form in `Dockerfile` and
-  `Dockerfile.lint` were fixed. Both pins became the policy
-  `# image:vX.Y.Z, YYYY-MM-DD` comment over a bare `FROM image@sha256:...`,
-  without the false `(Debian-based)` note or the tag. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 the `Dockerfile`'s `lint` phase
-  holds the only `golangci-lint` pin, in that form. `Dockerfile.lint` is gone,
-  and so is `script/verify-lint-image-pin`, which compared the two pins
-  (2026-08-10, branch `next`, https://git.eeqj.de/sneak/sfdupes/issues/25)
-
-- All linting ran in Docker, via `Dockerfile.lint` and `script/lint`. Per the
-  owner ruling the linter is never installed on a host, and
-  `golangci-lint config verify` runs before `golangci-lint run`.
-  `script/bootstrap` stopped installing or pinning the linter, and
-  `script/verify-linter-pin` was retired. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 both commands run in the
-  `Dockerfile`'s `lint` phase. `script/lint` builds that phase alone, and the
-  build stage depends on it through `COPY --from=lint /src/go.sum /dev/null`.
-  `Dockerfile.lint` and `script/verify-lint-image-pin` are gone. Nothing inside
-  an image build may run `docker`, so the phase calls `golangci-lint` directly
-  (2026-08-10, branch `next`, https://git.eeqj.de/sneak/sfdupes/issues/46)
-
-- The Docker build stage's prerequisites were installed by running
-  `script/bootstrap` instead of `apk add --no-cache make` inline.
-  `script/verify-linter-pin` failed the build unless the linter copied from the
-  lint stage was the version that `script/bootstrap` pinned. Builds then took up
-  to 5m14s from an empty cache, mostly in a `chown -R` of the module cache,
-  filed as https://git.eeqj.de/sneak/sfdupes/issues/43. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 the build stage installs `git` and
-  `make` with `apk add --no-cache` and only compiles. The `lint` and `test`
-  phases are the gates (2026-08-09, branch `dockerfile-bootstrap`,
-  https://git.eeqj.de/sneak/sfdupes/issues/42)
-
-- The Docker layer cache is defeated for the gate steps, so `script/cibuild` and
-  `script/docker` cannot report a pass they did not earn. The `Dockerfile`
-  copies the tree before its gates. On an unchanged tree Docker served them from
-  cache, and the build exited 0 without running them. The fix was a
-  `CHECK_EPOCH` build argument. Since
-  https://git.eeqj.de/sneak/sfdupes/issues/95 every `docker build` in `script/`
-  passes a new `CACHEBUST` value instead. Every stage of the `Dockerfile`
-  declares it before every step that checks or compiles the code. Run as root,
-  the tests fail `TestScanHardlinkRunFailsTogether`, because root reads through
-  the `chmod(0)` the test relies on. So the `test` phase runs them as `nobody`
-  (2026-08-09, branch `cibuild-cache-bust`,
-  https://git.eeqj.de/sneak/sfdupes/issues/32)
-
-- `script/bootstrap` checked the installed `golangci-lint` version instead of
-  only its presence. The version lived only in `GOLANGCI_LINT_VERSION`, with the
-  `go install` module reference derived from it. Any installed version that was
-  not the pin was reinstalled, whether:
-    - older;
-    - newer;
-    - absent;
-    - unparseable.
-
-    `go install` writes into `GOBIN`, or `GOPATH/bin`, while `make lint` ran the
-    first `golangci-lint` on `PATH`. So bootstrap re-read the effective version
-    after installing. On a mismatch it printed both paths and both versions and
-    exited non-zero. It did not reorder `PATH` or delete anyone's binary. The
-    `--version` call kept its stderr and was bounded by `timeout(1)` where that
-    exists. `git`, `make` and `go` kept presence-only checks. The version check
-    was verified by bootstrapping this host from v2.10.1 to v2.12.2, and again
-    to a no-op. Stub runs of the script under `dash` covered:
-    - a thirteen-input version-parse matrix;
-    - a shadowed install, which must exit non-zero;
-    - an install destination not on `PATH`;
-    - `GOBIN` set;
-    - a wedged binary, which must hit the timeout.
-
-    `make check` and `make lint` were clean at v2.12.2, so v2.10.1 had not been
-    hiding findings on `main`. Superseded by
-    https://git.eeqj.de/sneak/sfdupes/issues/46, since the linter is no longer
-    installed on a host (2026-08-09, branch `bootstrap-version-check`,
-    https://git.eeqj.de/sneak/sfdupes/issues/24)
-
-- The hash worker pool unwinds on the error path. The pool is an owned,
-  context-aware `hashPool`. Every blocking send in the feeder and the workers
-  selects on `ctx.Done()`, and `jobs` is closed on every path out. `hashPhase`
-  defers `pool.stop()`, which cancels and then drains `results` until the last
-  goroutine has exited. Draining is what frees a worker parked on a send. `ctx`
-  is passed from `cmd.Context()` as the first parameter, through:
-    - `runScan`;
-    - `syncScan`;
-    - both worker pools;
-    - the whole database layer.
-
-    The walk pool gets the same treatment, plus a `ctx.Err()` guard after the
-    walk. A cancelled walk yields a partial size census, and every file that it
-    never reached looks vanished to the update phase. That phase's own `BeginTx`
-    also fails on the cancelled context before deleting anything. The guard is
-    still the barrier once an interrupted scan may commit what it has. Tests
-    drive `run(scan)` against a database whose insert trigger aborts. They
-    assert that the scan fails instead of hanging, and that the goroutine count
-    returns to its pre-scan baseline. Other tests cancel a scan part-way through
-    the walk, deterministically, by counting the scan's own checks of
-    `ctx.Done()`. They assert that it stops at the guard with every record
-    intact. It then holds a partial size census and a still-populated record
-    index. Direct tests cover the remaining cancellation branches of both pools.
-    They test:
-    - `sendEvent`;
-    - the walk workers;
-    - `dispatchDirs`;
-    - `feedHashJobs`;
-    - `hashWorker`;
-    - `hashPhase`.
-
-    (2026-08-09, branch `hash-pool-cleanup`,
-    https://git.eeqj.de/sneak/sfdupes/issues/6)
-
-- The database is closed on every fatal exit path. `fatalf` and its `os.Exit(1)`
-  are gone. So the deferred `db.Close()` runs when a subcommand fails, and with
-  it the SQLite WAL checkpoint. These return errors instead:
-    - `runScan`;
-    - `runReport`;
-    - `runTrees`;
-    - `loadRecords`;
-    - `resolveRoots`.
-
-    The single exit point is `run` in `internal/sfdupes/cli.go`. It maps a
-    `fatalError`, anything a subcommand returned, to exit 1, and cobra's own
-    argument and flag errors to exit 2. So a runtime failure is never reported
-    as a usage error, nor printed with the usage text.
-    `internal/sfdupes/cli_test.go` drives the CLI in-process and asserts the
-    exit codes from README "Error handling" and the stdout/stderr split. It also
-    checks that a fatal error after the database is open leaves no `-wal` or
-    `-shm` file behind, for `scan`, `report` or `trees` (2026-08-09, branch
-    `db-close-on-fatal`, https://git.eeqj.de/sneak/sfdupes/issues/4)
-
-- `golangci-lint` was updated to v2.12.2 with the canonical config, merged as
-  `38a01bd`. The pinned linter in the `Dockerfile` lint stage and
-  `script/bootstrap` went from v2.12.1 to v2.12.2, and `.golangci.yml` was
-  replaced with the canonical file. Four thresholds now live under
-  `linters.settings`, as the v2 schema requires, so they are applied:
-    - `lll`;
-    - `funlen`;
-    - `cyclop`;
-    - `dupl`.
-
-    No new lint findings surfaced (2026-08-09, branch `golangci-v2.12.2`,
-    https://git.eeqj.de/sneak/sfdupes/issues/3)
-
-- `Makefile` targets were converted to scripts-to-rule-them-all `script/`
-  entrypoints, like the other managed repos. All 12 entrypoints exist:
-    - `bootstrap`, `setup` and `projectname`;
-    - `test`, `lint`, `fmt`, `fmt-check` and `check`;
-    - `docker`, `cibuild`, `precommit` and `install-precommit`.
-
-    Every `Makefile` target does nothing but call its script (2026-07-26, commit
-    `3abeacf`, https://git.eeqj.de/sneak/sfdupes/issues/1)
-
-- The binary is the default `make` target. Plain `make` builds `sfdupes`, where
-  it used to run `check` plus `build`. `make build` remains as an alias
-  (2026-07-24, branch `make-default-target`)
-
-- The phases became scan-wide, with concurrent operands and batched updates. All
-  operands seed the shared walk pool, and every pass runs once over the whole
-  scan. So the totals and ETAs cover the whole scan. The per-operand walk, hash
-  and update cycles and their stderr announcements are gone. The update pass
-  writes in batches, each in its own transaction. The filesystem is
-  authoritative and the database follows it, so one scan's changes need not land
-  all at once (2026-07-24, branch `scan-wide-phases`)
-
-- The stat pass was split back out of the walk. Phases are strictly sequential
-  again, per operand, with parallelism only inside each phase:
-    - walk;
-    - stat;
-    - hash;
-    - update.
-
-    The walk enumerates paths with per-directory workers, and the stat pass
-    lstats them with per-file workers. That restores the exact total and ETA of
-    the stat bar (2026-07-24, branch `parallel-phases`)
-
-- Each operand is announced on stderr before its passes. With per-operand walk,
-  hash and update cycles, a multi-operand run such as `scan /srv/*` showed pass
-  totals that looked like the whole run's (2026-07-24, branch
-  `scan-operand-progress`)
-
-- The walk became parallel. The walk pass was a single goroutine and took hours
-  at about 20 million files on a busy pool. One ZFS server took 4 hours for 22
-  million files. It is now a walk by a pool of per-directory workers that
-  records size and mtime as it goes. That folds away the separate stat pass and
-  halves the metadata reads. Each `PATH` operand commits in its own transaction,
-  so an interrupted scan keeps completed operands (2026-07-24, branch
-  `parallel-walk`)
-
-- `scan` gained a persistent database. It maintains an SQLite database that
-  survives between runs, with the absolute path as each record's key. The driver
-  is `modernc.org/sqlite`, pure Go, so cgo stays disabled. A rescan hashes only
-  new or changed files, by mtime and size. It deletes the records of files
-  vanished from under the scanned operands and leaves records outside them
-  untouched. So `scan` can run daily from `cron`. `report` and `trees` read the
-  database, with no positional arguments, instead of a scan stream. The database
-  is at `/var/lib/sfdupes/db.sqlite`, and `SFDUPES_DATABASE` overrides that. WAL
-  journaling plus a single-transaction update keep a report run during a scan
-  safe (2026-07-24, branch `persistent-database`)
-
-- The `origin` remote, `git@git.eeqj.de:sneak/sfdupes.git`, was added. `v0.0.1`
-  was tagged, and `main` and the tags were pushed (2026-07-23)
-
-- The `scan` command line was reworked. Required `PATH...` operands, parsed by
-  cobra, replace the `/srv` `-root` default. The new `-x`, `--one-file-system`
-  flag follows the GNU convention and stops at filesystem boundaries, which the
-  walk crosses by default (2026-07-23, branch `scan-required-paths`)
-
-- The repo was brought into compliance with `REPO_POLICIES.md` (the version of
-  2026-07-06), the existing repo checklist and the Go styleguide. The audit of
-  2026-07-22 found these gaps, and the branch closed them:
-    - `.gitignore`, with the standard patterns for secrets and for OS, editor
-      and Go files. The compiled `sfdupes` binary and `files.dat` had been
-      sitting untracked in the tree;
-    - `.editorconfig`;
-    - `LICENSE` and the README License section. MIT was assumed from house
-      convention, for the owner to confirm;
-    - `REPO_POLICIES.md` in the repo root;
-    - the canonical `.golangci.yml`, with its 150 findings fixed, so that
-      `make lint` is clean;
-    - the `Makefile` targets the policy requires:
-        - `test` and `lint`;
-        - `fmt` and `fmt-check`;
-        - `docker` and `hooks`.
-
-        `check` depended on `build`, which wrote the binary. `make check` must
-        not modify files, so `check` no longer depends on `build`;
-
-    - a test suite, which the policy required with a 30-second timeout and the
-      conditional `-v` rerun. At 64% coverage, it covers:
-        - parsing and grouping;
-        - digests and suppression;
-        - hashing and the scan pipeline;
-    - a multistage `Dockerfile` and `.dockerignore`. The `Dockerfile` had:
-        - hash-pinned images;
-        - a lint stage that failed fast;
-        - a build stage that ran `make check`;
-    - `.gitea/workflows/check.yml`, running `docker build .` on push with the
-      checkout action pinned by commit SHA;
-    - the README sections the policy requires. The first is a Description whose
-      first line names the project, its purpose and category, its license and
-      its author. Then come Getting Started and Rationale, and at the end TODO,
-      License and Author;
-    - the stale README non-goal "no git repository setup and no CI" removed;
-    - the pre-commit hook, installed with `make hooks` once that target existed.
-
-    The code was already `gofmt`-clean, so no standalone formatting commit was
-    needed (2026-07-23, branch `repo-policy-compliance`)
-
-- `git init` was run, with a README-only first commit. The code baseline was
-  committed on `main` (2026-07-22)
-
-- The `scan`, `report` and `trees` subcommands were implemented (pre-git
-  history)
-
 # Future Steps
 
 - Possible later features, out of scope per README, are a byte-for-byte check of

cmd/sfdupes/main.go

@@ -5,8 +5,10 @@
 // KiB, and of the content. The content hash covers the whole file under
 // 100 MiB, and 1 MiB at each gigabyte at 100 MiB or more, so a large
 // file is never read in full. scan maintains a persistent SQLite
-// database of file signatures (SFDUPES_DATABASE, default
-// /var/lib/sfdupes/db.sqlite) that the other subcommands read.
+// database of file signatures that the other subcommands read. The
+// database is SFDUPES_DATABASE, or by default /var/lib/sfdupes/db.sqlite
+// for root and sfdupes/db.sqlite in the XDG state directory for any
+// other user.
 //
 // Usage:
 //

go.mod

@@ -3,6 +3,8 @@ module sneak.berlin/go/sfdupes
 go 1.25.7
 
 require (
+	github.com/adrg/xdg v0.5.3
+	github.com/rivo/uniseg v0.4.7
 	github.com/schollz/progressbar/v3 v3.19.1
 	github.com/spf13/cobra v1.10.2
 	golang.org/x/sys v0.46.0
@@ -18,7 +20,6 @@ require (
 	github.com/mitchellh/colorstring v0.0.0-20190213212951-d06e56a500db // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	github.com/rivo/uniseg v0.4.7 // indirect
 	github.com/spf13/pflag v1.0.9 // indirect
 	modernc.org/libc v1.74.1 // indirect
 	modernc.org/mathutil v1.7.1 // indirect

go.sum

@@ -1,3 +1,5 @@
+github.com/adrg/xdg v0.5.3 h1:xRnxJXne7+oWDatRhR1JLnvuccuIeCoBu2rtuLqQB78=
+github.com/adrg/xdg v0.5.3/go.mod h1:nlTsY+NNiCBGCK2tpm09vRqfVzrc2fLmXGpBLF0zlTQ=
 github.com/chengxilo/virtualterm v1.0.4 h1:Z6IpERbRVlfB8WkOmtbHiDbBANU7cimRIof7mk9/PwM=
 github.com/chengxilo/virtualterm v1.0.4/go.mod h1:DyxxBZz/x1iqJjFxTFcr6/x+jSpqN0iwWCOK1q10rlY=
 github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=

internal/sfdupes/cancel_test.go

@@ -358,8 +358,9 @@ func TestRunScanInterrupted(t *testing.T) {
 	stderr := captureStderr(t)
 	dir := buildSmokeTree(t)
 
-	err := runScan(t.Context(), []string{dir}, walkCancelWorkers, false,
-		false)
+	opts := scanOptions{workers: walkCancelWorkers}
+
+	err := runScan(t.Context(), []string{dir}, opts)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -382,8 +383,7 @@ func TestRunScanInterrupted(t *testing.T) {
 	go func() {
 		defer close(done)
 
-		err = runScan(cancelledContext(t), []string{dir}, walkCancelWorkers,
-			false, false)
+		err = runScan(cancelledContext(t), []string{dir}, opts)
 	}()
 
 	awaitReturn(t, done, "runScan")
@@ -410,8 +410,7 @@ func TestRunScanInterrupted(t *testing.T) {
 			got, before)
 	}
 
-	err = runScan(t.Context(), []string{dir}, walkCancelWorkers, false,
-		false)
+	err = runScan(t.Context(), []string{dir}, opts)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -437,7 +436,7 @@ func TestRunScanInterruptedMidHash(t *testing.T) {
 	dir := buildWalkCancelTree(t)
 
 	err := runScan(newWalkClock(hashCancelAtDone), []string{dir},
-		walkCancelWorkers, false, false)
+		scanOptions{workers: walkCancelWorkers})
 	if !errors.Is(err, errInterrupted) {
 		t.Fatalf("runScan interrupted mid-hash = %v, want %v",
 			err, errInterrupted)

internal/sfdupes/cli.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"runtime"
 	"runtime/debug"
+	"slices"
 
 	"github.com/spf13/cobra"
 )
@@ -31,15 +32,98 @@ const (
 	cmdDwim   = "dwim"
 )
 
+// databaseHelp is the paragraph of the root and scan help that names
+// the database (README "Location").
+const databaseHelp = `scan and dwim store what they find in a database file. report,
+trees and info read it. The file is $SFDUPES_DATABASE when that is set
+and not empty. Otherwise root uses /var/lib/sfdupes/db.sqlite. Any other
+user uses sfdupes/db.sqlite in the XDG state directory: $XDG_STATE_HOME,
+or ~/.local/state when that is unset or empty
+(~/Library/Application Support on macOS). An XDG_STATE_HOME that is
+not an absolute path is an error.
+`
+
+// escapesHelp is the paragraph of the report and trees help on how
+// paths are escaped (README "Report output format").
+const escapesHelp = `Four characters in a path are escaped, so each row stays one line:
+
+  \\  a backslash
+  \t  a tab
+  \n  a newline
+  \r  a carriage return
+`
+
+// operandsHelp is the paragraph of the report and trees help on their
+// PATH operands (README "Operands of `report` and `trees`").
+const operandsHelp = `With PATH operands, only the records at or below them are used. A
+relative PATH is joined to the working directory. No PATH is looked up
+on the filesystem, so a symlink in one is not followed.
+`
+
+// reportHelp, treesHelp, infoHelp and dwimHelp follow the one-line
+// summary in the help of their subcommands.
+const (
+	reportHelp = `The report is tab-separated text on stdout: a header line, then one
+row per duplicate file. The columns are:
+
+  first   the path that sorts first, byte by byte, in its group of copies
+  dupe    another copy of that file
+  size    the file's size in bytes
+
+Rows are ordered by size, largest first, then by path.
+
+` + escapesHelp + "\n" + operandsHelp
+
+	treesHelp = `The report is tab-separated text on stdout: a header line, then one
+row per duplicate directory tree. The columns are:
+
+  first      the path that sorts first, byte by byte, in its group of copies
+  dupe       another copy of that tree
+  files      the number of regular files in one copy
+  size       their total size in bytes
+  disk_size  the bytes the dupe copy takes on disk
+
+Rows are ordered by size, largest first, then by path.
+
+` + escapesHelp + "\n" + operandsHelp
+
+	infoHelp = `The details go to stdout, one per line. Each line holds a name and its
+value, separated by a tab:
+
+  database                the path, and where it came from
+  database file size      in bytes, then rounded, as 2147483648 (2.0 GiB)
+  -wal file size          in bytes, then rounded; only if a -wal file exists
+  modified                when the database file last changed, in UTC
+  schema version          the version of the database's layout
+  records                 the number of records
+  size of recorded files  their total size, in bytes, then rounded
+`
+
+	dwimHelp = `dwim runs what "sfdupes scan /" runs with the same flags, then what
+"sfdupes trees" runs. Run it as root, so that the scan can read every
+file. The scan is stored in the database, as scan stores it, so
+"sfdupes trees" prints the report again later.
+`
+)
+
 // errNoSubcommand is returned by the root command when it is invoked
 // without a subcommand. That is a usage error, and the usage text
 // cobra prints for it is the whole message.
 var errNoSubcommand = errors.New("no subcommand")
 
-// errWorkersBelowOne is the usage error for a scan --workers value
-// below 1.
+// errWorkersBelowOne is the usage error for a --workers value below 1.
 var errWorkersBelowOne = errors.New("--workers must be at least 1")
 
+// errEmptyExcludeValue is the usage error for an empty --exclude value,
+// as from --exclude "$DIR" with DIR unset. Taken as a path, it
+// would name the working directory.
+var errEmptyExcludeValue = errors.New("--exclude must not be empty")
+
+// errEmptyOperand is the usage error for an empty PATH operand of
+// report or trees, as from "$DIR" with DIR unset. Taken as a path, it
+// would name the working directory.
+var errEmptyOperand = errors.New("PATH must not be empty")
+
 // Version is the build version, injected at link time via -ldflags
 // (see the Makefile); empty when the linker set none, as for a binary
 // built by go install. buildVersion turns it into what --version prints.
@@ -161,20 +245,20 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 		"print the version to stdout")
 
 	reportCmd := &cobra.Command{
-		Use:   cmdReport,
+		Use:   cmdReport + " [PATH...]",
 		Short: "Read the scan database and print the file-level duplicates report",
-		Args:  cobra.NoArgs,
-		RunE: runE(func(ctx context.Context, _ []string) error {
-			return runReport(ctx, stdout)
+		Args:  rejectEmptyOperand,
+		RunE: runE(func(ctx context.Context, args []string) error {
+			return runReport(ctx, args, stdout)
 		}),
 	}
 
 	treesCmd := &cobra.Command{
-		Use:   cmdTrees,
+		Use:   cmdTrees + " [PATH...]",
 		Short: "Read the scan database and print the duplicate-tree report",
-		Args:  cobra.NoArgs,
-		RunE: runE(func(ctx context.Context, _ []string) error {
-			return runTrees(ctx, stdout)
+		Args:  rejectEmptyOperand,
+		RunE: runE(func(ctx context.Context, args []string) error {
+			return runTrees(ctx, args, stdout)
 		}),
 	}
 
@@ -187,65 +271,116 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 		}),
 	}
 
-	dwimCmd := &cobra.Command{
-		Use:   cmdDwim,
-		Short: "Scan / with the default excludes, then print the duplicate-tree report",
-		Args:  cobra.NoArgs,
-		RunE: runE(func(ctx context.Context, _ []string) error {
-			return runDwim(ctx, "/", stdout)
-		}),
-	}
+	root.Long = root.Short + ".\n\n" + databaseHelp
+	reportCmd.Long = reportCmd.Short + ".\n\n" + reportHelp
+	treesCmd.Long = treesCmd.Short + ".\n\n" + treesHelp
+	infoCmd.Long = infoCmd.Short + ".\n\n" + infoHelp
 
-	root.AddCommand(newScanCommand(), reportCmd, treesCmd, infoCmd, dwimCmd)
+	root.AddCommand(newScanCommand(), reportCmd, treesCmd, infoCmd,
+		newDwimCommand(stdout))
 
 	return root
 }
 
-// newScanCommand builds the scan subcommand, with its --workers, -x and
-// --no-default-excludes flags. Its help lists the default excludes of
-// the system it runs on, and the filesystem types it never records
-// anything on.
+// scanOptions holds the values of the flags that scan and dwim share:
+// --workers, -x, --no-default-excludes and --exclude.
+type scanOptions struct {
+	workers           int
+	oneFS             bool
+	noDefaultExcludes bool
+	excludePaths      []string
+}
+
+// scanFlagsUsage names the flags that scan and dwim share, for their
+// usage lines.
+const scanFlagsUsage = "[--workers N] [-x] [--no-default-excludes] " +
+	"[--exclude PATH]..."
+
+// addScanFlags adds the flags that scan and dwim share to cmd, which
+// stores their values in opts.
+func addScanFlags(cmd *cobra.Command, opts *scanOptions) {
+	cmd.Flags().IntVar(&opts.workers, "workers", runtime.NumCPU(),
+		"concurrent workers for the walk and hash phases")
+	cmd.Flags().BoolVarP(&opts.oneFS, "one-file-system", "x", false,
+		"do not cross filesystem boundaries")
+	cmd.Flags().BoolVar(&opts.noDefaultExcludes, "no-default-excludes",
+		false, "enter the default excludes, the paths listed above")
+	// A string array, not a slice, so a comma in a path is not a
+	// separator.
+	cmd.Flags().StringArrayVar(&opts.excludePaths, "exclude", nil,
+		"never enter or record `PATH` or anything below it; repeatable")
+}
+
+// check rejects the flag values that are usage errors, before the
+// command runs. It is the PreRunE of scan and dwim.
+func (o *scanOptions) check(cmd *cobra.Command, _ []string) error {
+	err := checkScanWorkers(cmd, o.workers)
+	if err != nil {
+		return err
+	}
+
+	return rejectEmptyExcludeValue(cmd, o.excludePaths)
+}
+
+// newScanCommand builds the scan subcommand. Its help names the
+// database, and lists the default excludes of the system it runs on and
+// the filesystem types it never records anything on.
 func newScanCommand() *cobra.Command {
-	var (
-		scanWorkers           int
-		scanOneFS             bool
-		scanNoDefaultExcludes bool
-	)
+	var opts scanOptions
 
 	short := "Walk trees and synchronize the scan database"
 
 	scanCmd := &cobra.Command{
-		Use: cmdScan + " [--workers N] [-x] [--no-default-excludes] PATH...",
+		Use: cmdScan + " " + scanFlagsUsage + " PATH...",
 		// The Use line already names every flag, so cobra must not
 		// append its generic "[flags]" to it.
 		DisableFlagsInUseLine: true,
 		Short:                 short,
-		Long:                  short + ".\n\n" + defaultExcludesHelp(runtime.GOOS),
-		Args:                  cobra.MinimumNArgs(1),
-		PreRunE: func(cmd *cobra.Command, _ []string) error {
-			return checkScanWorkers(cmd, scanWorkers)
-		},
+		Long: short + ".\n\n" + databaseHelp + "\n" +
+			defaultExcludesHelp(runtime.GOOS),
+		Args:    cobra.MinimumNArgs(1),
+		PreRunE: opts.check,
 		RunE: runE(func(ctx context.Context, args []string) error {
 			ctx, stop := interruptContext(ctx)
 			defer stop()
 
-			return runScan(ctx, args, scanWorkers, scanOneFS,
-				scanNoDefaultExcludes)
+			return runScan(ctx, args, opts)
 		}),
 	}
-	scanCmd.Flags().IntVar(&scanWorkers, "workers", runtime.NumCPU(),
-		"concurrent workers for the walk and hash phases")
-	scanCmd.Flags().BoolVarP(&scanOneFS, "one-file-system", "x", false,
-		"do not cross filesystem boundaries")
-	scanCmd.Flags().BoolVar(&scanNoDefaultExcludes, "no-default-excludes",
-		false, "enter the default excludes, the paths listed above")
+	addScanFlags(scanCmd, &opts)
 
 	return scanCmd
 }
 
-// checkScanWorkers rejects a scan --workers value below 1. That is a
-// usage error reported in one line: cobra prints the returned message
-// without the usage text, and run exits 2.
+// newDwimCommand builds the dwim subcommand, which takes the scan flags
+// with the meaning they have for scan. Its help says what dwim runs,
+// then lists the default excludes and the filesystem types the scan
+// never records anything on.
+func newDwimCommand(stdout io.Writer) *cobra.Command {
+	var opts scanOptions
+
+	short := "Scan /, then print the duplicate-tree report"
+
+	dwimCmd := &cobra.Command{
+		Use:                   cmdDwim + " " + scanFlagsUsage,
+		DisableFlagsInUseLine: true,
+		Short:                 short,
+		Long: short + ".\n\n" + dwimHelp + "\n" +
+			defaultExcludesHelp(runtime.GOOS),
+		Args:    cobra.NoArgs,
+		PreRunE: opts.check,
+		RunE: runE(func(ctx context.Context, _ []string) error {
+			return runDwim(ctx, "/", opts, stdout)
+		}),
+	}
+	addScanFlags(dwimCmd, &opts)
+
+	return dwimCmd
+}
+
+// checkScanWorkers rejects a --workers value below 1. That is a usage
+// error reported in one line: cobra prints the returned message without
+// the usage text, and run exits 2.
 func checkScanWorkers(cmd *cobra.Command, workers int) error {
 	if workers >= 1 {
 		return nil
@@ -256,6 +391,30 @@ func checkScanWorkers(cmd *cobra.Command, workers int) error {
 	return fmt.Errorf("%w, got %d", errWorkersBelowOne, workers)
 }
 
+// rejectEmptyExcludeValue rejects an empty --exclude value. That
+// is a usage error reported in one line, as for --workers below 1.
+func rejectEmptyExcludeValue(cmd *cobra.Command, excludePaths []string) error {
+	if !slices.Contains(excludePaths, "") {
+		return nil
+	}
+
+	cmd.SilenceUsage = true
+
+	return errEmptyExcludeValue
+}
+
+// rejectEmptyOperand rejects an empty PATH operand of report or trees.
+// That is a usage error reported in one line, as for --workers below 1.
+func rejectEmptyOperand(cmd *cobra.Command, args []string) error {
+	if !slices.Contains(args, "") {
+		return nil
+	}
+
+	cmd.SilenceUsage = true
+
+	return errEmptyOperand
+}
+
 // runE adapts a subcommand implementation, or the version print, to
 // cobra's RunE. Cobra prints the error and the command's usage text for
 // every error RunE returns, but a subcommand that ran and failed has no

internal/sfdupes/cli_test.go

@@ -15,6 +15,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/adrg/xdg"
 	"golang.org/x/sys/unix"
 )
 
@@ -22,6 +23,12 @@ import (
 // error prints and a runtime failure must not.
 const usageMarker = "Usage:"
 
+// helpFlag asks for the help text.
+const helpFlag = "--help"
+
+// workersFlag is the scan and dwim flag that sets the worker count.
+const workersFlag = "--workers"
+
 // TestMain stops the tests at once, saying why, when their temporary
 // directory is not on a persistent filesystem: scan records nothing
 // there (README "Rules for the walk"), so most scan tests would fail
@@ -553,13 +560,17 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	// line.
 	t.Setenv(databaseEnv, "")
 
-	// Whether scan calls the database new depends on that machine too.
-	scanWant := "scan: database " + defaultDatabasePath + " (default)\n"
+	dbPath, err := defaultDatabasePath(os.Geteuid())
+	if err != nil {
+		t.Fatal(err)
+	}
 
-	_, err := os.Stat(defaultDatabasePath)
+	// Whether scan calls the database new depends on that machine too.
+	scanWant := "scan: database " + dbPath + " (default)\n"
+
+	_, err = os.Stat(dbPath)
 	if errors.Is(err, fs.ErrNotExist) {
-		scanWant = "scan: new database " + defaultDatabasePath +
-			" (default)\n"
+		scanWant = "scan: new database " + dbPath + " (default)\n"
 	}
 
 	missing := filepath.Join(t.TempDir(), "nope")
@@ -570,29 +581,30 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	}{
 		cmdScan: {
 			run: func() error {
-				return runScan(t.Context(), []string{missing}, 1, false, false)
+				return runScan(t.Context(), []string{missing},
+					scanOptions{workers: 1})
 			},
 			want: scanWant,
 		},
 		cmdReport: {
 			run: func() error {
-				return runReport(cancelledContext(t), io.Discard)
+				return runReport(cancelledContext(t), nil, io.Discard)
 			},
-			want: "report: database " + defaultDatabasePath + " (default)\n" +
+			want: "report: database " + dbPath + " (default)\n" +
 				reportOpenLine + "\n" + reportOpenLine + "\n",
 		},
 		cmdTrees: {
 			run: func() error {
-				return runTrees(cancelledContext(t), io.Discard)
+				return runTrees(cancelledContext(t), nil, io.Discard)
 			},
-			want: "trees: database " + defaultDatabasePath + " (default)\n" +
+			want: "trees: database " + dbPath + " (default)\n" +
 				treesOpenLine + "\n" + treesOpenLine + "\n",
 		},
 		cmdInfo: {
 			run: func() error {
 				return runInfo(cancelledContext(t), io.Discard)
 			},
-			want: "info: database " + defaultDatabasePath + " (default)\n" +
+			want: "info: database " + dbPath + " (default)\n" +
 				infoOpenLine + "\n" + infoOpenLine + "\n",
 		},
 	}
@@ -619,8 +631,6 @@ func TestRunUsageErrors(t *testing.T) {
 	}{
 		"no subcommand":      {[]string{}, usageMarker},
 		"scan without paths": {[]string{cmdScan}, usageMarker},
-		"report with args":   {[]string{cmdReport, "x"}, usageMarker},
-		"trees with args":    {[]string{cmdTrees, "x"}, usageMarker},
 		"info with args":     {[]string{cmdInfo, "x"}, usageMarker},
 		"dwim with args":     {[]string{cmdDwim, "x"}, usageMarker},
 		"unknown flag":       {[]string{cmdScan, "--nope", "/"}, usageMarker},
@@ -661,7 +671,7 @@ func TestRunScanRejectsWorkersBelowOne(t *testing.T) {
 
 			var stdout, stderr bytes.Buffer
 
-			args := []string{cmdScan, "--workers", workers, t.TempDir()}
+			args := []string{cmdScan, workersFlag, workers, t.TempDir()}
 
 			code := run(args, &stdout, &stderr)
 			if code != exitUsage {
@@ -692,7 +702,7 @@ func TestRunHelp(t *testing.T) {
 
 	// README §Settings: help goes to stderr, exits 0, and leaves stdout
 	// empty.
-	cases := [][]string{{"--help"}, {"-h"}, {cmdScan, "--help"}}
+	cases := [][]string{{helpFlag}, {"-h"}, {cmdScan, helpFlag}}
 
 	for _, args := range cases {
 		var stdout, stderr bytes.Buffer
@@ -711,6 +721,29 @@ func TestRunHelp(t *testing.T) {
 			t.Errorf("run(%v) stdout = %q, want nothing (data only)",
 				args, got)
 		}
+
+		// README §Location: the help of sfdupes and of scan says where
+		// the database is, and that XDG_STATE_HOME must be an absolute
+		// path.
+		for _, want := range []string{
+			"scan and dwim store", databaseEnv, rootDatabasePath,
+			stateHomeEnv, "~/.local/state", "not an absolute path is an error",
+		} {
+			if !strings.Contains(stderr.String(), want) {
+				t.Errorf("run(%v) stderr = %q, want it to name %q",
+					args, stderr.String(), want)
+			}
+		}
+	}
+
+	// The scan help lists --exclude with its argument.
+	var stdout, stderr bytes.Buffer
+
+	run([]string{cmdScan, "-h"}, &stdout, &stderr)
+
+	if !strings.Contains(stderr.String(), excludeFlag+" PATH") {
+		t.Errorf("scan -h stderr = %q, want %s PATH", stderr.String(),
+			excludeFlag)
 	}
 }
 
@@ -719,7 +752,8 @@ func TestRunScanUsageLine(t *testing.T) {
 
 	// README §Subcommands shows this usage line for scan, and the help
 	// and the usage error both print it.
-	const want = "\n  sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...\n"
+	const want = "\n  sfdupes scan [--workers N] [-x] [--no-default-excludes] " +
+		"[--exclude PATH]... PATH...\n"
 
 	cases := map[string][]string{
 		"help":          {cmdScan, "-h"},
@@ -742,6 +776,74 @@ func TestRunScanUsageLine(t *testing.T) {
 	}
 }
 
+func TestRunHelpDescribesOutput(t *testing.T) {
+	t.Parallel()
+
+	// README §Settings: the help of report, trees, info and dwim says
+	// what each prints. The help of report and trees also explains
+	// their PATH operands. TestRunHelp checks the database paragraph
+	// of the root and scan help.
+	escapes := []string{`\\`, `\t`, `\n`, `\r`}
+	operands := []string{
+		"With PATH operands", "at or below them", "working directory",
+		"No PATH is looked up", "not followed",
+	}
+
+	// The subcommand whose help is printed.
+	cases := []struct {
+		subcommand []string
+		want       []string
+	}{
+		{
+			[]string{cmdReport},
+			slices.Concat([]string{
+				"tab-separated", "\n  first ", "\n  dupe ", "\n  size ",
+				"largest first",
+			}, escapes, operands),
+		},
+		{
+			[]string{cmdTrees},
+			slices.Concat([]string{
+				"tab-separated", "\n  first ", "\n  dupe ", "\n  files ",
+				"\n  size ", "\n  disk_size ", "largest first",
+			}, escapes, operands),
+		},
+		{
+			[]string{cmdInfo},
+			[]string{
+				"\n  database ", "\n  database file size ",
+				"\n  -wal file size ", "\n  modified ",
+				"\n  schema version ", "\n  records ",
+				"\n  size of recorded files ",
+			},
+		},
+		{
+			[]string{cmdDwim},
+			[]string{
+				"sfdupes scan /", "as root", "sfdupes trees", "database",
+			},
+		},
+	}
+
+	for _, tc := range cases {
+		var stdout, stderr bytes.Buffer
+
+		args := slices.Concat(tc.subcommand, []string{helpFlag})
+
+		code := run(args, &stdout, &stderr)
+		if code != exitOK {
+			t.Fatalf("run(%v) = %d, want %d", args, code, exitOK)
+		}
+
+		for _, want := range tc.want {
+			if !strings.Contains(stderr.String(), want) {
+				t.Errorf("run(%v) help lacks %q:\n%s",
+					args, want, stderr.String())
+			}
+		}
+	}
+}
+
 func TestRunVersion(t *testing.T) {
 	t.Parallel()
 
@@ -911,8 +1013,7 @@ func TestRunScanSucceedsDespiteWarnings(t *testing.T) {
 
 	stderr := scanOK(t, dir)
 
-	warning := "hash " + unreadable + ": open " + unreadable +
-		": permission denied\n"
+	warning := "scan: skipped " + unreadable + " (permission denied)\n"
 	if !strings.Contains(stderr, warning) {
 		t.Errorf("stderr = %q, want %q", stderr, warning)
 	}
@@ -1184,7 +1285,7 @@ func TestRunScanSkipsSymlinkToFIFOOperand(t *testing.T) {
 
 	stderr := scanOK(t, link)
 
-	warning := "walk " + fifo + ": skipping FIFO operand\n"
+	warning := "scan: skipped " + fifo + " (a FIFO)\n"
 	if !strings.Contains(stderr, warning) {
 		t.Errorf("stderr = %q, want %q", stderr, warning)
 	}
@@ -1219,7 +1320,7 @@ func assertOperandSkipped(t *testing.T, dbPath, operand, kind,
 
 	stderr := scanOK(t, operand)
 
-	warning := "walk " + operand + ": skipping " + kind + " operand\n"
+	warning := "scan: skipped " + operand + " (a " + kind + ")\n"
 	if !strings.Contains(stderr, warning) {
 		t.Errorf("stderr = %q, want %q", stderr, warning)
 	}
@@ -1253,7 +1354,7 @@ func TestRunScanSkipsOperandInsideZFS(t *testing.T) {
 	writeFile(t, hourly, "f", pattern(1, 10))
 	f := writeFile(t, dir, "f", pattern(2, 10))
 
-	warning := "walk " + hourly + ": skipping operand inside a .zfs directory\n"
+	warning := "scan: skipped " + hourly + " (inside a .zfs directory)\n"
 
 	for _, tc := range []struct {
 		operands []string
@@ -1305,7 +1406,7 @@ func TestRunScanSkipsOperandThroughSymlinkIntoZFS(t *testing.T) {
 	stderr := scanOK(t, filepath.Join(snaps, "daily"))
 
 	daily := filepath.Join(dir, "pool", ".zfs", "snapshot", "daily")
-	warning := "walk " + daily + ": skipping operand inside a .zfs directory\n"
+	warning := "scan: skipped " + daily + " (inside a .zfs directory)\n"
 	summary := "scan: 0 files seen (0 added, 0 updated, 0 removed, " +
 		"0 unchanged), 1 skipped\n"
 
@@ -1524,6 +1625,100 @@ func TestRunDatabasePathUsedAsGiven(t *testing.T) {
 	})
 }
 
+func TestRunNonRootUsesStateDirectory(t *testing.T) {
+	// README §Location: without SFDUPES_DATABASE, a user other than
+	// root keeps the database in the XDG state directory. scan creates
+	// the missing directories with mode 0700, and report and trees read
+	// the same file.
+	if os.Geteuid() == 0 {
+		t.Skip("root uses " + rootDatabasePath)
+	}
+
+	t.Cleanup(xdg.Reload)
+
+	state := filepath.Join(t.TempDir(), "state")
+	t.Setenv(stateHomeEnv, state)
+	t.Setenv(databaseEnv, "")
+	xdg.Reload()
+
+	assertRunsUseDatabase(t, filepath.Join(state, "sfdupes"), "db.sqlite")
+
+	for _, dir := range []string{state, filepath.Join(state, "sfdupes")} {
+		fi, err := os.Stat(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if got := fi.Mode().Perm(); got != stateDirPerm {
+			t.Errorf("%s mode = %#o, want %#o", dir, got, stateDirPerm)
+		}
+	}
+}
+
+func TestRunNonRootDatabaseEnvDirectoryMode(t *testing.T) {
+	// README §Location: for SFDUPES_DATABASE, scan creates each missing
+	// directory with mode 755, for a user other than root too.
+	if os.Geteuid() == 0 {
+		t.Skip("needs a user other than root")
+	}
+
+	top := filepath.Join(t.TempDir(), "missing")
+	sub := filepath.Join(top, "sub")
+	t.Setenv(databaseEnv, filepath.Join(sub, "db.sqlite"))
+
+	scanOK(t, t.TempDir())
+
+	for _, dir := range []string{top, sub} {
+		fi, err := os.Stat(dir)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		if got := fi.Mode().Perm(); got != dbDirPerm {
+			t.Errorf("%s mode = %#o, want %#o", dir, got, dbDirPerm)
+		}
+	}
+}
+
+func TestRunRelativeStateHomeFails(t *testing.T) {
+	// README §Location: without SFDUPES_DATABASE, a user other than
+	// root whose XDG_STATE_HOME is not an absolute path gets an error
+	// naming it from every subcommand, which exits 1 before it does
+	// anything else.
+	if os.Geteuid() == 0 {
+		t.Skip("root does not use " + stateHomeEnv)
+	}
+
+	t.Setenv(stateHomeEnv, "relative/state")
+	t.Setenv(databaseEnv, "")
+
+	want := "sfdupes: " + stateHomeEnv +
+		" must be an absolute path, got \"relative/state\"\n"
+
+	// A scan that got past the check would stop at this missing
+	// operand, before it creates anything.
+	missing := filepath.Join(t.TempDir(), "missing")
+
+	for _, args := range [][]string{
+		{cmdScan, missing}, {cmdReport}, {cmdTrees}, {cmdInfo},
+	} {
+		var stdout, stderr bytes.Buffer
+
+		code := run(args, &stdout, &stderr)
+		if code != exitFatal || stderr.String() != want {
+			t.Errorf("run(%v) = %d with stderr %q, want %d and %q",
+				args, code, stderr.String(), exitFatal, want)
+		}
+	}
+
+	// dwim scans /, so runDwim gets the missing operand instead.
+	err := runDwim(t.Context(), missing, scanOptions{workers: 1}, io.Discard)
+	if !errors.Is(err, errStateHomeNotAbsolute) {
+		t.Errorf("runDwim: error = %v, want %v", err,
+			errStateHomeNotAbsolute)
+	}
+}
+
 // holdScanLock takes the lock on the database at path, as a running
 // scan does, and holds it until the test ends. It fails the test when
 // the lock is already held.
@@ -1754,9 +1949,13 @@ func TestStdoutWriteErrorPropagates(t *testing.T) {
 	scanFixture(t)
 
 	cases := map[string]func(context.Context, io.Writer) error{
-		cmdReport: runReport,
-		cmdTrees:  runTrees,
-		cmdInfo:   runInfo,
+		cmdReport: func(ctx context.Context, w io.Writer) error {
+			return runReport(ctx, nil, w)
+		},
+		cmdTrees: func(ctx context.Context, w io.Writer) error {
+			return runTrees(ctx, nil, w)
+		},
+		cmdInfo: runInfo,
 	}
 
 	for name, fn := range cases {

internal/sfdupes/db.go

@@ -15,28 +15,40 @@ import (
 	"strings"
 	"time"
 
+	"github.com/adrg/xdg"
 	"golang.org/x/sys/unix"
 	// The pure-Go SQLite driver, registered as "sqlite"; keeps cgo
 	// disabled.
 	_ "modernc.org/sqlite"
 )
 
-// defaultDatabasePath is where the persistent scan database lives when
-// SFDUPES_DATABASE is not set.
-const defaultDatabasePath = "/var/lib/sfdupes/db.sqlite"
+// rootDatabasePath is the database root uses when SFDUPES_DATABASE is
+// not set.
+const rootDatabasePath = "/var/lib/sfdupes/db.sqlite"
 
 // databaseEnv is the environment variable that overrides the database
 // path.
 const databaseEnv = "SFDUPES_DATABASE"
 
+// stateHomeEnv is the environment variable that names the XDG state
+// directory.
+const stateHomeEnv = "XDG_STATE_HOME"
+
 // schemaVersion is the database schema version this build reads and
 // writes, stored in PRAGMA user_version.
 const schemaVersion = 1
 
-// dbDirPerm is the mode for a database parent directory created by
-// scan.
+// dbDirPerm is the mode for a directory scan creates for root's
+// default database or for SFDUPES_DATABASE.
 const dbDirPerm = 0o755
 
+// stateDirPerm is the mode for a directory scan creates for the
+// database at a user's default path in the XDG state directory. It is
+// the mode the XDG Base Directory specification asks for, and it keeps
+// other users from reading the database, which lists every scanned
+// path.
+const stateDirPerm = 0o700
+
 // dbFilePerm is the mode for a database file created by scan, the mode
 // SQLite gives a database file it creates.
 const dbFilePerm = 0o644
@@ -144,15 +156,48 @@ var errScanRunning = errors.New("another scan is running")
 // 32 bytes of a SHA-256.
 var errHashSize = errors.New("hash is not 32 bytes")
 
+// errStateHomeNotAbsolute reports an XDG_STATE_HOME that is set to a
+// path that is not absolute.
+var errStateHomeNotAbsolute = errors.New(
+	stateHomeEnv + " must be an absolute path")
+
+// defaultDatabasePath is the database used when SFDUPES_DATABASE is not
+// set: rootDatabasePath for root (euid 0), and db.sqlite in the
+// directory sfdupes under the XDG state directory for any other user.
+// xdg reads XDG_STATE_HOME and HOME once, when the program starts. For
+// any other user, an XDG_STATE_HOME that is set and not an absolute
+// path is an error, where xdg would use its default instead. The path
+// is joined as text, not cleaned, for the reason absolutePath gives.
+func defaultDatabasePath(euid int) (string, error) {
+	if euid == 0 {
+		return rootDatabasePath, nil
+	}
+
+	stateHome := os.Getenv(stateHomeEnv)
+	if stateHome != "" && !filepath.IsAbs(stateHome) {
+		return "", fmt.Errorf("%w, got %q", errStateHomeNotAbsolute,
+			stateHome)
+	}
+
+	stateDir := strings.TrimSuffix(xdg.StateHome, "/")
+
+	return stateDir + "/sfdupes/db.sqlite", nil
+}
+
 // databasePath resolves the database location: SFDUPES_DATABASE when
-// set and non-empty, the compiled-in default otherwise. It returns the
+// set and non-empty, defaultDatabasePath otherwise. It returns the
 // path, made absolute by absolutePath, and where the path came from,
 // "default" or "from SFDUPES_DATABASE", for the line each subcommand
 // prints first on stderr.
 func databasePath() (string, string, error) {
-	path, source := defaultDatabasePath, "default"
-	if p := os.Getenv(databaseEnv); p != "" {
-		path, source = p, "from "+databaseEnv
+	path, source := os.Getenv(databaseEnv), "from "+databaseEnv
+	if path == "" {
+		def, err := defaultDatabasePath(os.Geteuid())
+		if err != nil {
+			return "", "", err
+		}
+
+		path, source = def, "default"
 	}
 
 	abs, err := absolutePath(path)
@@ -188,15 +233,23 @@ func absolutePath(path string) (string, error) {
 
 // createDatabaseDirectory creates the directory the database at path
 // goes in, and any directory above it, if missing. path is absolute, as
-// databasePath returns it. The directory is path with its last element
-// cut off as text, by filepath.Split. filepath.Dir would clean it,
-// dropping each ".." together with the directory name before it; when
-// that name is a symlink, the cleaned path names another directory than
-// the one the system reaches when it opens path.
+// databasePath returns it. Each directory gets mode dbDirPerm, or
+// stateDirPerm when databasePath uses a user's default path in the XDG
+// state directory: the user is not root and SFDUPES_DATABASE is unset
+// or empty. The directory is path with its last element cut off as
+// text, by filepath.Split. filepath.Dir would clean it, dropping each
+// ".." together with the directory name before it; when that name is a
+// symlink, the cleaned path names another directory than the one the
+// system reaches when it opens path.
 func createDatabaseDirectory(path string) error {
 	dir, _ := filepath.Split(path)
 
-	err := os.MkdirAll(dir, dbDirPerm)
+	perm := os.FileMode(dbDirPerm)
+	if os.Geteuid() != 0 && os.Getenv(databaseEnv) == "" {
+		perm = stateDirPerm
+	}
+
+	err := os.MkdirAll(dir, perm)
 	if err != nil {
 		return fmt.Errorf("create database directory: %w", err)
 	}

internal/sfdupes/db_test.go

@@ -9,10 +9,13 @@ import (
 	"io/fs"
 	"os"
 	"path/filepath"
+	"runtime"
 	"slices"
 	"strings"
 	"testing"
 	"time"
+
+	"github.com/adrg/xdg"
 )
 
 // testDBPath returns a database path inside a fresh temp dir, with
@@ -56,11 +59,16 @@ func TestDatabasePath(t *testing.T) {
 
 	t.Chdir(dir)
 
+	defaultPath, err := defaultDatabasePath(os.Geteuid())
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	fromEnv := "from " + databaseEnv
 	cases := []struct {
 		env, path, source string
 	}{
-		{"", defaultDatabasePath, "default"},
+		{"", defaultPath, "default"},
 		{"/custom/place.sqlite", "/custom/place.sqlite", fromEnv},
 		{"/custom/link/../place.sqlite", "/custom/link/../place.sqlite",
 			fromEnv},
@@ -89,6 +97,48 @@ func TestDatabasePath(t *testing.T) {
 	}
 }
 
+func TestDefaultDatabasePath(t *testing.T) {
+	// README §Location: root's database is /var/lib/sfdupes/db.sqlite.
+	// Any other user's is db.sqlite in sfdupes under XDG_STATE_HOME, or
+	// under ~/.local/state when XDG_STATE_HOME is unset or empty. For
+	// that user, an XDG_STATE_HOME that is not an absolute path is an
+	// error.
+	if runtime.GOOS != goosLinux {
+		t.Skip("macOS has another default state directory")
+	}
+
+	t.Cleanup(xdg.Reload)
+	t.Setenv("HOME", "/home/someone")
+
+	cases := []struct {
+		euid      int
+		stateHome string
+		want      string
+		wantErr   error
+	}{
+		{0, "", rootDatabasePath, nil},
+		{0, "/elsewhere/state", rootDatabasePath, nil},
+		{0, "relative/state", rootDatabasePath, nil},
+		{1000, "", "/home/someone/.local/state/sfdupes/db.sqlite", nil},
+		{1000, "relative/state", "", errStateHomeNotAbsolute},
+		{1000, "~/state", "", errStateHomeNotAbsolute},
+		{1000, "/elsewhere/state", "/elsewhere/state/sfdupes/db.sqlite", nil},
+		{1000, "/elsewhere/state/", "/elsewhere/state/sfdupes/db.sqlite", nil},
+	}
+
+	for _, tc := range cases {
+		t.Setenv(stateHomeEnv, tc.stateHome)
+		xdg.Reload()
+
+		got, err := defaultDatabasePath(tc.euid)
+		if got != tc.want || !errors.Is(err, tc.wantErr) {
+			t.Errorf("defaultDatabasePath(%d) with %s=%q = %q, %v; want %q, %v",
+				tc.euid, stateHomeEnv, tc.stateHome, got, err, tc.want,
+				tc.wantErr)
+		}
+	}
+}
+
 func TestOpenScanDatabaseCreates(t *testing.T) {
 	t.Parallel()
 

internal/sfdupes/dwim.go

@@ -5,19 +5,20 @@ import (
 	"fmt"
 	"io"
 	"os"
-	"runtime"
 )
 
 // runDwim implements the dwim subcommand: it runs what "sfdupes scan
-// root" runs with no flags, then what "sfdupes trees" runs, and ends
-// with a line on stderr saying how to print the report again. The dwim
-// command passes "/" as root. It catches SIGINT and SIGTERM as scan
-// does, but only during the scan. An error from the scan, an interrupt
-// included, ends it before the report. During the report, a signal
-// ends it at once, as it ends trees.
-func runDwim(ctx context.Context, root string, stdout io.Writer) error {
+// root" runs with the scan flags in opts, then what "sfdupes trees"
+// runs, and ends with a line on stderr saying how to print the report
+// again. The dwim command passes "/" as root. It catches SIGINT and
+// SIGTERM as scan does, but only during the scan. An error from the
+// scan, an interrupt included, ends it before the report. During the
+// report, a signal ends it at once, as it ends trees.
+func runDwim(
+	ctx context.Context, root string, opts scanOptions, stdout io.Writer,
+) error {
 	scanCtx, stop := interruptContext(ctx)
-	err := runScan(scanCtx, []string{root}, runtime.NumCPU(), false, false)
+	err := runScan(scanCtx, []string{root}, opts)
 
 	stop()
 
@@ -25,7 +26,7 @@ func runDwim(ctx context.Context, root string, stdout io.Writer) error {
 		return err
 	}
 
-	err = runTrees(ctx, stdout)
+	err = runTrees(ctx, nil, stdout)
 	if err != nil {
 		return err
 	}

internal/sfdupes/dwim_test.go

@@ -2,7 +2,10 @@ package sfdupes //nolint:testpackage // calls the unexported runDwim
 
 import (
 	"bytes"
+	"errors"
+	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -28,7 +31,7 @@ func TestRunDwimPrintsLargestTreesFirst(t *testing.T) {
 
 	stderr := captureStderr(t)
 
-	err := runDwim(t.Context(), root, &stdout)
+	err := runDwim(t.Context(), root, scanOptions{workers: 1}, &stdout)
 	if err != nil {
 		t.Fatalf("runDwim: %v; stderr: %s", err, stderr())
 	}
@@ -67,7 +70,7 @@ func TestRunDwimScanErrorPrintsNoReport(t *testing.T) {
 
 	stderr := captureStderr(t)
 
-	err := runDwim(t.Context(), missing, &stdout)
+	err := runDwim(t.Context(), missing, scanOptions{workers: 1}, &stdout)
 	if err == nil {
 		t.Fatal("runDwim on a missing root: error = nil, want it fatal")
 	}
@@ -81,3 +84,127 @@ func TestRunDwimScanErrorPrintsNoReport(t *testing.T) {
 		t.Errorf("stderr = %q, want nothing from trees or dwim", got)
 	}
 }
+
+func TestRunDwimExcludeLeavesTreeOut(t *testing.T) {
+	// dwim passes its --exclude paths to the scan: an excluded copy is
+	// not recorded, so its pair is not reported.
+	t.Setenv(databaseEnv, testDBPath(t))
+
+	root := realTempDir(t)
+
+	kept := []string{
+		filepath.Dir(writeFile(t, root, "a1/f.bin", pattern(1, 300))),
+		filepath.Dir(writeFile(t, root, "a2/f.bin", pattern(1, 300))),
+	}
+	writeFile(t, root, "b1/f.bin", pattern(2, 5000))
+	excluded := filepath.Dir(writeFile(t, root, "b2/f.bin", pattern(2, 5000)))
+
+	var stdout bytes.Buffer
+
+	stderr := captureStderr(t)
+
+	opts := scanOptions{workers: 1, excludePaths: []string{excluded}}
+
+	err := runDwim(t.Context(), root, opts, &stdout)
+	if err != nil {
+		t.Fatalf("runDwim: %v; stderr: %s", err, stderr())
+	}
+
+	lines := strings.Split(strings.TrimSuffix(stdout.String(), "\n"), "\n")
+	if len(lines) != 2 {
+		t.Fatalf("stdout = %q, want a header and one group",
+			stdout.String())
+	}
+
+	want := kept[0] + "\t" + kept[1] + "\t1\t300\t"
+	if !strings.HasPrefix(lines[1], want) {
+		t.Errorf("group = %q, want it to start %q", lines[1], want)
+	}
+
+	leftOut := "scan: left out " + excluded + " (--exclude)\n"
+	if got := stderr(); !strings.Contains(got, leftOut) {
+		t.Errorf("stderr = %q, want it to contain %q", got, leftOut)
+	}
+}
+
+func TestRunDwimTakesScanFlags(t *testing.T) {
+	// The dwim command parses and checks the scan flags as scan does.
+	// Each case stops before the walk of / begins, and before the
+	// database is created. "dwim --exclude /" excludes its own operand.
+	cases := map[string]struct {
+		args []string
+		code int
+		want string
+	}{
+		"--exclude /": {
+			[]string{cmdDwim, excludeFlag, "/"},
+			exitFatal,
+			"sfdupes: scan /: operand excluded by --exclude /\n",
+		},
+		"--exclude empty": {
+			[]string{cmdDwim, excludeFlag, ""},
+			exitUsage,
+			"Error: --exclude must not be empty\n",
+		},
+		"--workers 0": {
+			[]string{cmdDwim, workersFlag, "0", "-x", "--no-default-excludes"},
+			exitUsage,
+			"Error: --workers must be at least 1, got 0\n",
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			path := testDBPath(t)
+			t.Setenv(databaseEnv, path)
+
+			stdout := captureStdout(t)
+			stderr := captureStderr(t)
+
+			code := run(tc.args, os.Stdout, os.Stderr)
+			if code != tc.code {
+				t.Errorf("run(%q) = %d, want %d", tc.args, code, tc.code)
+			}
+
+			if !strings.HasSuffix(stderr(), tc.want) || stdout() != "" {
+				t.Errorf("stdout, stderr = %q, %q, want \"\" and stderr "+
+					"ending %q", stdout(), stderr(), tc.want)
+			}
+
+			_, err := os.Lstat(path)
+			if !errors.Is(err, os.ErrNotExist) {
+				t.Errorf("lstat %s: %v, want no database", path, err)
+			}
+		})
+	}
+}
+
+func TestRunDwimHelpListsScanFlags(t *testing.T) {
+	t.Parallel()
+
+	// README "Subcommands" shows this usage line for dwim.
+	const want = "\n  sfdupes dwim [--workers N] [-x] [--no-default-excludes] " +
+		"[--exclude PATH]...\n"
+
+	var stdout, stderr bytes.Buffer
+
+	run([]string{cmdDwim, "-h"}, &stdout, &stderr)
+
+	if !strings.Contains(stderr.String(), want) {
+		t.Errorf("dwim -h stderr = %q, want the usage line %q",
+			stderr.String(), want)
+	}
+
+	if !strings.Contains(stderr.String(), excludeFlag+" PATH") {
+		t.Errorf("dwim -h stderr = %q, want %s PATH", stderr.String(),
+			excludeFlag)
+	}
+
+	// README "Settings": the help lists the default excludes and the
+	// filesystem types the walk never records anything on.
+	help := strings.TrimSpace(defaultExcludesHelp(runtime.GOOS))
+	if !strings.Contains(stderr.String(), help) {
+		t.Errorf("dwim -h stderr = %q, want it to hold %q", stderr.String(),
+			help)
+	}
+}

internal/sfdupes/exclude.go

@@ -1,8 +1,9 @@
 package sfdupes
 
-// This file holds two rules for the walk (README "Rules for the walk"):
+// This file holds three rules for the walk (README "Rules for the
+// walk"): the paths given with --exclude, which the scan never enters;
 // the default excludes, the paths a scan's walk does not enter below an
-// operand unless scan is given --no-default-excludes, and the
+// operand unless scan is given --no-default-excludes; and the
 // filesystems that are not persistent, on which the scan never records
 // or reads anything.
 
@@ -12,6 +13,7 @@ import (
 	"fmt"
 	"io"
 	"io/fs"
+	"os"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -43,17 +45,25 @@ const typesPerHelpLine = 6
 // readMountInfo reads.
 var errMountInfoLine = errors.New("unexpected line")
 
+// errExcludedOperand reports an operand at or below a path given with
+// --exclude.
+var errExcludedOperand = errors.New("operand excluded by --exclude")
+
 // walkRules holds the rules for the walk that a scan sets up as it
-// starts. excludes are the default excludes, or none with
-// --no-default-excludes. nonPersistent maps the device ID of every
-// mounted filesystem that is not persistent to its type; the scan
-// records and reads nothing on one of those, whatever the flags.
-// databaseFiles are the scan's own database files, which the walk
-// passes over (see databaseFiles in db.go).
+// starts. excludePaths are the paths given with --exclude, each with
+// the device ID and inode number of the file it names (see
+// excludePathInodes), which the walk neither enters nor records,
+// whatever path leads to them. defaultExcludes are the default
+// excludes, or none with --no-default-excludes. nonPersistent maps the
+// device ID of every mounted filesystem that is not persistent to its
+// type; the scan records and reads nothing on one of those, whatever
+// the flags. databaseFiles are the scan's own database files, which the
+// walk passes over (see databaseFiles in db.go).
 type walkRules struct {
-	excludes      []string
-	nonPersistent map[uint64]string
-	databaseFiles []fileRec
+	excludePaths    []fileRec
+	defaultExcludes []string
+	nonPersistent   map[uint64]string
+	databaseFiles   []fileRec
 }
 
 // defaultExcludes returns the default excludes of the system goos
@@ -62,14 +72,31 @@ type walkRules struct {
 func defaultExcludes(goos string) []string {
 	switch goos {
 	case goosLinux:
-		return []string{"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"}
+		// The kernel's view of the running system, temporary files, and
+		// caches that programs and package managers fill again on their
+		// own: the system's, root's and those in each home in /home.
+		return []string{
+			"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp", "/var/cache",
+			"/root/.cache", "/root/.npm", "/root/.cargo/registry",
+			"/root/go/pkg/mod",
+			"/home/*/.cache", "/home/*/.npm", "/home/*/.cargo/registry",
+			"/home/*/go/pkg/mod",
+		}
 	case goosDarwin:
 		// A walk of / reaches everything in /System/Volumes/Data through
 		// the firmlinks, so walking it as well would record each of
-		// those files twice.
+		// those files twice. /private/var/folders holds each user's
+		// temporary files and caches. The walk reaches root's home,
+		// /var/root, as /private/var/root.
 		return []string{
 			"/dev", "/private/tmp", "/private/var/tmp", "/private/var/vm",
 			"/System/Volumes/VM", "/System/Volumes/Data",
+			"/private/var/folders", "/Library/Caches",
+			"/private/var/root/Library/Caches", "/private/var/root/.cache",
+			"/private/var/root/.npm", "/private/var/root/.cargo/registry",
+			"/private/var/root/go/pkg/mod",
+			"/Users/*/Library/Caches", "/Users/*/.cache", "/Users/*/.npm",
+			"/Users/*/.cargo/registry", "/Users/*/go/pkg/mod",
 		}
 	case goosWindows:
 		// Relative to the root of the drive an operand lies on. The
@@ -97,6 +124,88 @@ func isDefaultExclude(path string, excludes []string) bool {
 	return false
 }
 
+// excludePathInodes returns each of paths, the --exclude values, as its
+// real path (see realPaths) with the device ID and inode number of the
+// file there. The walk compares what it reaches with those, as it does
+// with the database files, so it also leaves out an --exclude path
+// spelled another way, such as in another letter case on macOS.
+func excludePathInodes(paths []string) ([]fileRec, error) {
+	resolved, err := realPaths(paths)
+	if err != nil {
+		return nil, err
+	}
+
+	files := make([]fileRec, 0, len(resolved))
+
+	for _, path := range resolved {
+		fi, err := os.Lstat(path)
+		if err != nil {
+			return nil, err
+		}
+
+		dev, ino := inodeOfInfo(fi)
+		files = append(files, fileRec{path: path, dev: dev, ino: ino})
+	}
+
+	return files, nil
+}
+
+// matchingExcludePath returns the path of the one of excludePaths with
+// the device ID and inode number of fi, an lstat result, or "" when
+// there is none.
+func matchingExcludePath(fi fs.FileInfo, excludePaths []fileRec) string {
+	dev, ino := inodeOfInfo(fi)
+	f := fileRec{dev: dev, ino: ino}
+
+	for _, exclude := range excludePaths {
+		if sameInode(exclude, f) {
+			return exclude.path
+		}
+	}
+
+	return ""
+}
+
+// checkOperandsNotExcluded returns an error when one of roots, the
+// operands, or a directory above it is one of excludePaths, the
+// --exclude paths, by device ID and inode number. The error names the
+// operand and the --exclude path. The roots are absolute, so the
+// directories above each one end at /. A path here that lstat cannot
+// read is an error too.
+func checkOperandsNotExcluded(roots []string, excludePaths []fileRec) error {
+	for _, root := range roots {
+		for path := root; ; path = filepath.Dir(path) {
+			fi, err := os.Lstat(path)
+			if err != nil {
+				return fmt.Errorf("scan %s: %w", root, err)
+			}
+
+			exclude := matchingExcludePath(fi, excludePaths)
+			if exclude != "" {
+				return fmt.Errorf("scan %s: %w %s", root, errExcludedOperand,
+					exclude)
+			}
+
+			if path == "/" {
+				break
+			}
+		}
+	}
+
+	return nil
+}
+
+// excludePathLine returns the line the scan prints for path when fi,
+// its lstat, makes it one of rules.excludePaths, and "" when it does
+// not.
+func (rules walkRules) excludePathLine(path string, fi fs.FileInfo) string {
+	if matchingExcludePath(fi, rules.excludePaths) == "" {
+		return ""
+	}
+
+	return fmt.Sprintf("scan: left out %s (--exclude)", path)
+}
+
 // nonPersistentFilesystemTypes returns the filesystem types that are
 // not persistent: filesystems that the kernel makes up or that are kept
 // in memory. devfs is the macOS filesystem of /dev; the others are
@@ -124,7 +233,7 @@ func (rules walkRules) nonPersistentLine(path string, fi fs.FileInfo) string {
 		return ""
 	}
 
-	return fmt.Sprintf("scan: skipped %s (%s, not a persistent filesystem)",
+	return fmt.Sprintf("scan: left out %s (%s, not a persistent filesystem)",
 		path, fsType)
 }
 
@@ -162,7 +271,7 @@ func readMountInfo(mountinfo io.Reader) (map[uint64]string, error) {
 	return devices, lines.Err()
 }
 
-// defaultExcludesHelp describes, for the scan command's help, the
+// defaultExcludesHelp describes, for the scan and dwim help, the
 // default excludes of the system goos names and, where the scan lists
 // the mounted filesystems, the types it never records anything on.
 func defaultExcludesHelp(goos string) string {

internal/sfdupes/exclude_test.go

@@ -14,6 +14,9 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// excludeFlag is the scan flag that names a path the walk never enters.
+const excludeFlag = "--exclude"
+
 func TestDefaultExcludes(t *testing.T) {
 	t.Parallel()
 
@@ -21,10 +24,22 @@ func TestDefaultExcludes(t *testing.T) {
 	// on any other system. The Windows list exists in the code only: the
 	// binary does not build for Windows, so the README leaves it out.
 	cases := map[string][]string{
-		goosLinux: {"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp"},
+		goosLinux: {
+			"/proc", "/sys", "/dev", "/run", "/tmp", "/var/tmp", "/var/cache",
+			"/root/.cache", "/root/.npm", "/root/.cargo/registry",
+			"/root/go/pkg/mod",
+			"/home/*/.cache", "/home/*/.npm", "/home/*/.cargo/registry",
+			"/home/*/go/pkg/mod",
+		},
 		goosDarwin: {
 			"/dev", "/private/tmp", "/private/var/tmp", "/private/var/vm",
 			"/System/Volumes/VM", "/System/Volumes/Data",
+			"/private/var/folders", "/Library/Caches",
+			"/private/var/root/Library/Caches", "/private/var/root/.cache",
+			"/private/var/root/.npm", "/private/var/root/.cargo/registry",
+			"/private/var/root/go/pkg/mod",
+			"/Users/*/Library/Caches", "/Users/*/.cache", "/Users/*/.npm",
+			"/Users/*/.cargo/registry", "/Users/*/go/pkg/mod",
 		},
 		goosWindows: {
 			"pagefile.sys", "swapfile.sys", "hiberfil.sys",
@@ -60,6 +75,29 @@ func TestIsDefaultExclude(t *testing.T) {
 			t.Errorf("isDefaultExclude(%q) = %v, want %v", path, got, want)
 		}
 	}
+
+	// The Linux list matches the cache directories in /root and in each
+	// home in /home, but nothing below them and no .cache directory
+	// elsewhere.
+	linux := map[string]bool{
+		"/home/u/.cache":           true,
+		"/home/u/go/pkg/mod":       true,
+		"/root/.cache":             true,
+		"/var/cache":               true,
+		"/home/u/.cache/pip":       false,
+		"/home/u/src/.cache":       false,
+		"/srv/home/u/.cache":       false,
+		"/home/u/.cargo/bin":       false,
+		"/home/u/go/pkg/mod/cache": false,
+	}
+
+	for path, want := range linux {
+		got := isDefaultExclude(path, defaultExcludes(goosLinux))
+		if got != want {
+			t.Errorf("isDefaultExclude(%q, Linux list) = %v, want %v", path,
+				got, want)
+		}
+	}
 }
 
 func TestReadMountInfo(t *testing.T) {
@@ -188,7 +226,7 @@ func TestScanDoesNotEnterDefaultExclude(t *testing.T) {
 	unseen := writeFile(t, excluded, "new", pattern(2, 10))
 
 	st, err := syncScan(t.Context(), db, []string{dir}, 1, false,
-		walkRules{excludes: []string{excluded}})
+		walkRules{defaultExcludes: []string{excluded}})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -204,7 +242,7 @@ func TestScanDoesNotEnterDefaultExclude(t *testing.T) {
 		t.Errorf("%s recorded: the walk entered %s", unseen, excluded)
 	}
 
-	line := "\nscan: skipped " + excluded + " (default exclude)\n"
+	line := "\nscan: left out " + excluded + " (default exclude)\n"
 	if !strings.Contains(stderr(), line) {
 		t.Errorf("stderr = %q, want %q", stderr(), line)
 	}
@@ -234,7 +272,7 @@ func TestScanWalksOperandInDefaultExclude(t *testing.T) {
 			db := openTestDB(t)
 
 			st, err := syncScan(t.Context(), db, roots, 1, false,
-				walkRules{excludes: []string{excluded}})
+				walkRules{defaultExcludes: []string{excluded}})
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -262,7 +300,7 @@ func TestScanNamesNoDefaultExcludeThatIsAnOperand(t *testing.T) {
 	f := writeFile(t, excluded, "f", pattern(1, 10))
 
 	st, err := syncScan(t.Context(), db, []string{dir, excluded}, 1, false,
-		walkRules{excludes: []string{excluded}})
+		walkRules{defaultExcludes: []string{excluded}})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -273,7 +311,7 @@ func TestScanNamesNoDefaultExcludeThatIsAnOperand(t *testing.T) {
 
 	recordByPath(t, dbRecords(t, db), f)
 
-	if strings.Contains(stderr(), "scan: skipped") {
+	if strings.Contains(stderr(), "scan: left out") {
 		t.Errorf("stderr = %q, want no line for %s", stderr(), excluded)
 	}
 }
@@ -287,15 +325,15 @@ func TestRunScanTempDirOperand(t *testing.T) {
 	t.Setenv(databaseEnv, path)
 
 	dir := realTempDir(t)
-	if !underAnyRoot(dir, defaultExcludes(runtime.GOOS)) {
+	if !insideDefaultExclude(dir, defaultExcludes(runtime.GOOS)) {
 		t.Skip(dir + " is not inside a default exclude")
 	}
 
 	f := writeFile(t, dir, "sub/f", pattern(1, 10))
 
 	stderr := scanOK(t, dir)
-	if strings.Contains(stderr, "scan: skipped") {
-		t.Errorf("stderr = %q, want no line saying a path was skipped",
+	if strings.Contains(stderr, "scan: left out") {
+		t.Errorf("stderr = %q, want no line saying a path was left out",
 			stderr)
 	}
 
@@ -339,7 +377,7 @@ func TestRunScanNoDefaultExcludes(t *testing.T) {
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)
 
-	line := "scan: skipped /var/tmp (default exclude)\n"
+	line := "scan: left out /var/tmp (default exclude)\n"
 
 	if stderr := scanOK(t, "/var"); !strings.Contains(stderr, line) {
 		t.Errorf("scan /var: stderr = %q, want %q", stderr, line)
@@ -356,6 +394,217 @@ func TestRunScanNoDefaultExcludes(t *testing.T) {
 	assertRecorded(t, path, f, true)
 }
 
+// TestRunScanExcludeNested scans a tree through the command with an
+// --exclude path two levels below the operand. The walk records nothing
+// at or below it and says so. The record an earlier scan left there is
+// kept although its file is gone.
+func TestRunScanExcludeNested(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := realTempDir(t)
+	excluded := filepath.Join(dir, "a", "b")
+	kept := writeFile(t, dir, "a/f", pattern(1, 10))
+	gone := writeFile(t, excluded, "gone", pattern(2, 10))
+
+	scanOK(t, dir)
+
+	err := os.Remove(gone)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	unseen := writeFile(t, excluded, "c/new", pattern(3, 10))
+
+	stderr := scanOK(t, excludeFlag, excluded, dir)
+
+	line := "scan: left out " + excluded + " (--exclude)\n"
+	if !strings.Contains(stderr, line) {
+		t.Errorf("stderr = %q, want %q", stderr, line)
+	}
+
+	assertRecorded(t, path, kept, true)
+	assertRecorded(t, path, gone, true)
+	assertRecorded(t, path, unseen, false)
+}
+
+// TestRunScanExcludeRepeated gives --exclude twice, once for a
+// directory and once for a file, the file through a symlink to it. The
+// walk records neither, and records the file beside them.
+func TestRunScanExcludeRepeated(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := realTempDir(t)
+	inDir := writeFile(t, dir, "skip/f", pattern(1, 10))
+	file := writeFile(t, dir, "skip-file", pattern(2, 10))
+	kept := writeFile(t, dir, "keep", pattern(3, 10))
+
+	link := filepath.Join(t.TempDir(), "link")
+	symlink(t, file, link)
+
+	scanOK(t, excludeFlag, filepath.Dir(inDir), excludeFlag, link, dir)
+
+	assertRecorded(t, path, inDir, false)
+	assertRecorded(t, path, file, false)
+	assertRecorded(t, path, kept, true)
+}
+
+// TestRunScanExcludeReachedByAnotherPath gives --exclude a directory
+// through a symlink and a file through "..", so the walk reaches
+// neither by the path given. The walk also reaches the file through a
+// hard link. It records none of these, prints a line naming each, and
+// records the file beside them.
+func TestRunScanExcludeReachedByAnotherPath(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := realTempDir(t)
+	inDir := writeFile(t, dir, "a/cache/f", pattern(1, 10))
+	file := writeFile(t, dir, "a/big", pattern(2, 10))
+	kept := writeFile(t, dir, "keep", pattern(3, 10))
+
+	hardLink := filepath.Join(dir, "big-link")
+
+	err := os.Link(file, hardLink)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	link := filepath.Join(t.TempDir(), "link")
+	symlink(t, filepath.Join(dir, "a"), link)
+
+	// Joined as text, since filepath.Join would drop the "..".
+	dotDot := dir + "/a/cache/../big"
+
+	stderr := scanOK(t, excludeFlag, filepath.Join(link, "cache"),
+		excludeFlag, dotDot, dir)
+
+	for _, leftOut := range []string{filepath.Dir(inDir), file, hardLink} {
+		line := "scan: left out " + leftOut + " (--exclude)\n"
+		if !strings.Contains(stderr, line) {
+			t.Errorf("stderr = %q, want %q", stderr, line)
+		}
+	}
+
+	assertRecorded(t, path, inDir, false)
+	assertRecorded(t, path, file, false)
+	assertRecorded(t, path, hardLink, false)
+	assertRecorded(t, path, kept, true)
+}
+
+// TestRunScanExcludedOperand checks that an operand at or below an
+// --exclude path is a fatal error that names both, and so is an
+// --exclude path that does not exist. An operand that is a hard link to
+// an --exclude file is at it. Neither scan writes a database.
+func TestRunScanExcludedOperand(t *testing.T) {
+	dir := realTempDir(t)
+	sub := filepath.Join(dir, "sub")
+	f := writeFile(t, sub, "f", pattern(1, 10))
+
+	hardLink := filepath.Join(dir, "f-link")
+
+	err := os.Link(f, hardLink)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cases := map[string]struct {
+		args []string
+		want string
+	}{
+		"at it": {
+			[]string{excludeFlag, sub, sub},
+			"scan " + sub + ": operand excluded by --exclude " + sub + "\n",
+		},
+		"below it": {
+			[]string{excludeFlag, dir, sub},
+			"scan " + sub + ": operand excluded by --exclude " + dir + "\n",
+		},
+		"after an operand it does not exclude": {
+			[]string{dir, excludeFlag, sub, sub},
+			"scan " + sub + ": operand excluded by --exclude " + sub + "\n",
+		},
+		"a hard link to it": {
+			[]string{excludeFlag, f, hardLink},
+			"scan " + hardLink + ": operand excluded by --exclude " + f + "\n",
+		},
+		"exclude missing": {
+			[]string{excludeFlag, filepath.Join(dir, "nope"), dir},
+			"--exclude: resolve " + filepath.Join(dir, "nope") + ": ",
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			path := testDBPath(t)
+			t.Setenv(databaseEnv, path)
+
+			stdout := captureStdout(t)
+			stderr := captureStderr(t)
+
+			code := run(append([]string{cmdScan}, tc.args...), os.Stdout,
+				os.Stderr)
+			if code != exitFatal {
+				t.Errorf("run(scan %q) = %d, want %d", tc.args, code,
+					exitFatal)
+			}
+
+			assertFatalOutput(t, stderr(), stdout())
+
+			if !strings.Contains(stderr(), "sfdupes: "+tc.want) {
+				t.Errorf("stderr = %q, want %q", stderr(), tc.want)
+			}
+
+			_, err := os.Lstat(path)
+			if !errors.Is(err, os.ErrNotExist) {
+				t.Errorf("lstat %s: %v, want no database", path, err)
+			}
+		})
+	}
+}
+
+// TestRunScanRejectsEmptyExclude checks that an empty --exclude value,
+// given as its own argument, after "=" or after another --exclude, is
+// a usage error reported in one line before anything is scanned
+// (README "Settings").
+func TestRunScanRejectsEmptyExclude(t *testing.T) {
+	dir := t.TempDir()
+
+	cases := map[string][]string{
+		"separate":     {excludeFlag, "", dir},
+		"joined by =":  {excludeFlag + "=", dir},
+		"after a path": {excludeFlag, t.TempDir(), excludeFlag, "", dir},
+	}
+
+	for name, args := range cases {
+		t.Run(name, func(t *testing.T) {
+			path := testDBPath(t)
+			t.Setenv(databaseEnv, path)
+
+			var stdout, stderr bytes.Buffer
+
+			code := run(append([]string{cmdScan}, args...), &stdout,
+				&stderr)
+			if code != exitUsage {
+				t.Errorf("run(scan %q) = %d, want %d", args, code,
+					exitUsage)
+			}
+
+			want := "Error: --exclude must not be empty\n"
+			if stdout.String() != "" || stderr.String() != want {
+				t.Errorf("stdout, stderr = %q, %q, want \"\", %q",
+					stdout.String(), stderr.String(), want)
+			}
+
+			_, err := os.Lstat(path)
+			if !errors.Is(err, os.ErrNotExist) {
+				t.Errorf("lstat %s: %v, want no database", path, err)
+			}
+		})
+	}
+}
+
 // TestRunScanSkipsNonPersistentOperand scans, through the command, a
 // directory on a filesystem that is not persistent, with and without
 // --no-default-excludes: either way it is skipped, with a line saying
@@ -366,7 +615,7 @@ func TestRunScanSkipsNonPersistentOperand(t *testing.T) {
 
 	t.Setenv(databaseEnv, testDBPath(t))
 
-	line := "scan: skipped " + shm + " (" + fsType +
+	line := "scan: left out " + shm + " (" + fsType +
 		", not a persistent filesystem)\n"
 	summary := "scan: 0 files seen"
 
@@ -420,7 +669,7 @@ func TestScanKeepsRecordsOnNonPersistent(t *testing.T) {
 		t.Errorf("%s recorded", unseen)
 	}
 
-	line := "scan: skipped " + shm + " (" + fsType +
+	line := "scan: left out " + shm + " (" + fsType +
 		", not a persistent filesystem)\n"
 	if !strings.Contains(stderr(), line) {
 		t.Errorf("stderr = %q, want %q", stderr(), line)
@@ -453,7 +702,7 @@ func TestWalkLeavesOutNonPersistent(t *testing.T) {
 
 		leftOut = append(leftOut, ev.leftOut)
 
-		want := "scan: skipped " + ev.leftOut + " (" + fsType +
+		want := "scan: left out " + ev.leftOut + " (" + fsType +
 			", not a persistent filesystem)"
 		if ev.warn != want {
 			t.Errorf("line = %q, want %q", ev.warn, want)
@@ -497,6 +746,20 @@ func shmTempDir(t *testing.T) (string, string, walkRules) {
 	return dir, fsType, walkRules{nonPersistent: nonPersistent}
 }
 
+// insideDefaultExclude reports whether path, which is absolute and
+// clean, or a directory above it is one of excludes.
+func insideDefaultExclude(path string, excludes []string) bool {
+	for dir := path; ; dir = filepath.Dir(dir) {
+		if isDefaultExclude(dir, excludes) {
+			return true
+		}
+
+		if dir == "/" {
+			return false
+		}
+	}
+}
+
 // assertRecorded checks that the database at dbPath holds a record for
 // path when want is set, and none when it is not.
 func assertRecorded(t *testing.T, dbPath, path string, want bool) {

internal/sfdupes/progress.go

@@ -9,8 +9,10 @@ import (
 	"strings"
 	"sync"
 	"time"
+	"unicode"
 	"unicode/utf8"
 
+	"github.com/rivo/uniseg"
 	"github.com/schollz/progressbar/v3"
 	"golang.org/x/term"
 )
@@ -116,7 +118,8 @@ type drawnCount struct {
 // terminal's width. A phase with an exact total ends the line with its
 // percent and bar, at the terminal's right edge, when the terminal has
 // room for them. It is drawn again every redrawInterval, so its elapsed
-// time keeps moving while the phase waits for its next item. When
+// time keeps moving while the phase waits for its next item. Each draw
+// of the walk shows the path it reached last on the line below. When
 // stderr is not a TTY it emits no ANSI redraws: it prints line as the
 // phase starts, every plainInterval while it runs, and as it ends.
 //
@@ -141,6 +144,7 @@ type progress struct {
 	estimate   bool
 	waitingFor string
 	stalledOn  string
+	path       string // the path the walk reached last; "" for none
 	count      int64
 	start      time.Time // when the phase started, for the elapsed time
 	rateStart  time.Time // when the count started, for the time remaining
@@ -239,6 +243,82 @@ func (p *progress) draw() {
 	// less the terminal's last column, which spinnerMargin keeps clear. A
 	// bar's cells take one column each but more than one byte.
 	p.widest = max(p.widest, utf8.RuneCountInString(text)+spinnerMargin-1)
+
+	if p.path != "" {
+		// The library draws the path on the line below and returns to
+		// the phase's line. The path is escaped as warnings are, and its
+		// other control characters are written out, so that the terminal
+		// shows them instead of acting on them. It stays clear of the
+		// last column, so that no terminal wraps it.
+		shown := escapeControlChars(escapePath(p.path))
+		_ = p.bar.AddDetail(cutToColumns(shown, width-1))
+	}
+}
+
+// escapeControlChars returns s with each control character written as
+// \x and its code in two hex digits, as \x1b for the escape character.
+// The control characters are U+0000 to U+001F, U+007F and U+0080 to
+// U+009F, as unicode.IsControl reports them. A byte from 0x80 to 0x9F
+// that is not part of a valid UTF-8 character is written the same way,
+// as \x9b, since a terminal not in UTF-8 mode acts on it. Every other
+// byte is kept as it is.
+func escapeControlChars(s string) string {
+	var b strings.Builder
+
+	for rest := s; rest != ""; {
+		char, size := utf8.DecodeRuneInString(rest)
+		if char == utf8.RuneError && size == 1 {
+			// Not part of a valid UTF-8 character. A terminal not in
+			// UTF-8 mode reads the byte as the character with its code,
+			// so that 0x9b is a control character there and 0xe9 is not.
+			char = rune(rest[0])
+		}
+
+		if unicode.IsControl(char) {
+			fmt.Fprintf(&b, `\x%02x`, char)
+		} else {
+			b.WriteString(rest[:size])
+		}
+
+		rest = rest[size:]
+	}
+
+	return b.String()
+}
+
+// cutToColumns returns s cut to at most columns terminal columns, at a
+// character boundary. A wide character, as in Chinese, takes two
+// columns.
+func cutToColumns(s string, columns int) string {
+	used, end, state := 0, 0, -1
+
+	for rest := s; rest != ""; {
+		char, next, width, nextState := uniseg.FirstGraphemeClusterInString(
+			rest, state)
+		if used+width > columns {
+			break
+		}
+
+		used += width
+		end += len(char)
+		rest, state = next, nextState
+	}
+
+	return s[:end]
+}
+
+// clear erases the display from the terminal: its line, and the path on
+// the line below when one is shown. p.mu must be held.
+func (p *progress) clear() {
+	if p.bar == nil {
+		return
+	}
+
+	if p.path != "" {
+		_ = p.bar.AddDetail("")
+	}
+
+	_ = p.bar.Clear()
 }
 
 // newBar has the library make what draw draws: its spinner, followed by
@@ -263,6 +343,8 @@ func (p *progress) newBar(width int) {
 		// The text carries the elapsed time and the time remaining.
 		progressbar.OptionSetElapsedTime(false),
 		progressbar.OptionSetPredictTime(false),
+		// The line below the phase's line, for the walk's path.
+		progressbar.OptionSetMaxDetailRow(1),
 	)
 	p.widest = 0
 }
@@ -314,6 +396,19 @@ func (p *progress) increment() {
 	p.lastItem = now
 }
 
+// setPath records the path the walk reached last, which a display on a
+// terminal shows at its next draw.
+func (p *progress) setPath(path string) {
+	if p == nil {
+		return
+	}
+
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	p.path = path
+}
+
 // warnf prints a one-line warning to stderr without corrupting the
 // display. The whole message is escaped like a report's path columns,
 // so a path holding a newline cannot split the warning.
@@ -327,11 +422,9 @@ func (p *progress) warnf(format string, args ...any) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 
-	if p.bar != nil {
-		// The warning takes the display's line; the next draw shows
-		// the display again, on the line below.
-		_ = p.bar.Clear()
-	}
+	// The warning takes the display's line; the next draw shows the
+	// display again, on the line below.
+	p.clear()
 
 	fmt.Fprintln(os.Stderr, msg)
 }
@@ -392,9 +485,7 @@ func (w *progressWriter) Flush() error {
 		w.p.mu.Lock()
 		defer w.p.mu.Unlock()
 
-		if w.p.bar != nil {
-			_ = w.p.bar.Clear()
-		}
+		w.p.clear()
 	}
 
 	_, err := w.out.Write(w.buf)
@@ -414,6 +505,13 @@ func (p *progress) finish() {
 	close(p.stop)
 	<-p.done
 
+	// The walk's path is only for someone watching the walk. Its line is
+	// erased, and the next phase is drawn on it.
+	p.mu.Lock()
+	p.clear()
+	p.path = ""
+	p.mu.Unlock()
+
 	p.draw()
 
 	if p.width != nil {

internal/sfdupes/progress_test.go

@@ -6,6 +6,7 @@ import (
 	"math"
 	"os"
 	"path/filepath"
+	"regexp"
 	"slices"
 	"strings"
 	"sync"
@@ -642,9 +643,9 @@ func TestTerminalStalledLines(t *testing.T) {
 		{label: scanPhase("hash"), stalledOn: hashStalledOn},
 		{label: scanPhase("update"), stalledOn: "db"},
 		{label: treesPhase("read"), stalledOn: "db"},
-		{label: treesPhase("group"), stalledOn: "cpu"},
+		{label: treesPhase("group"), stalledOn: "cpu/disk"},
 		{label: treesPhase("write"), stalledOn: "out"},
-		{label: reportPhase("read"), stalledOn: "db/out"},
+		{label: reportPhase("read"), stalledOn: "db/disk/out"},
 	}
 
 	for _, params := range phases {
@@ -1138,3 +1139,164 @@ func TestWarningAfterLastItemKeepsBar(t *testing.T) {
 		t.Errorf("terminal shows %q, want a count of 2 of 2", shown)
 	}
 }
+
+func TestCutToColumns(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		s       string
+		columns int
+		want    string
+	}{
+		{"/srv/a", 10, "/srv/a"},
+		{"/srv/photos", 8, "/srv/pho"},
+		// Each of these two fullwidth letters takes two columns.
+		{"/ＡＢ/a", 4, "/Ａ"},
+		{"/ＡＢ/a", 5, "/ＡＢ"},
+		// An accent that combines with the letter before stays with it.
+		{"/café/a", 5, "/café"},
+		{"/srv", 0, ""},
+	}
+
+	for _, tc := range cases {
+		if got := cutToColumns(tc.s, tc.columns); got != tc.want {
+			t.Errorf("cutToColumns(%q, %d) = %q, want %q",
+				tc.s, tc.columns, got, tc.want)
+		}
+	}
+}
+
+func TestEscapeControlChars(t *testing.T) {
+	t.Parallel()
+
+	cases := map[string]string{
+		"/srv/music":    "/srv/music",
+		"/a\x1b[2Jb":    `/a\x1b[2Jb`,
+		"/a\x00\a\x7fb": `/a\x00\x07\x7fb`,
+		// U+009B takes two bytes in UTF-8.
+		"/a\u009b2Jb": `/a\x9b2Jb`,
+		// Byte 0x9b alone is not valid UTF-8. A terminal not in UTF-8
+		// mode acts on it, as on every byte from 0x80 to 0x9f.
+		"/a\x9b2Jb":       `/a\x9b2Jb`,
+		"/a\x80\x9f\xa0b": `/a\x80\x9f` + "\xa0b",
+		// In ě, which is c4 9b in UTF-8, byte 0x9b is part of a valid
+		// character.
+		"/aě2Jb":          "/aě2Jb",
+		"/café/ＡＢ":        "/café/ＡＢ",
+		"/latin1-caf\xe9": "/latin1-caf\xe9",
+	}
+	for in, want := range cases {
+		if got := escapeControlChars(in); got != want {
+			t.Errorf("escapeControlChars(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+// pathLines returns the text of each line the library wrote below a
+// display's line. It writes one as an erase to the end of the line, a
+// carriage return, the text and a newline.
+func pathLines(out string) []string {
+	matches := regexp.MustCompile("\x1b\\[K\r([^\n]*)\n").
+		FindAllStringSubmatch(out, -1)
+	lines := make([]string, 0, len(matches))
+
+	for _, m := range matches {
+		lines = append(lines, m[1])
+	}
+
+	return lines
+}
+
+// TestTerminalWalkShowsPathBelow checks that the walk's display on a
+// terminal 30 columns wide shows the path it reached last on the line
+// below, cut to 29 columns. That line is erased before a warning is
+// printed, and again when the walk ends, so the next phase is drawn on
+// an empty line.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestTerminalWalkShowsPathBelow(t *testing.T) {
+	stderr := captureStderr(t)
+	p := startProgress(progressParams{
+		label: scanPhase("walk"), unit: scanUnit, total: -1,
+	}, func() int { return 30 }, testRedraw, time.Now)
+
+	const shown = "/srv/photos/2026/holiday/beac"
+
+	p.setPath("/srv/photos/2026/holiday/beach.jpg")
+	p.increment()
+	awaitOutput(t, stderr, shown, 1)
+
+	p.warnf("warning")
+
+	before, _, _ := strings.Cut(stderr(), "warning")
+	if lines := pathLines(before); lines[len(lines)-1] != "" {
+		t.Errorf("line below shows %q as the warning is printed, want it "+
+			"empty", lines[len(lines)-1])
+	}
+
+	// Drawn again below the warning.
+	awaitOutput(t, stderr, shown, strings.Count(before, shown)+1)
+	p.finish()
+
+	lines := pathLines(stderr())
+	if !slices.Contains(lines, shown) {
+		t.Errorf("lines below the display = %q, want %q among them",
+			lines, shown)
+	}
+
+	if last := lines[len(lines)-1]; last != "" {
+		t.Errorf("line below the display shows %q after the walk, want it "+
+			"empty", last)
+	}
+}
+
+// TestTerminalWalkEscapesControlChars checks that a walked path holding
+// escape characters, which would clear the screen and set the window
+// title, puts none on the line below the walk's display. The line shows
+// each control character written out, as \x1b.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestTerminalWalkEscapesControlChars(t *testing.T) {
+	stderr := captureStderr(t)
+	p := startProgress(progressParams{
+		label: scanPhase("walk"), unit: scanUnit, total: -1,
+	}, func() int { return 80 }, testRedraw, time.Now)
+
+	p.setPath("/srv/\x1b[2J\x1b]0;title\a/beach.jpg")
+	p.increment()
+	awaitOutput(t, stderr, "beach.jpg", 1)
+	p.finish()
+
+	lines := pathLines(stderr())
+	for _, line := range lines {
+		if strings.Contains(line, "\x1b") {
+			t.Errorf("line below the display = %q, want no escape "+
+				"character", line)
+		}
+	}
+
+	const shown = `/srv/\x1b[2J\x1b]0;title\x07/beach.jpg`
+	if !slices.Contains(lines, shown) {
+		t.Errorf("lines below the display = %q, want %q among them",
+			lines, shown)
+	}
+}
+
+// TestPlainWalkShowsNoPath checks that the walk's display prints no path
+// when stderr is not a terminal.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestPlainWalkShowsNoPath(t *testing.T) {
+	stderr := captureStderr(t)
+	p := startProgress(progressParams{
+		label: scanPhase("walk"), unit: scanUnit, total: -1,
+	}, nil, testRedraw, time.Now)
+
+	p.setPath("/srv/a")
+	p.increment()
+	p.finish()
+
+	if strings.Contains(stderr(), "/srv/a") {
+		t.Errorf("stderr = %q, want no path", stderr())
+	}
+}

internal/sfdupes/report.go

@@ -4,11 +4,15 @@ import (
 	"context"
 	"crypto/sha256"
 	"database/sql"
+	"errors"
 	"fmt"
 	"io"
+	"io/fs"
 	"os"
+	"path/filepath"
 	"slices"
 	"strings"
+	"syscall"
 	"time"
 )
 
@@ -41,8 +45,10 @@ func reportPhase(name string) string {
 }
 
 // runReport implements the report subcommand: it prints the file-level
-// duplicates report as TSV on stdout. It never touches the scanned
-// filesystem; its only I/O is the database, stdout, and stderr. It
+// duplicates report as TSV on stdout, from the records at or below the
+// operands in args, or from every record when args is empty. Of the
+// scanned filesystem it only checks, with pathExists, that each path in
+// a duplicate group is still there; it never looks up an operand. It
 // names the database on stderr before it opens it. Any database
 // problem, including a missing database, is fatal.
 //
@@ -52,7 +58,12 @@ func reportPhase(name string) string {
 // order, writing each size's duplicate groups to stdout once that size
 // is read; see readReport). Each display ends before the next phase
 // starts, and before an error is returned to be printed.
-func runReport(ctx context.Context, stdout io.Writer) error {
+func runReport(ctx context.Context, args []string, stdout io.Writer) error {
+	operands, err := cleanOperands(args)
+	if err != nil {
+		return err
+	}
+
 	dbPath, source, err := databasePath()
 	if err != nil {
 		return err
@@ -77,13 +88,15 @@ func runReport(ctx context.Context, stdout io.Writer) error {
 
 	defer func() { _ = db.Close() }()
 
-	// The read phase writes the report to stdout while it reads the
-	// records, so it may wait on either.
+	// The read phase checks the paths of each duplicate group on disk
+	// and writes the report to stdout while it reads the records, so it
+	// may wait on any of the three.
 	prog = newProgress(progressParams{
 		label: reportPhase("read"), unit: "records", total: -1,
-		waitingFor: countingRecords, stalledOn: "db/out",
+		waitingFor: countingRecords, stalledOn: "db/disk/out",
 	})
-	report, err := readReport(ctx, db, dbPath, stdout, prog)
+	report, err := readReport(ctx, db, dbPath, operands, stdout, pathExists,
+		prog)
 
 	prog.finish()
 
@@ -104,14 +117,17 @@ func runReport(ctx context.Context, stdout io.Writer) error {
 // dbPath, as its total, then reads every record in signature order,
 // counting each on prog, and writes the report to stdout: the header
 // line, then the rows of each size's duplicate groups once that size is
-// read. stdout may be the terminal prog is drawn on, so the lines go
+// read. Only the records that underOperand finds under operands join a
+// group, and a path for which exists returns false is left out of its
+// group. stdout may be the terminal prog is drawn on, so the lines go
 // through prog's writer. It returns what it wrote, with the totals for
 // the summary line.
 func readReport(ctx context.Context, db *sql.DB, dbPath string,
-	stdout io.Writer, prog *progress,
+	operands []string, stdout io.Writer, exists func(path string) bool,
+	prog *progress,
 ) (*reportWriter, error) {
 	out := prog.writer(stdout)
-	report := &reportWriter{out: out}
+	report := &reportWriter{out: out, exists: exists}
 
 	_, err := fmt.Fprintln(out, "first\tdupe\tsize")
 	if err != nil {
@@ -123,6 +139,12 @@ func readReport(ctx context.Context, db *sql.DB, dbPath string,
 	err = loadSignatureRows(ctx, db, prog.setTotal, func(r scanRec) error {
 		prog.increment()
 
+		report.records++
+
+		if !underOperand(r.path, operands) {
+			return nil
+		}
+
 		writeErr = report.add(r)
 
 		return writeErr
@@ -154,10 +176,14 @@ func readReport(ctx context.Context, db *sql.DB, dbPath string,
 // records of one size, so it holds only the duplicate groups of the
 // size being read, and writes them, ordered by first path, once a
 // record of a smaller size comes. It counts what the summary line
-// reports.
+// reports; readReport counts the records read.
 type reportWriter struct {
 	out io.Writer
 
+	// exists reports whether a path is still on disk. A path that is
+	// gone is left out of its group.
+	exists func(path string) bool
+
 	sig    scanRec    // the signature being read; its path is not used
 	paths  []string   // the paths read so far with sig
 	groups [][]string // the duplicate groups of sig.size, each sorted
@@ -170,8 +196,6 @@ type reportWriter struct {
 // its paths; any other ends them, and one of a smaller size first
 // writes the groups of the size before.
 func (w *reportWriter) add(r scanRec) error {
-	w.records++
-
 	if r.size == w.sig.size && r.head == w.sig.head &&
 		r.tail == w.sig.tail && r.content == w.sig.content {
 		w.paths = append(w.paths, r.path)
@@ -194,10 +218,18 @@ func (w *reportWriter) add(r scanRec) error {
 	return nil
 }
 
-// endGroup ends the paths read with w.sig: two or more are a duplicate
-// group, held with its paths sorted until writeGroups writes it. Empty
-// files form no group, since removing one frees nothing.
+// endGroup ends the paths read with w.sig: two or more that are still
+// on disk are a duplicate group, held with its paths sorted until
+// writeGroups writes it. Empty files form no group, since removing one
+// frees nothing. Only the paths of a possible group are checked, never
+// a path with no duplicate or an empty file.
 func (w *reportWriter) endGroup() {
+	if len(w.paths) >= minGroupSize && w.sig.size > 0 {
+		w.paths = slices.DeleteFunc(w.paths, func(p string) bool {
+			return !w.exists(p)
+		})
+	}
+
 	if len(w.paths) < minGroupSize || w.sig.size == 0 {
 		// The paths are dropped, and their slice reused.
 		w.paths = w.paths[:0]
@@ -246,6 +278,57 @@ func (w *reportWriter) finish() error {
 	return w.writeGroups()
 }
 
+// pathExists reports whether path is still on disk, for report and
+// trees. It is false only when lstat finds that the path, or a
+// directory above it, is gone. Any other error counts as present, so
+// a user who may not look inside a directory still sees its copies.
+func pathExists(path string) bool {
+	_, err := os.Lstat(path)
+
+	return !errors.Is(err, fs.ErrNotExist) && !errors.Is(err, syscall.ENOTDIR)
+}
+
+// cleanOperands returns the operands of report or trees, each made
+// absolute by absolutePath and then cleaned. The records hold real
+// paths, with no "." or ".." element and no repeated or trailing
+// slash, so an operand is cleaned to match them. No operand is looked
+// up on the filesystem. None is empty: rejectEmptyOperand refuses an
+// empty operand on the command line.
+func cleanOperands(args []string) ([]string, error) {
+	operands := make([]string, 0, len(args))
+
+	for _, a := range args {
+		abs, err := absolutePath(a)
+		if err != nil {
+			return nil, fmt.Errorf("resolve path %s: %w", escapePath(a), err)
+		}
+
+		operands = append(operands, filepath.Clean(abs))
+	}
+
+	return operands, nil
+}
+
+// underOperand reports whether path is one of operands or lies below
+// one of them. Each operand is clean and absolute, as cleanOperands
+// returns it. With no operands, a report covers the whole database, so
+// underOperand is true for every path.
+func underOperand(path string, operands []string) bool {
+	if len(operands) == 0 {
+		return true
+	}
+
+	for _, o := range operands {
+		// Only the root directory's clean path ends in "/".
+		dir := strings.TrimSuffix(o, "/") + "/"
+		if path == o || strings.HasPrefix(path, dir) {
+			return true
+		}
+	}
+
+	return false
+}
+
 // escapePath returns a path as it is written in a report column (README
 // "Report output format"): a backslash, tab, newline or carriage return
 // becomes \\, \t, \n or \r, and every other byte is kept as it is.

internal/sfdupes/report_test.go

@@ -72,15 +72,38 @@ type dupeGroup struct {
 	paths []string
 }
 
+// everyPathExists stands in for pathExists where a test's records name
+// made-up paths that are not on disk.
+func everyPathExists(string) bool { return true }
+
+// recsOnDisk returns a new directory and recs with each path moved
+// under it, after creating an empty file at each moved path: report and
+// trees then find every path still on disk.
+func recsOnDisk(t *testing.T, recs []scanRec) (string, []scanRec) {
+	t.Helper()
+
+	dir := realTempDir(t)
+	moved := make([]scanRec, len(recs))
+
+	for i, r := range recs {
+		r.path = writeFile(t, dir, r.path, nil)
+		moved[i] = r
+	}
+
+	return dir, moved
+}
+
 // dupeGroups returns the duplicate groups report reads from db, in
 // report order, from the rows it writes: a group's rows come together,
-// each naming the group's first path.
+// each naming the group's first path. Every path counts as still on
+// disk.
 func dupeGroups(t *testing.T, db *sql.DB) []dupeGroup {
 	t.Helper()
 
 	var out bytes.Buffer
 
-	_, err := readReport(t.Context(), db, "", &out, nil)
+	_, err := readReport(t.Context(), db, "", nil, &out, everyPathExists,
+		nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -126,8 +149,139 @@ func dupeGroupsOf(t *testing.T, recs []scanRec) []dupeGroup {
 	return dupeGroups(t, db)
 }
 
+// TestRunReportOperands runs report with operands on three copies of
+// one file, two of them under srv/photos, and a fourth copy in
+// srv/photos2, all in a new directory. Only the records at or below the
+// operands form the groups and the totals, however each operand is
+// written.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
+func TestRunReportOperands(t *testing.T) {
+	h := testHash("h")
+
+	recs := make([]scanRec, 0, 4)
+	for _, p := range []string{
+		"/a/copy", "/srv/photos/x", "/srv/photos/y", "/srv/photos2/z",
+	} {
+		recs = append(recs,
+			scanRec{size: 7, head: h, tail: h, content: h, path: p})
+	}
+
+	dir, recs := recsOnDisk(t, recs)
+	a, x, y, z := recs[0].path, recs[1].path, recs[2].path, recs[3].path
+
+	t.Setenv(databaseEnv, seedDatabase(t, recs))
+
+	header := "first\tdupe\tsize\n"
+	row := func(first, dupe string) string {
+		return first + "\t" + dupe + "\t7\n"
+	}
+	photos := header + row(x, y)
+
+	cases := map[string]struct {
+		operands []string
+		want     string
+		summary  string
+	}{
+		"directory": {[]string{dir + "/srv/photos"}, photos,
+			"1 duplicate groups, 1 dupe files, 7 B reclaimable"},
+		"trailing slash":  {[]string{dir + "/srv/photos/"}, photos, ""},
+		"dot and dot-dot": {[]string{dir + "/srv/./a/../photos"}, photos, ""},
+		"files":           {[]string{a, z}, header + row(a, z), ""},
+		"overlapping": {[]string{dir + "/srv", dir + "/srv/photos"},
+			header + row(x, y) + row(x, z), ""},
+		"root": {[]string{"/"},
+			header + row(a, x) + row(a, y) + row(a, z), ""},
+		"no record": {[]string{dir + "/srv/photo"}, header,
+			"0 duplicate groups, 0 dupe files, 0 B reclaimable"},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			var stdout bytes.Buffer
+
+			stderr := captureStderr(t)
+
+			code := run(append([]string{cmdReport}, tc.operands...),
+				&stdout, os.Stderr)
+			if code != exitOK {
+				t.Fatalf("run(report %q) = %d, want %d; stderr: %s",
+					tc.operands, code, exitOK, stderr())
+			}
+
+			if got := stdout.String(); got != tc.want {
+				t.Errorf("stdout = %q, want %q", got, tc.want)
+			}
+
+			// The records read are all four, whatever the operands.
+			summary := "report: 4 records read, " + tc.summary + "\n"
+			if got := stderr(); tc.summary != "" &&
+				!strings.HasSuffix(got, summary) {
+				t.Errorf("stderr = %q, want it to end %q", got, summary)
+			}
+		})
+	}
+}
+
+//nolint:paralleltest // t.Chdir changes the process-wide working directory
+func TestCleanOperandsMakesRelativePathsAbsolute(t *testing.T) {
+	dir := t.TempDir()
+	t.Chdir(dir)
+
+	wd, err := absolutePath(".")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := cleanOperands([]string{"photos/", "a/../b", "/c//d"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	want := []string{
+		filepath.Join(wd, "photos"), filepath.Join(wd, "b"), "/c/d",
+	}
+	if !slices.Equal(got, want) {
+		t.Errorf("cleanOperands = %q, want %q", got, want)
+	}
+}
+
+// TestRunReportsRejectEmptyOperand checks that an empty PATH operand of
+// report or trees, alone or after another operand, is a usage error
+// reported in one line before the database is opened (README "Error
+// handling and exit codes").
+func TestRunReportsRejectEmptyOperand(t *testing.T) {
+	cases := map[string][]string{
+		"report":           {cmdReport, ""},
+		"trees":            {cmdTrees, ""},
+		"after an operand": {cmdTrees, t.TempDir(), ""},
+	}
+
+	for name, args := range cases {
+		t.Run(name, func(t *testing.T) {
+			// No database exists there, so opening one would be a fatal
+			// error, exit 1.
+			t.Setenv(databaseEnv, testDBPath(t))
+
+			var stdout, stderr bytes.Buffer
+
+			code := run(args, &stdout, &stderr)
+			if code != exitUsage {
+				t.Errorf("run(%q) = %d, want %d", args, code, exitUsage)
+			}
+
+			want := "Error: PATH must not be empty\n"
+			if stdout.String() != "" || stderr.String() != want {
+				t.Errorf("stdout, stderr = %q, %q, want \"\", %q",
+					stdout.String(), stderr.String(), want)
+			}
+		})
+	}
+}
+
 func TestRunReportEscapesPaths(t *testing.T) {
-	t.Setenv(databaseEnv, seedDatabase(t, awkwardPairRecs()))
+	dir, recs := recsOnDisk(t, awkwardPairRecs())
+	t.Setenv(databaseEnv, seedDatabase(t, recs))
 
 	var stdout, stderr bytes.Buffer
 
@@ -138,7 +292,7 @@ func TestRunReportEscapesPaths(t *testing.T) {
 	}
 
 	want := "first\tdupe\tsize\n" +
-		`/d/\tone\ntwo\rthree\\four/f` + "\t/d/A/f\t5\n"
+		dir + `/d/\tone\ntwo\rthree\\four/f` + "\t" + dir + "/d/A/f\t5\n"
 	if got := stdout.String(); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
@@ -172,7 +326,8 @@ func TestReportStdoutFailsWhileReading(t *testing.T) {
 	// Counts the records read, and draws nothing.
 	prog := &progress{now: time.Now}
 
-	_, err = readReport(t.Context(), db, "", failingWriter{}, prog)
+	_, err = readReport(t.Context(), db, "", nil, failingWriter{},
+		everyPathExists, prog)
 	if !errors.Is(err, errWriteFailed) ||
 		!strings.HasPrefix(err.Error(), "write stdout: ") {
 		t.Errorf("error = %v, want write stdout: %v", err, errWriteFailed)
@@ -203,7 +358,8 @@ const smokeReport = "first\tdupe\tsize\n" +
 // it ends; read counts the 6 records before it reads them. The summary
 // comes last, and stdout holds the report alone.
 func TestRunReportProgressNamesEachPhase(t *testing.T) {
-	path := seedDatabase(t, smokeTreeRecs())
+	dir, recs := recsOnDisk(t, smokeTreeRecs())
+	path := seedDatabase(t, recs)
 	t.Setenv(databaseEnv, path)
 
 	stderr := captureStderr(t)
@@ -216,8 +372,9 @@ func TestRunReportProgressNamesEachPhase(t *testing.T) {
 			code, exitOK, stderr())
 	}
 
-	if got := stdout.String(); got != smokeReport {
-		t.Errorf("stdout = %q, want %q", got, smokeReport)
+	report := strings.ReplaceAll(smokeReport, "/d/", dir+"/d/")
+	if got := stdout.String(); got != report {
+		t.Errorf("stdout = %q, want %q", got, report)
 	}
 
 	// How each line starts; the rate at the end of a phase varies.
@@ -266,7 +423,8 @@ func TestReadReportOnTerminal(t *testing.T) {
 		waitingFor: countingRecords,
 	}, terminalWidth, time.Hour, time.Now)
 
-	_, err = readReport(t.Context(), db, "", os.Stderr, p)
+	_, err = readReport(t.Context(), db, "", nil, os.Stderr,
+		everyPathExists, p)
 
 	p.finish()
 
@@ -301,6 +459,8 @@ func TestRunReportsIgnoreInsertionOrder(t *testing.T) {
 		scanRec{size: 50, head: a, tail: a, content: a, path: "/x/1"},
 	)
 
+	_, recs = recsOnDisk(t, recs)
+
 	reversed := slices.Clone(recs)
 	slices.Reverse(reversed)
 
@@ -455,6 +615,120 @@ func TestDupeGroupsLeavesOutEmptyFiles(t *testing.T) {
 	}
 }
 
+// TestReadReportChecksOnlyPossibleDuplicates checks that report asks
+// whether a path is on disk only when its signature has two or more
+// records, and never for an empty file.
+func TestReadReportChecksOnlyPossibleDuplicates(t *testing.T) {
+	t.Parallel()
+
+	a, u, e := testHash("a"), testHash("u"), sha256.Sum256(nil)
+	recs := []scanRec{
+		{size: 9, head: a, tail: a, content: a, path: "/copy/1"},
+		{size: 9, head: a, tail: a, content: a, path: "/copy/2"},
+		{size: 9, head: a, tail: a, content: a, path: "/copy/3"},
+		{size: 9, head: u, tail: u, content: u, path: "/lonely"},
+		{size: 0, head: e, tail: e, content: e, path: "/empty1"},
+		{size: 0, head: e, tail: e, content: e, path: "/empty2"},
+	}
+
+	db := openTestDB(t)
+
+	err := applyChanges(t.Context(), db, recs, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var checked []string
+
+	exists := func(path string) bool {
+		checked = append(checked, path)
+
+		return true
+	}
+
+	_, err = readReport(t.Context(), db, "", nil, io.Discard, exists, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	slices.Sort(checked)
+
+	want := []string{"/copy/1", "/copy/2", "/copy/3"}
+	if !slices.Equal(checked, want) {
+		t.Errorf("paths checked on disk = %q, want %q", checked, want)
+	}
+}
+
+// TestReadReportDropsGroupLeftWithOnePath checks that a group of two
+// paths, one of them gone, is no duplicate group. If kept, it would
+// write no row, but the summary line would count it.
+func TestReadReportDropsGroupLeftWithOnePath(t *testing.T) {
+	t.Parallel()
+
+	a := testHash("a")
+	recs := []scanRec{
+		{size: 9, head: a, tail: a, content: a, path: "/a/kept"},
+		{size: 9, head: a, tail: a, content: a, path: "/a/gone"},
+	}
+
+	db := openTestDB(t)
+
+	err := applyChanges(t.Context(), db, recs, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	exists := func(path string) bool { return path != "/a/gone" }
+
+	report, err := readReport(t.Context(), db, "", nil, io.Discard, exists,
+		nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if report.dupeGroups != 0 {
+		t.Errorf("duplicate groups = %d, want 0", report.dupeGroups)
+	}
+}
+
+// TestReadReportKeepsGroupLeftWithTwoPaths checks that a group of three
+// paths, one of them gone, is a duplicate group of the other two.
+func TestReadReportKeepsGroupLeftWithTwoPaths(t *testing.T) {
+	t.Parallel()
+
+	a := testHash("a")
+	recs := []scanRec{
+		{size: 9, head: a, tail: a, content: a, path: "/b/kept1"},
+		{size: 9, head: a, tail: a, content: a, path: "/b/gone"},
+		{size: 9, head: a, tail: a, content: a, path: "/b/kept2"},
+	}
+
+	db := openTestDB(t)
+
+	err := applyChanges(t.Context(), db, recs, nil, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	exists := func(path string) bool { return path != "/b/gone" }
+
+	var out bytes.Buffer
+
+	report, err := readReport(t.Context(), db, "", nil, &out, exists, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if report.dupeGroups != 1 {
+		t.Errorf("duplicate groups = %d, want 1", report.dupeGroups)
+	}
+
+	want := "first\tdupe\tsize\n/b/kept1\t/b/kept2\t9\n"
+	if got := out.String(); got != want {
+		t.Errorf("stdout = %q, want %q", got, want)
+	}
+}
+
 func TestDupeGroupsContentSeparates(t *testing.T) {
 	t.Parallel()
 

internal/sfdupes/scan.go

@@ -98,44 +98,47 @@ func scanPhase(name string) string {
 // runs, and the lock is released after it. When ctx is cancelled, as by
 // the SIGINT or SIGTERM that interruptContext catches, the scan keeps
 // what it has hashed (see syncScan), prints how many files its walk
-// reached, and returns errInterrupted. workers must be at least 1; the
-// scan command rejects anything less. Before the lock is taken, the
-// scan lists the filesystems that are not persistent, on which the walk
-// records nothing; unless noDefaultExcludes is set, the walk does not
-// enter the default excludes either. Once the database is open, the
-// scan notes its files, which the walk passes over.
-func runScan(ctx context.Context, roots []string, workers int,
-	oneFS, noDefaultExcludes bool,
-) error {
+// reached, and returns errInterrupted. opts.workers must be at least 1;
+// the scan and dwim commands reject anything less. Before the lock is
+// taken, the scan lists the filesystems that are not persistent, on
+// which the walk records nothing; unless opts.noDefaultExcludes is set,
+// the walk does not enter the default excludes either. The walk neither
+// enters nor records the paths in opts.excludePaths, the --exclude
+// values; an operand at or below one is a fatal error. Once the
+// database is open, the scan notes its files, which the walk passes
+// over.
+func runScan(ctx context.Context, roots []string, opts scanOptions) error {
 	dbPath, source, err := databasePath()
 	if err != nil {
 		return err
 	}
 
-	// A database file missing now is created by lockScanDatabase below.
-	what := "database"
+	printDatabasePath(dbPath, source)
 
-	_, err = os.Stat(dbPath)
-	if errors.Is(err, fs.ErrNotExist) {
-		what = "new database"
-	}
-
-	fmt.Fprintf(os.Stderr, "scan: %s %s (%s)\n", what, escapePath(dbPath),
-		source)
-
-	roots, err = resolveRoots(roots)
+	roots, err = realPaths(roots)
 	if err != nil {
 		return err
 	}
 
-	nonPersistent, err := nonPersistentFilesystems()
+	var rules walkRules
+
+	rules.excludePaths, err = excludePathInodes(opts.excludePaths)
+	if err != nil {
+		return fmt.Errorf("--exclude: %w", err)
+	}
+
+	err = checkOperandsNotExcluded(roots, rules.excludePaths)
+	if err != nil {
+		return err
+	}
+
+	rules.nonPersistent, err = nonPersistentFilesystems()
 	if err != nil {
 		return fmt.Errorf("list the mounted filesystems: %w", err)
 	}
 
-	rules := walkRules{nonPersistent: nonPersistent}
-	if !noDefaultExcludes {
-		rules.excludes = defaultExcludes(runtime.GOOS)
+	if !opts.noDefaultExcludes {
+		rules.defaultExcludes = defaultExcludes(runtime.GOOS)
 	}
 
 	lock, err := lockScanDatabase(dbPath)
@@ -163,7 +166,7 @@ func runScan(ctx context.Context, roots []string, workers int,
 		return err
 	}
 
-	st, err := syncScan(ctx, db, roots, workers, oneFS, rules)
+	st, err := syncScan(ctx, db, roots, opts.workers, opts.oneFS, rules)
 	if errors.Is(err, context.Canceled) {
 		return interrupted(st.walked)
 	}
@@ -181,6 +184,21 @@ func runScan(ctx context.Context, roots []string, workers int,
 	return nil
 }
 
+// printDatabasePath prints on stderr the line that names the database
+// at dbPath, with source, saying whether it is new. A database file
+// missing now is new: lockScanDatabase creates it.
+func printDatabasePath(dbPath, source string) {
+	what := "database"
+
+	_, err := os.Stat(dbPath)
+	if errors.Is(err, fs.ErrNotExist) {
+		what = "new database"
+	}
+
+	fmt.Fprintf(os.Stderr, "scan: %s %s (%s)\n", what, escapePath(dbPath),
+		source)
+}
+
 // interruptContext returns a copy of ctx that the first SIGINT or
 // SIGTERM cancels; the scan command runs the scan under it. stop
 // releases the signals.
@@ -209,24 +227,25 @@ func interrupted(walked int) error {
 	return errInterrupted
 }
 
-// resolveRoots converts each PATH operand to its real path: absolute,
-// with every symlink in it followed. Database records are keyed by
-// that path, so scan results depend neither on the working directory
-// nor on the symlinks an operand was given through. An operand that
-// cannot be resolved, because it or a symlink's target does not exist
-// or cannot be read, is a fatal error before any scanning.
-func resolveRoots(roots []string) ([]string, error) {
-	resolved := make([]string, 0, len(roots))
+// realPaths returns the real path of each of paths, the PATH operands
+// or the --exclude values: absolute, with every symlink in it followed.
+// Database records are keyed by that path, so scan results depend
+// neither on the working directory nor on the symlinks an operand was
+// given through. A path that cannot be resolved, because it or a
+// symlink's target does not exist or cannot be read, is a fatal error
+// before any scanning.
+func realPaths(paths []string) ([]string, error) {
+	resolved := make([]string, 0, len(paths))
 
-	for _, root := range roots {
-		a, err := absolutePath(root)
+	for _, path := range paths {
+		a, err := absolutePath(path)
 		if err != nil {
-			return nil, fmt.Errorf("resolve %s: %w", root, err)
+			return nil, fmt.Errorf("resolve %s: %w", path, err)
 		}
 
 		r, err := filepath.EvalSymlinks(a)
 		if err != nil {
-			return nil, fmt.Errorf("resolve %s: %w", root, err)
+			return nil, fmt.Errorf("resolve %s: %w", path, err)
 		}
 
 		resolved = append(resolved, r)
@@ -295,7 +314,7 @@ func walkReaches(above, r string, oneFS bool, rules walkRules,
 	}
 
 	for ; dir != above; dir = filepath.Dir(dir) {
-		if isDefaultExclude(dir, rules.excludes) {
+		if isDefaultExclude(dir, rules.defaultExcludes) {
 			return false
 		}
 
@@ -418,7 +437,7 @@ func (s *scanState) runPhases(ctx context.Context, roots []string,
 
 // walkableRoots returns the operands the walk can start from: regular
 // files, and directories not named .zfs, none of them inside a
-// directory named .zfs. The operands are real paths from resolveRoots,
+// directory named .zfs. The operands are real paths from realPaths,
 // so these checks apply to what a symlink operand points at. Every
 // other operand is warned about, counted as skipped, and dropped. A
 // dropped operand is no longer a root, so the records stored beneath it
@@ -493,10 +512,11 @@ func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 // as verified. When its disk_size differs from the record's, it is
 // queued for the update phase, which writes the new disk_size. A record
 // at or below a path the walk left out, because -x stopped at it, it is
-// a default exclude, or it is not on a persistent filesystem, counts as
-// outside the roots: it is not deleted. That does not hold below a root
-// at or inside that path, since that root's own walk verifies the
-// records below it; nor is a line printed for such a root.
+// an --exclude path or a default exclude, or it is not on a persistent
+// filesystem, counts as outside the roots: it is not deleted. That does
+// not hold below a root at or inside that path, since that root's own
+// walk verifies the records below it; nor is a line printed for such a
+// root.
 func (s *scanState) walkPhase(events <-chan walkEvent, roots []string) {
 	// The records the last scan left under the roots estimate how many
 	// files the walk will find. A first scan has none, and no estimate.
@@ -542,10 +562,11 @@ func (s *scanState) walkPhase(events <-chan walkEvent, roots []string) {
 
 		s.st.walked++
 
+		prog.setPath(ev.rec.path)
 		prog.increment()
 
 		old, ok := s.existing[ev.rec.path]
-		if !ok || old.size != ev.rec.size || mtimeAfter(ev.rec.mtime, old.mtime) {
+		if !ok || old.size != ev.rec.size || !ev.rec.mtime.Equal(old.mtime) {
 			s.toHash = append(s.toHash, ev.rec)
 
 			continue
@@ -643,8 +664,8 @@ func isDatabaseFile(f fileRec, rules walkRules) bool {
 // inode run, committing completed records to the database in batches
 // as results arrive, so a long scan persists its progress as it goes
 // (an interrupted scan resumes cheaply: the next run skips everything
-// already recorded). A run that fails to hash is warned about and
-// counted as skipped; stale records for its paths, if any, are deleted
+// already recorded). Each path of a run that fails to hash is warned
+// about and counted as skipped; stale records for them, if any, are deleted
 // by the update phase. A run whose file no longer exists gets no record
 // and no warning, and is not counted as skipped. The total counts actual
 // reads, so the display's time remaining is meaningful.
@@ -685,9 +706,12 @@ func (s *scanState) hashPhase(ctx context.Context, workers int) error {
 		}
 
 		if r.err != nil {
-			s.st.skipped += len(r.run)
+			// Every path in the run is counted, so every path gets a line.
+			for _, rec := range r.run {
+				s.st.skipped++
 
-			prog.warnf("hash %s: %v", r.run[0].path, r.err)
+				prog.warnf("%s", skippedLine(rec.path, r.err))
+			}
 
 			continue
 		}
@@ -780,16 +804,6 @@ func (s *scanState) updatePhase(ctx context.Context) error {
 	return applyChanges(ctx, s.db, nil, deletes, prog)
 }
 
-// mtimeAfter reports whether mtime a is later than mtime b.
-// Not a.After(b): time.Time wraps an mtime past year 292 billion; Unix() undoes it.
-func mtimeAfter(a, b time.Time) bool {
-	if a.Unix() != b.Unix() {
-		return a.Unix() > b.Unix()
-	}
-
-	return a.Nanosecond() > b.Nanosecond()
-}
-
 // underAnyRoot reports whether path is any of the roots or lies under
 // one of them.
 func underAnyRoot(path string, roots []string) bool {
@@ -844,9 +858,9 @@ type dirJob struct {
 // file's statted record, a warning when fail is set, or a path the walk
 // left out: when otherFSDir is set, a directory -x did not enter
 // because it is on another filesystem than its operand, and when
-// leftOut is set, a default exclude or a directory or file on a
-// filesystem that is not persistent, with warn holding the line that
-// says so.
+// leftOut is set, an --exclude path, a default exclude or a directory or
+// file on a filesystem that is not persistent, with warn holding the
+// line that says so.
 type walkEvent struct {
 	rec        fileRec
 	warn       string
@@ -897,15 +911,14 @@ func sendEvent(ctx context.Context, events chan<- walkEvent,
 // operandWarning returns the one-line warning for an operand the walk
 // does not start from, naming the path and what it is, or "" for one it
 // does: a regular file, or a directory not named .zfs, that is not
-// inside a directory named .zfs. resolveRoots has already followed the
+// inside a directory named .zfs. realPaths has already followed the
 // operand's symlinks, so a symlink here is an operand replaced by one
 // after it was resolved, and is not followed.
 func operandWarning(root string, fi fs.FileInfo) string {
 	// The walk never enters a .zfs directory, so it starts from nothing
 	// inside one either.
 	if slices.Contains(strings.Split(filepath.Dir(root), "/"), ".zfs") {
-		return fmt.Sprintf("walk %s: skipping operand inside a .zfs directory",
-			root)
+		return fmt.Sprintf("scan: skipped %s (inside a .zfs directory)", root)
 	}
 
 	var kind string
@@ -931,7 +944,21 @@ func operandWarning(root string, fi fs.FileInfo) string {
 		kind = "non-regular file"
 	}
 
-	return fmt.Sprintf("walk %s: skipping %s operand", root, kind)
+	return fmt.Sprintf("scan: skipped %s (a %s)", root, kind)
+}
+
+// skippedLine returns the warning for a path the scan skips because of
+// err. The reason is the error without its path, which the line already
+// names.
+func skippedLine(path string, err error) string {
+	reason := err.Error()
+
+	var pathErr *fs.PathError
+	if errors.As(err, &pathErr) {
+		reason = pathErr.Err.Error()
+	}
+
+	return fmt.Sprintf("scan: skipped %s (%s)", path, reason)
 }
 
 // seedRoot turns one PATH operand into the walk's starting state: a
@@ -954,7 +981,7 @@ func seedRoot(ctx context.Context, root string, rules walkRules,
 
 	if err != nil {
 		sendEvent(ctx, events, walkEvent{
-			warn: fmt.Sprintf("walk %s: %v", root, err),
+			warn: skippedLine(root, err),
 			fail: true,
 		})
 
@@ -1083,7 +1110,7 @@ func walkOneDir(ctx context.Context, job dirJob, oneFS bool,
 
 	if err != nil {
 		sendEvent(ctx, events, walkEvent{
-			warn: fmt.Sprintf("walk %s: %v", job.path, err),
+			warn: skippedLine(job.path, err),
 			fail: true,
 		})
 
@@ -1128,8 +1155,9 @@ func walkOneDir(ctx context.Context, job dirJob, oneFS bool,
 // no longer exists, and one that has stopped being a regular file is
 // skipped. A file on a filesystem in rules that is not persistent, as
 // through a bind mount of one file, is not recorded: it gets an event
-// naming it instead. One of the scan's own database files in rules gets
-// no event at all.
+// naming it instead, and so does a file given with --exclude, reached
+// by any path, a hard link to it included. One of the scan's own
+// database files in rules gets no event at all.
 func emitFile(ctx context.Context, p string, e fs.DirEntry,
 	rules walkRules, events chan<- walkEvent,
 ) {
@@ -1141,7 +1169,7 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 
 	if err != nil {
 		sendEvent(ctx, events, walkEvent{
-			warn: fmt.Sprintf("stat %s: %v", p, err),
+			warn: skippedLine(p, err),
 			fail: true,
 		})
 
@@ -1153,6 +1181,10 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 	}
 
 	line := rules.nonPersistentLine(p, info)
+	if line == "" {
+		line = rules.excludePathLine(p, info)
+	}
+
 	if line != "" {
 		sendEvent(ctx, events, walkEvent{leftOut: p, warn: line})
 
@@ -1177,11 +1209,11 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 // subdirJob applies the descent rules to directory p: never enter
 // .zfs (ZFS snapshot pseudo-dirs would list every file once per
 // snapshot), never enter a directory on a filesystem in rules that is
-// not persistent, nor a default exclude in rules, and with -x never
-// enter a directory on a different filesystem than its operand. For the
-// last three it sends an event that names the directory, so the scan
-// keeps the records below it, except those below an operand at or
-// inside it.
+// not persistent, nor an --exclude path or a default exclude in rules,
+// and with -x never enter a directory on a different filesystem than
+// its operand. For all but .zfs it sends an event that names the
+// directory, so the scan keeps the records below it, except those below
+// an operand at or inside it.
 func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	parent dirJob, oneFS bool, rules walkRules,
 	events chan<- walkEvent,
@@ -1198,7 +1230,7 @@ func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 
 	if err != nil {
 		sendEvent(ctx, events, walkEvent{
-			warn: fmt.Sprintf("walk %s: %v", p, err),
+			warn: skippedLine(p, err),
 			fail: true,
 		})
 
@@ -1206,16 +1238,20 @@ func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	}
 
 	line := rules.nonPersistentLine(p, info)
+	if line == "" {
+		line = rules.excludePathLine(p, info)
+	}
+
 	if line != "" {
 		sendEvent(ctx, events, walkEvent{leftOut: p, warn: line})
 
 		return dirJob{}, false
 	}
 
-	if isDefaultExclude(p, rules.excludes) {
+	if isDefaultExclude(p, rules.defaultExcludes) {
 		sendEvent(ctx, events, walkEvent{
 			leftOut: p,
-			warn:    fmt.Sprintf("scan: skipped %s (default exclude)", p),
+			warn:    fmt.Sprintf("scan: left out %s (default exclude)", p),
 		})
 
 		return dirJob{}, false

internal/sfdupes/scan_test.go

@@ -752,8 +752,9 @@ func TestWalkPassesOverRemovedPaths(t *testing.T) {
 
 // TestScanWarnsAboutUnreadablePaths checks that a permission error is
 // still warned about and counted as skipped, unlike a removed path. The
-// walk reaches an unreadable directory, and a file in a directory that
-// can be read but not searched. Another such file is an operand.
+// walk reaches an unreadable directory. It also reaches a file and a
+// subdirectory in a directory that can be read but not searched. Another
+// such file is an operand.
 //
 //nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
 func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
@@ -765,6 +766,7 @@ func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
 	dir := t.TempDir()
 	tree := filepath.Join(dir, "tree")
 	walked := writeFile(t, tree, "unsearchable/f", []byte("x"))
+	subdir := filepath.Join(filepath.Dir(walked), "d")
 	operand := writeFile(t, dir, "unsearchable/f", []byte("y"))
 	unreadable := filepath.Join(tree, "unreadable")
 
@@ -773,6 +775,11 @@ func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	err = os.Mkdir(subdir, 0o750)
+	if err != nil {
+		t.Fatal(err)
+	}
+
 	modes := map[string]fs.FileMode{
 		unreadable:            0,
 		filepath.Dir(walked):  0o400,
@@ -793,15 +800,16 @@ func TestScanWarnsAboutUnreadablePaths(t *testing.T) {
 	}
 
 	st := syncTree(t, openTestDB(t), tree, operand)
-	if st != (scanStats{skipped: 3}) {
-		t.Errorf("stats = %+v, want 3 skipped and nothing else", st)
+	if st != (scanStats{skipped: 4}) {
+		t.Errorf("stats = %+v, want 4 skipped and nothing else", st)
 	}
 
 	got := stderr()
 	for _, warning := range []string{
-		"walk " + unreadable + ": open " + unreadable + ": permission denied\n",
-		"stat " + walked + ": lstat " + walked + ": permission denied\n",
-		"walk " + operand + ": lstat " + operand + ": permission denied\n",
+		"scan: skipped " + unreadable + " (permission denied)\n",
+		"scan: skipped " + walked + " (permission denied)\n",
+		"scan: skipped " + subdir + " (permission denied)\n",
+		"scan: skipped " + operand + " (permission denied)\n",
 	} {
 		if !strings.Contains(got, warning) {
 			t.Errorf("stderr = %q, want %q", got, warning)
@@ -1215,7 +1223,7 @@ func assertSmokeTreeGroups(t *testing.T, dir string, db *sql.DB) {
 
 	super, dirs := dbTree(t, db)
 
-	tg := collectTreeGroups(dirs, super, nil)
+	tg := collectTreeGroups(dirs, super, everyPathExists, nil)
 	if len(tg) != 1 {
 		t.Fatalf("len(tree groups) = %d, want 1", len(tg))
 	}
@@ -1384,6 +1392,47 @@ func TestSyncScanMtimeBump(t *testing.T) {
 	}
 }
 
+// TestSyncScanMtimeMovedBack replaces a file with other content of the
+// same size and an older mtime, as cp -p or a restore from backup does.
+// The next scan must hash the file again.
+func TestSyncScanMtimeMovedBack(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	db := openTestDB(t)
+	a := writeFile(t, dir, "a.bin", pattern(1, 500))
+
+	recorded := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
+
+	err := os.Chtimes(a, recorded, recorded)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	syncTree(t, db, dir)
+
+	replaced := pattern(3, 500)
+	writeFile(t, dir, "a.bin", replaced)
+
+	older := recorded.Add(-time.Hour)
+
+	err = os.Chtimes(a, older, older)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	st := syncTree(t, db, dir)
+	if st != (scanStats{walked: 1, updated: 1}) {
+		t.Fatalf("rescan stats = %+v, want 1 updated", st)
+	}
+
+	assertWholeFileHashed(t, db, a, replaced)
+
+	if r := recordByPath(t, dbRecords(t, db), a); !r.mtime.Equal(older) {
+		t.Fatalf("mtime = %v, want %v", r.mtime, older)
+	}
+}
+
 // assertWholeFileHashed fails unless the record for path holds the
 // whole-file hash of data as its head, tail, and content.
 func assertWholeFileHashed(t *testing.T, db *sql.DB, path string,
@@ -1900,14 +1949,19 @@ func TestScanHardlinksReadOnce(t *testing.T) {
 	}
 }
 
+// TestScanHardlinkRunFailsTogether makes a file with two hard links
+// unreadable. Both paths are counted as skipped, and each gets its own
+// warning.
+//
+//nolint:paralleltest // captureStderr replaces the process-wide os.Stderr
 func TestScanHardlinkRunFailsTogether(t *testing.T) {
-	t.Parallel()
-
+	stderr := captureStderr(t)
 	dir := t.TempDir()
 	db := openTestDB(t)
 	a := writeFile(t, dir, "a.bin", pattern(1, 300))
+	b := filepath.Join(dir, "b.bin")
 
-	err := os.Link(a, filepath.Join(dir, "b.bin"))
+	err := os.Link(a, b)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -1923,6 +1977,15 @@ func TestScanHardlinkRunFailsTogether(t *testing.T) {
 	if st.skipped != 2 || st.added != 0 {
 		t.Fatalf("stats = %+v, want both hardlink paths skipped", st)
 	}
+
+	got := stderr()
+
+	for _, p := range []string{a, b} {
+		warning := "scan: skipped " + p + " (permission denied)\n"
+		if !strings.Contains(got, warning) {
+			t.Errorf("stderr = %q, want %q", got, warning)
+		}
+	}
 }
 
 // TestHashPhaseSkipsFileReplacedByFIFO replaces a walked regular file
@@ -1960,7 +2023,7 @@ func TestHashPhaseSkipsFileReplacedByFIFO(t *testing.T) {
 			s.st.skipped, len(s.batch))
 	}
 
-	warning := "hash " + p + ": not a regular file\n"
+	warning := "scan: skipped " + p + " (not a regular file)\n"
 	if got := stderr(); strings.Count(got, warning) != 1 {
 		t.Errorf("stderr = %q, want %q once", got, warning)
 	}
@@ -2155,7 +2218,7 @@ func TestScanHashWriteFailureUnwindsPool(t *testing.T) {
 	var stderr bytes.Buffer
 
 	code := run([]string{
-		cmdScan, "--workers", strconv.Itoa(hashLeakWorkers), dir,
+		cmdScan, workersFlag, strconv.Itoa(hashLeakWorkers), dir,
 	}, io.Discard, &stderr)
 	if code != exitFatal {
 		t.Fatalf("run(scan) = %d, want %d; stderr: %s",
@@ -2248,7 +2311,7 @@ func TestPruneRoots(t *testing.T) {
 
 	// The walk of /p does not enter /p/q when it is a default exclude, so
 	// /p/q is kept, without -x too; /p/q/r lies under it and is dropped.
-	got = pruneRoots(nested, false, walkRules{excludes: []string{pqRoot}},
+	got = pruneRoots(nested, false, walkRules{defaultExcludes: []string{pqRoot}},
 		device)
 	if !slices.Equal(got, want) {
 		t.Fatalf("pruneRoots excluding %s = %q, want %q", pqRoot, got, want)
@@ -2266,23 +2329,86 @@ func TestPruneRoots(t *testing.T) {
 	}
 }
 
-func TestReportsNeverTouchFilesystem(t *testing.T) {
-	t.Parallel()
+// TestRunReportsLeaveOutRemovedCopies scans the smoke tree, then
+// removes copies the reports name. report leaves out each removed file,
+// and a group left with one path. trees checks only the root of each
+// duplicate tree: with a file under t2 removed it still names t2, and
+// with t2 removed it names nothing.
+func TestRunReportsLeaveOutRemovedCopies(t *testing.T) {
+	t.Setenv(databaseEnv, testDBPath(t))
 
 	dir := buildSmokeTree(t)
-	db := openTestDB(t)
+	p := func(name string) string { return filepath.Join(dir, name) }
 
-	syncTree(t, db, dir)
+	scanOK(t, dir)
 
-	// Remove the scanned tree entirely; the analysis must be
-	// unaffected because it reads the database alone.
-	err := os.RemoveAll(dir)
+	for _, name := range []string{"b/copy2.bin", "tiny2", "t2/sub/f2"} {
+		err := os.Remove(p(name))
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	want := "first\tdupe\tsize\n" +
+		p("t1/f1") + "\t" + p("t2/f1") + "\t3000\n" +
+		p("t1/f1") + "\t" + p("t3/f1") + "\t3000\n" +
+		p("a/one.bin") + "\t" + p("b/copy.bin") + "\t2000\n" +
+		p("t1/sub/f2") + "\t" + p("t3/sub/f2renamed") + "\t100\n"
+	if got := runStdout(t, cmdReport); got != want {
+		t.Errorf("report stdout = %q, want %q", got, want)
+	}
+
+	// disk_size, the last column, depends on the filesystem.
+	header := "first\tdupe\tfiles\tsize\tdisk_size\n"
+
+	want = header + p("t1") + "\t" + p("t2") + "\t2\t3100\t"
+	if got := runStdout(t, cmdTrees); !strings.HasPrefix(got, want) ||
+		strings.Count(got, "\n") != 2 {
+		t.Errorf("trees stdout = %q, want one row starting %q", got, want)
+	}
+
+	err := os.RemoveAll(p("t2"))
 	if err != nil {
 		t.Fatal(err)
 	}
 
-	assertSmokeDupeGroups(t, dir, db)
-	assertSmokeTreeGroups(t, dir, db)
+	if got := runStdout(t, cmdTrees); got != header {
+		t.Errorf("trees stdout after removing t2 = %q, want %q", got, header)
+	}
+}
+
+func TestPathExists(t *testing.T) {
+	t.Parallel()
+
+	dir := t.TempDir()
+	file := writeFile(t, dir, "file", nil)
+	locked := writeFile(t, dir, "locked/file", nil)
+
+	// Without search permission on its directory, a file cannot be
+	// looked at by anyone but root.
+	err := os.Chmod(filepath.Dir(locked), 0)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Runs before t.TempDir's own cleanup, which must delete the files.
+	t.Cleanup(func() {
+		//nolint:gosec // removing the directory needs its search bit back
+		_ = os.Chmod(filepath.Dir(locked), 0o700)
+	})
+
+	cases := map[string]bool{
+		dir:                                 true,
+		file:                                true,
+		locked:                              true,
+		filepath.Join(dir, "gone"):          false,
+		filepath.Join(file, "under-a-file"): false,
+	}
+	for path, want := range cases {
+		if got := pathExists(path); got != want {
+			t.Errorf("pathExists(%q) = %v, want %v", path, got, want)
+		}
+	}
 }
 
 func TestUnderRoot(t *testing.T) {

internal/sfdupes/trees.go

@@ -37,11 +37,14 @@ func treesPhase(name string) string {
 
 // runTrees implements the trees subcommand: it reads every record from
 // the database in path order, reconstructs the directory hierarchy from
-// the record paths, computes a Merkle-style digest per directory, and
-// prints maximal duplicate-tree groups as TSV on stdout. It never
-// touches the scanned filesystem; its only I/O is the database, stdout,
-// and stderr. It names the database on stderr before it opens it. Any
-// database problem, including a missing database, is fatal.
+// the paths of the records at or below the operands in args (of every
+// record when args is empty), computes a Merkle-style digest per
+// directory, and prints maximal duplicate-tree groups as TSV on stdout.
+// Of the scanned filesystem it only checks, with pathExists, that the
+// root of each duplicate tree is still there, never the files below it;
+// it never looks up an operand. It names the database on stderr before
+// it opens it. Any database problem, including a missing database, is
+// fatal.
 //
 // The work runs in four phases, each under a progress display of its
 // own on stderr: open (open the database), read (read the number of
@@ -49,7 +52,12 @@ func treesPhase(name string) string {
 // hierarchy), group (group the directories by digest) and write (write
 // the groups to stdout). Each display ends before the next phase
 // starts, and before an error is returned to be printed.
-func runTrees(ctx context.Context, stdout io.Writer) error {
+func runTrees(ctx context.Context, args []string, stdout io.Writer) error {
+	operands, err := cleanOperands(args)
+	if err != nil {
+		return err
+	}
+
 	dbPath, source, err := databasePath()
 	if err != nil {
 		return err
@@ -78,7 +86,7 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 		label: treesPhase("read"), unit: "records", total: -1,
 		waitingFor: countingRecords, stalledOn: "db",
 	})
-	records, super, allDirs, err := readTree(ctx, db, prog)
+	records, super, allDirs, err := readTree(ctx, db, operands, prog)
 
 	prog.finish()
 
@@ -86,13 +94,13 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 		return fmt.Errorf("database %s: %w", dbPath, err)
 	}
 
-	// Grouping works in memory, so it waits on nothing but the
-	// processor.
+	// Grouping works in memory, then checks the root of each duplicate
+	// tree on disk, so it waits on the processor or the disk.
 	prog = newProgress(progressParams{
 		label: treesPhase("group"), unit: "directories",
-		total: int64(len(allDirs)), stalledOn: "cpu",
+		total: int64(len(allDirs)), stalledOn: "cpu/disk",
 	})
-	dupes := collectTreeGroups(allDirs, super, prog)
+	dupes := collectTreeGroups(allDirs, super, pathExists, prog)
 
 	prog.finish()
 
@@ -100,7 +108,7 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 		label: treesPhase("write"), unit: "groups", total: int64(len(dupes)),
 		stalledOn: "out",
 	})
-	dupeTrees, reclaimable, err := writeTreeGroups(stdout, dupes, prog)
+	dupeTrees, err := writeTreeGroups(stdout, dupes, prog)
 
 	prog.finish()
 
@@ -111,16 +119,23 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 	fmt.Fprintf(os.Stderr,
 		"trees: %d records read, %d duplicate tree groups, %d dupe trees, "+
 			"%s reclaimable\n",
-		records, len(dupes), dupeTrees, humanBytes(reclaimable))
+		records, len(dupes), dupeTrees, humanBytes(reclaimableBytes(dupes)))
 
 	return nil
 }
 
 // readTree gives prog the number of records in db as its total, then
-// reads every record in path order into the directory hierarchy,
-// counting each on prog. It returns the number of records read, the
-// super-root and every directory.
-func readTree(ctx context.Context, db *sql.DB,
+// reads every record in path order, counting each on prog. It adds the
+// records that underOperand finds under operands to the directory
+// hierarchy. It returns the number of records read, the super-root and
+// every directory at or below one of operands.
+//
+// Any other directory lies above an operand and can hold files that are
+// not used, so its digest does not describe it. It is left out of the
+// directories returned. A directory whose parent is left out gets the
+// super-root as its parent, so suppressed never judges its group by
+// that parent's digest.
+func readTree(ctx context.Context, db *sql.DB, operands []string,
 	prog *progress,
 ) (int, *treeNode, []*treeNode, error) {
 	records := 0
@@ -130,7 +145,10 @@ func readTree(ctx context.Context, db *sql.DB,
 		records++
 
 		prog.increment()
-		tree.add(r)
+
+		if underOperand(r.path, operands) {
+			tree.add(r)
+		}
 	})
 	if err != nil {
 		return 0, nil, nil, err
@@ -138,29 +156,40 @@ func readTree(ctx context.Context, db *sql.DB,
 
 	super, allDirs := tree.finish()
 
-	return records, super, allDirs, nil
+	dirs := allDirs[:0]
+
+	for _, d := range allDirs {
+		if !underOperand(d.path, operands) {
+			continue
+		}
+
+		if !underOperand(d.parent.path, operands) {
+			d.parent = super
+		}
+
+		dirs = append(dirs, d)
+	}
+
+	return records, super, dirs, nil
 }
 
 // writeTreeGroups writes dupes to stdout as TSV, a header line and then
 // a row for every member of each group but its first, counting each
 // group on prog once its rows are written. The disk_size column is the
 // dupe's own. stdout may be the terminal prog is drawn on, so the lines
-// go through prog's writer. It returns the number of rows (dupe trees)
-// and the bytes they could reclaim.
+// go through prog's writer. It returns the number of rows (dupe trees).
 func writeTreeGroups(stdout io.Writer, dupes [][]*treeNode,
 	prog *progress,
-) (int, int64, error) {
+) (int, error) {
 	out := prog.writer(stdout)
 
 	_, err := fmt.Fprintln(out, "first\tdupe\tfiles\tsize\tdisk_size")
 	if err != nil {
-		return 0, 0, fmt.Errorf("write stdout: %w", err)
+		return 0, fmt.Errorf("write stdout: %w", err)
 	}
 
 	dupeTrees := 0
 
-	var reclaimable int64
-
 	for _, g := range dupes {
 		first := g[0]
 		for _, n := range g[1:] {
@@ -168,11 +197,10 @@ func writeTreeGroups(stdout io.Writer, dupes [][]*treeNode,
 				escapePath(first.path), escapePath(n.path),
 				first.fileCount, first.totalSize, n.diskSize)
 			if err != nil {
-				return 0, 0, fmt.Errorf("write stdout: %w", err)
+				return 0, fmt.Errorf("write stdout: %w", err)
 			}
 
 			dupeTrees++
-			reclaimable += first.totalSize
 		}
 
 		prog.increment()
@@ -180,10 +208,44 @@ func writeTreeGroups(stdout io.Writer, dupes [][]*treeNode,
 
 	err = out.Flush()
 	if err != nil {
-		return 0, 0, fmt.Errorf("write stdout: %w", err)
+		return 0, fmt.Errorf("write stdout: %w", err)
 	}
 
-	return dupeTrees, reclaimable, nil
+	return dupeTrees, nil
+}
+
+// reclaimableBytes returns the sum of the size column over the rows
+// that writeTreeGroups writes for dupes, leaving out a row whose dupe
+// lies inside another row's dupe.
+func reclaimableBytes(dupes [][]*treeNode) int64 {
+	isDupe := make(map[*treeNode]bool)
+
+	for _, g := range dupes {
+		for _, n := range g[1:] {
+			isDupe[n] = true
+		}
+	}
+
+	var total int64
+
+	for n := range isDupe {
+		if !insideDupe(n, isDupe) {
+			total += n.totalSize
+		}
+	}
+
+	return total
+}
+
+// insideDupe reports whether a directory above n is in isDupe.
+func insideDupe(n *treeNode, isDupe map[*treeNode]bool) bool {
+	for p := n.parent; p != nil; p = p.parent {
+		if isDupe[p] {
+			return true
+		}
+	}
+
+	return false
 }
 
 // treeBuilder reconstructs the directory hierarchy from records added
@@ -300,12 +362,14 @@ func fileEntry(name string, r scanRec) string {
 
 // collectTreeGroups groups directories by digest, counting each one on
 // prog as it is grouped, and returns every maximal group with two or
-// more members, each group's members sorted by path, groups ordered by
-// tree size descending then by first path ascending. A group whose
-// trees hold 0 bytes is left out, since removing one frees nothing. The
-// groups it implies hold 0 bytes too, so leaving it out hides nothing.
+// more members for which exists returns true, each group's members
+// sorted by path, groups ordered by tree size descending then by first
+// path ascending. A group whose trees hold 0 bytes is left out, since
+// removing one frees nothing. The groups it implies hold 0 bytes too,
+// so leaving it out hides nothing. exists is called only on the members
+// of maximal groups.
 func collectTreeGroups(allDirs []*treeNode, super *treeNode,
-	prog *progress,
+	exists func(path string) bool, prog *progress,
 ) [][]*treeNode {
 	groups := make(map[[sha256.Size]byte][]*treeNode)
 	for _, d := range allDirs {
@@ -323,6 +387,16 @@ func collectTreeGroups(allDirs []*treeNode, super *treeNode,
 			continue
 		}
 
+		// A member that is gone leaves the group. A suppressed group is
+		// still implied: its members still on disk have parents still
+		// on disk, and those parents' group is checked the same way.
+		g = slices.DeleteFunc(g, func(n *treeNode) bool {
+			return !exists(n.path)
+		})
+		if len(g) < minGroupSize {
+			continue
+		}
+
 		slices.SortFunc(g, func(a, b *treeNode) int {
 			return strings.Compare(a.path, b.path)
 		})

internal/sfdupes/trees_test.go

@@ -6,6 +6,7 @@ import (
 	"database/sql"
 	"os"
 	"slices"
+	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -179,7 +180,7 @@ func TestTreeNamesSortingBeforeSlash(t *testing.T) {
 		t.Fatalf("directory paths = %q, want %q", got, want)
 	}
 
-	groups := collectTreeGroups(dirs, super, nil)
+	groups := collectTreeGroups(dirs, super, everyPathExists, nil)
 
 	gotGroups := groupPaths(groups)
 	wantGroups := [][]string{{"/a", "/c"}}
@@ -195,7 +196,8 @@ func TestTreeNamesSortingBeforeSlash(t *testing.T) {
 }
 
 func TestRunTreesEscapesPaths(t *testing.T) {
-	t.Setenv(databaseEnv, seedDatabase(t, awkwardPairRecs()))
+	dir, recs := recsOnDisk(t, awkwardPairRecs())
+	t.Setenv(databaseEnv, seedDatabase(t, recs))
 
 	var stdout, stderr bytes.Buffer
 
@@ -206,12 +208,63 @@ func TestRunTreesEscapesPaths(t *testing.T) {
 	}
 
 	want := "first\tdupe\tfiles\tsize\tdisk_size\n" +
-		`/d/\tone\ntwo\rthree\\four` + "\t/d/A\t1\t5\t4096\n"
+		dir + `/d/\tone\ntwo\rthree\\four` + "\t" + dir + "/d/A\t1\t5\t4096\n"
 	if got := stdout.String(); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
 }
 
+// TestRunTreesOperands runs trees with operands on records in a new
+// directory. x/a and y/a are copies, and so are y/a and srv/b. x and y
+// also hold files that differ. Given x/a and y/a, trees reads x and y
+// as holding one copy each of a, yet neither is reported: only the
+// directories at or below an operand are.
+func TestRunTreesOperands(t *testing.T) {
+	f, g, k := testHash("f"), testHash("g"), testHash("k")
+	dir, recs := recsOnDisk(t, []scanRec{
+		{size: 10, head: f, tail: f, content: f, path: "/srv/b/f"},
+		{size: 10, head: f, tail: f, content: f, path: "/x/a/f"},
+		{size: 20, head: g, tail: g, content: g, path: "/x/g"},
+		{size: 10, head: f, tail: f, content: f, path: "/y/a/f"},
+		{size: 30, head: k, tail: k, content: k, path: "/y/k"},
+	})
+	b, xa, ya := dir+"/srv/b", dir+"/x/a", dir+"/y/a"
+
+	header := "first\tdupe\tfiles\tsize\tdisk_size\n"
+	row := func(first, dupe string) string {
+		return first + "\t" + dupe + "\t1\t10\t0\n"
+	}
+
+	cases := map[string]struct {
+		operands []string
+		want     string
+	}{
+		"none":       {nil, header + row(b, xa) + row(b, ya)},
+		"two copies": {[]string{xa, ya}, header + row(xa, ya)},
+		"parents":    {[]string{dir + "/x", dir + "/y"}, header + row(xa, ya)},
+		"one copy":   {[]string{b}, header},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			t.Setenv(databaseEnv, seedDatabase(t, recs))
+
+			var stdout, stderr bytes.Buffer
+
+			code := run(append([]string{cmdTrees}, tc.operands...),
+				&stdout, &stderr)
+			if code != exitOK {
+				t.Fatalf("run(trees %q) = %d, want %d; stderr: %s",
+					tc.operands, code, exitOK, stderr.String())
+			}
+
+			if got := stdout.String(); got != tc.want {
+				t.Errorf("stdout = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
 // TestRunTreesDiskSize runs trees on two copies of a tree whose files
 // take different space on disk. The row's disk_size is the sum over the
 // dupe copy's files, those in its subdirectory included.
@@ -224,9 +277,11 @@ func TestRunTreesDiskSize(t *testing.T) {
 		{size: 20, diskSize: 1024, head: g, tail: g, content: g, path: "/b/sub/g"},
 	}
 
+	dir, recs := recsOnDisk(t, recs)
 	t.Setenv(databaseEnv, seedDatabase(t, recs))
 
-	want := "first\tdupe\tfiles\tsize\tdisk_size\n/a\t/b\t2\t30\t1536\n"
+	want := "first\tdupe\tfiles\tsize\tdisk_size\n" +
+		dir + "/a\t" + dir + "/b\t2\t30\t1536\n"
 	if got := runStdout(t, cmdTrees); got != want {
 		t.Errorf("stdout = %q, want %q", got, want)
 	}
@@ -237,10 +292,11 @@ func TestRunTreesDiskSize(t *testing.T) {
 // there. After the line naming the database, each phase prints a line
 // naming it with its place among the four as it starts and another as
 // it ends. read counts the 6 records before it reads them, group counts
-// the 8 directories, and write the 1 group. The summary comes last, and
+// the directories, and write the 1 group. The summary comes last, and
 // stdout holds the report alone.
 func TestRunTreesProgressNamesEachPhase(t *testing.T) {
-	path := seedDatabase(t, smokeTreeRecs())
+	dir, recs := recsOnDisk(t, smokeTreeRecs())
+	path := seedDatabase(t, recs)
 	t.Setenv(databaseEnv, path)
 
 	stderr := captureStderr(t)
@@ -254,11 +310,15 @@ func TestRunTreesProgressNamesEachPhase(t *testing.T) {
 	}
 
 	report := "first\tdupe\tfiles\tsize\tdisk_size\n" +
-		"/d/t1\t/d/t2\t2\t3100\t8192\n"
+		dir + "/d/t1\t" + dir + "/d/t2\t2\t3100\t8192\n"
 	if got := stdout.String(); got != report {
 		t.Errorf("stdout = %q, want %q", got, report)
 	}
 
+	// The smoke-test records name 8 directories, / included. Moved under
+	// dir, they name one more for each slash in dir.
+	dirs := strconv.Itoa(8 + strings.Count(dir, "/"))
+
 	// How each line starts; the rate at the end of a phase varies.
 	want := []string{
 		"trees: database " + path + " (from " + databaseEnv + ")",
@@ -266,8 +326,9 @@ func TestRunTreesProgressNamesEachPhase(t *testing.T) {
 		treesOpenLine,
 		"[2/4] read: counting the records, elapsed 0s, remain unknown",
 		"[2/4] read: [6/6] 100%, ",
-		"[3/4] group: [0/8] 0%, 0 directories/s, elapsed 0s, remain unknown",
-		"[3/4] group: [8/8] 100%, ",
+		"[3/4] group: [0/" + dirs + "] 0%, 0 directories/s, elapsed 0s, " +
+			"remain unknown",
+		"[3/4] group: [" + dirs + "/" + dirs + "] 100%, ",
 		"[4/4] write: [0/1] 0%, 0 groups/s, elapsed 0s, remain unknown",
 		"[4/4] write: [1/1] 100%, ",
 		"trees: 6 records read, 1 duplicate tree groups, 1 dupe trees, ",
@@ -296,14 +357,14 @@ func TestRunTreesProgressNamesEachPhase(t *testing.T) {
 func TestWriteTreeGroupsOnTerminal(t *testing.T) {
 	stderr := captureStderr(t)
 	super, dirs := treeOf(t, smokeTreeRecs())
-	dupes := collectTreeGroups(dirs, super, nil)
+	dupes := collectTreeGroups(dirs, super, everyPathExists, nil)
 
 	// No timed draws: the display is drawn as it starts and as it ends.
 	p := startProgress(progressParams{
 		label: treesPhase("write"), unit: "groups", total: int64(len(dupes)),
 	}, terminalWidth, time.Hour, time.Now)
 
-	_, _, err := writeTreeGroups(os.Stderr, dupes, p)
+	_, err := writeTreeGroups(os.Stderr, dupes, p)
 
 	p.finish()
 
@@ -377,7 +438,7 @@ func TestCollectTreeGroupsMaximal(t *testing.T) {
 
 	super, dirs := treeOf(t, smokeTreeRecs())
 
-	groups := collectTreeGroups(dirs, super, nil)
+	groups := collectTreeGroups(dirs, super, everyPathExists, nil)
 
 	want := [][]string{{"/d/t1", "/d/t2"}}
 	if got := groupPaths(groups); !slices.EqualFunc(got, want,
@@ -401,14 +462,16 @@ func TestCollectTreeGroupsDeterministic(t *testing.T) {
 
 	super, dirs := treeOf(t, recs)
 
-	forward := groupPaths(collectTreeGroups(dirs, super, nil))
+	forward := groupPaths(collectTreeGroups(dirs, super, everyPathExists,
+		nil))
 
 	reversed := slices.Clone(recs)
 	slices.Reverse(reversed)
 
 	superR, dirsR := treeOf(t, reversed)
 
-	backward := groupPaths(collectTreeGroups(dirsR, superR, nil))
+	backward := groupPaths(collectTreeGroups(dirsR, superR, everyPathExists,
+		nil))
 	if !slices.EqualFunc(forward, backward, slices.Equal) {
 		t.Fatalf("output depends on record order: %v vs %v",
 			forward, backward)
@@ -428,7 +491,7 @@ func TestCollectTreeGroupsSiblings(t *testing.T) {
 
 	super, dirs := treeOf(t, recs)
 
-	got := groupPaths(collectTreeGroups(dirs, super, nil))
+	got := groupPaths(collectTreeGroups(dirs, super, everyPathExists, nil))
 
 	want := [][]string{{"/p/x1", "/p/x2"}}
 	if !slices.EqualFunc(got, want, slices.Equal) {
@@ -457,7 +520,7 @@ func TestCollectTreeGroupsLeavesOutEmptyTrees(t *testing.T) {
 
 	super, dirs := treeOf(t, recs)
 
-	got := groupPaths(collectTreeGroups(dirs, super, nil))
+	got := groupPaths(collectTreeGroups(dirs, super, everyPathExists, nil))
 
 	want := [][]string{{"/p/x1", "/p/x2"}}
 	if !slices.EqualFunc(got, want, slices.Equal) {
@@ -480,10 +543,159 @@ func TestCollectTreeGroupsDifferingParents(t *testing.T) {
 
 	super, dirs := treeOf(t, recs)
 
-	got := groupPaths(collectTreeGroups(dirs, super, nil))
+	got := groupPaths(collectTreeGroups(dirs, super, everyPathExists, nil))
 
 	want := [][]string{{"/p/a/x", "/q/b/x"}}
 	if !slices.EqualFunc(got, want, slices.Equal) {
 		t.Fatalf("groups = %v, want %v", got, want)
 	}
 }
+
+// TestCollectTreeGroupsChecksOnlyReportedCopies checks that trees asks
+// whether a directory is on disk only for the members of the groups it
+// returns. It never asks about /a/proj/sub, which lies inside a copy
+// and whose group the group of /a/proj implies.
+func TestCollectTreeGroupsChecksOnlyReportedCopies(t *testing.T) {
+	t.Parallel()
+
+	f, g, u := testHash("f"), testHash("g"), testHash("u")
+	recs := []scanRec{
+		{size: 10, head: f, tail: f, content: f, path: "/a/proj/f"},
+		{size: 20, head: g, tail: g, content: g, path: "/a/proj/sub/g"},
+		{size: 10, head: f, tail: f, content: f, path: "/b/proj/f"},
+		{size: 20, head: g, tail: g, content: g, path: "/b/proj/sub/g"},
+		// /a lacks this file, so /a and /b are no copies of each other.
+		{size: 30, head: u, tail: u, content: u, path: "/b/lonely"},
+	}
+
+	super, dirs := treeOf(t, recs)
+
+	var checked []string
+
+	exists := func(path string) bool {
+		checked = append(checked, path)
+
+		return true
+	}
+
+	got := groupPaths(collectTreeGroups(dirs, super, exists, nil))
+
+	want := [][]string{{"/a/proj", "/b/proj"}}
+	if !slices.EqualFunc(got, want, slices.Equal) {
+		t.Fatalf("groups = %v, want %v", got, want)
+	}
+
+	slices.Sort(checked)
+
+	if !slices.Equal(checked, want[0]) {
+		t.Errorf("directories checked on disk = %q, want %q",
+			checked, want[0])
+	}
+}
+
+// TestCollectTreeGroupsDropsGroupLeftWithOneCopy checks that a group of
+// two copies, one of them gone, is not returned. If kept, it would
+// write no row, but the summary line would count it.
+func TestCollectTreeGroupsDropsGroupLeftWithOneCopy(t *testing.T) {
+	t.Parallel()
+
+	f := testHash("f")
+	recs := []scanRec{
+		{size: 10, head: f, tail: f, content: f, path: "/c/kept/f"},
+		{size: 10, head: f, tail: f, content: f, path: "/c/gone/f"},
+	}
+
+	super, dirs := treeOf(t, recs)
+
+	exists := func(path string) bool { return path != "/c/gone" }
+
+	got := groupPaths(collectTreeGroups(dirs, super, exists, nil))
+	if len(got) != 0 {
+		t.Errorf("groups = %v, want none", got)
+	}
+}
+
+// TestCollectTreeGroupsKeepsGroupLeftWithTwoCopies checks that a group
+// of three copies, one of them gone, is returned with the other two.
+func TestCollectTreeGroupsKeepsGroupLeftWithTwoCopies(t *testing.T) {
+	t.Parallel()
+
+	f := testHash("f")
+	recs := []scanRec{
+		{size: 10, head: f, tail: f, content: f, path: "/d/kept1/f"},
+		{size: 10, head: f, tail: f, content: f, path: "/d/gone/f"},
+		{size: 10, head: f, tail: f, content: f, path: "/d/kept2/f"},
+	}
+
+	super, dirs := treeOf(t, recs)
+
+	exists := func(path string) bool { return path != "/d/gone" }
+
+	got := groupPaths(collectTreeGroups(dirs, super, exists, nil))
+
+	want := [][]string{{"/d/kept1", "/d/kept2"}}
+	if !slices.EqualFunc(got, want, slices.Equal) {
+		t.Errorf("groups = %v, want %v", got, want)
+	}
+}
+
+// TestReclaimableBytesCountsNestedDupeOnce checks that a dupe tree
+// inside another dupe tree adds nothing to the reclaimable bytes. /B/x
+// lies in /B, so the total adds the sizes of /B and /C/x, 1500 + 1000.
+func TestReclaimableBytesCountsNestedDupeOnce(t *testing.T) {
+	t.Parallel()
+
+	fh, ft, fc := testHash("fh"), testHash("ft"), testHash("fc")
+	gh, gt, gc := testHash("gh"), testHash("gt"), testHash("gc")
+	recs := []scanRec{
+		{size: 1000, head: fh, tail: ft, content: fc, path: "/A/x/f"},
+		{size: 500, head: gh, tail: gt, content: gc, path: "/A/y/g"},
+		{size: 1000, head: fh, tail: ft, content: fc, path: "/B/x/f"},
+		{size: 500, head: gh, tail: gt, content: gc, path: "/B/y/g"},
+		{size: 1000, head: fh, tail: ft, content: fc, path: "/C/x/f"},
+	}
+
+	super, dirs := treeOf(t, recs)
+	dupes := collectTreeGroups(dirs, super, everyPathExists, nil)
+
+	wantGroups := [][]string{{"/A", "/B"}, {"/A/x", "/B/x", "/C/x"}}
+	if got := groupPaths(dupes); !slices.EqualFunc(got, wantGroups,
+		slices.Equal) {
+		t.Fatalf("groups = %v, want %v", got, wantGroups)
+	}
+
+	if got := reclaimableBytes(dupes); got != 2500 {
+		t.Errorf("reclaimableBytes = %d, want 2500", got)
+	}
+}
+
+// TestReclaimableBytesCountsDupeInsideGrandparentDupeOnce checks that a
+// dupe tree adds nothing when a dupe tree above its parent holds it.
+// /B/m/x lies in /B, and /B/m is no dupe, since the group of /A and /B
+// implies that of /A/m and /B/m. The total adds the sizes of /B and
+// /C/x, 1300 + 1000.
+func TestReclaimableBytesCountsDupeInsideGrandparentDupeOnce(t *testing.T) {
+	t.Parallel()
+
+	f, w := testHash("f"), testHash("w")
+	recs := []scanRec{
+		{size: 1000, head: f, tail: f, content: f, path: "/A/m/x/f"},
+		{size: 300, head: w, tail: w, content: w, path: "/A/m/w"},
+		{size: 1000, head: f, tail: f, content: f, path: "/B/m/x/f"},
+		{size: 300, head: w, tail: w, content: w, path: "/B/m/w"},
+		{size: 1000, head: f, tail: f, content: f, path: "/C/x/f"},
+	}
+
+	super, dirs := treeOf(t, recs)
+	dupes := collectTreeGroups(dirs, super, everyPathExists, nil)
+
+	wantGroups := [][]string{{"/A", "/B"}, {"/A/m/x", "/B/m/x", "/C/x"}}
+	if got := groupPaths(dupes); !slices.EqualFunc(got, wantGroups,
+		slices.Equal) {
+		t.Fatalf("groups = %v, want %v", got, wantGroups)
+	}
+
+	if got := reclaimableBytes(dupes); got != 2300 {
+		t.Errorf("reclaimableBytes = %d, want 2300", got)
+	}
+}