Say "skipped" in scan's warnings for skipped paths (closes #170) (#195)

README.md

@@ -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
+Root keeps the database in `/var/lib/sfdupes/db.sqlite`. Any other user keeps it
-another file. The first `scan` creates 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
@@ -114,22 +117,27 @@ docker run --rm -v /srv:/srv:ro -v /var/lib/sfdupes:/var/lib/sfdupes \
 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
+  the database elsewhere, put `SFDUPES_DATABASE=/path/to/db.sqlite` directly
-  command on the same line.
+  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
+    - a line for each system, temporary or cache directory the walk leaves out
-      "Rules for the walk");
+      (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`;
@@ -148,8 +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
+  by every user. Another user can then run those three commands against root's
-  root's database.
+  database, with `SFDUPES_DATABASE=/var/lib/sfdupes/db.sqlite` before the
+  command.
 
 ### Reading the reports
 
@@ -258,8 +267,8 @@ In order:
 
 ```
 sfdupes scan [--workers N] [-x] [--no-default-excludes] [--exclude PATH]... PATH...
-sfdupes report > dupes.tsv
+sfdupes report [PATH...] > dupes.tsv
-sfdupes trees > dupetrees.tsv
+sfdupes trees [PATH...] > dupetrees.tsv
 sfdupes info
 sfdupes dwim [--workers N] [-x] [--no-default-excludes] [--exclude PATH]...
 sfdupes --version
@@ -288,9 +297,9 @@ 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
+  database is at its default path (see "Location"). There is no command-line
-  it. The path names the file exactly, whatever characters it holds. A relative
+  flag for it. The path names the file exactly, whatever characters it holds. A
-  path is relative to the working directory.
+  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
@@ -300,8 +309,9 @@ The reference below is also the program's specification.
   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 and temporary directories the walk otherwise
+  excludes. Those are the system, temporary and cache directories the walk
-  leaves out, such as `/var/tmp` on Linux (see "Rules for 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
@@ -553,7 +563,7 @@ 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
@@ -563,7 +573,7 @@ walk practical on large or busy storage pools.
   For each one it reaches, `scan` prints a line on stderr naming it:
 
     ```
-    scan: skipped /srv/cache (--exclude)
+    scan: left out /srv/cache (--exclude)
     ```
 
     The path is not counted as skipped. The scan keeps the records at and below
@@ -571,12 +581,17 @@ walk practical on large or busy storage pools.
     `--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 temporary files and the kernel's view of the running
+  default excludes hold the kernel's view of the running system, temporary
-  system. They depend on the system, and `sfdupes scan --help` lists those of
+  files, and caches. A cache here is data that programs and package managers
-  the system it runs on. On Linux they are:
+  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
+    /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:
@@ -584,13 +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,
+    `/private/var/folders` holds each user's temporary files and caches on
-    through firmlinks. Firmlinks are the links macOS uses to show the data
+    macOS. Root's home on macOS is `/var/root`, which the walk reaches as
-    volume inside the system volume. Walking `/System/Volumes/Data` as well
+    `/private/var/root`. A walk of `/` on macOS reaches everything in
-    would record each of those files twice. Any other system has no default
+    `/System/Volumes/Data` already, through firmlinks. Firmlinks are the links
-    excludes.
+    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.
@@ -599,7 +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
@@ -659,6 +682,18 @@ 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. 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. When a
+file with several hard links cannot be read, each of its paths gets a warning.
+
 #### Stopping a scan
 
 `scan` stops cleanly on `SIGINT`, the signal Ctrl-C sends, or `SIGTERM`, the
@@ -686,11 +721,12 @@ and `scan` leaves it ignored.
 
 ### `report` mode
 
-`report` reads every record from the database. It takes no positional arguments.
+`report` reads every record from the database. It takes optional `PATH`
-A positional argument is anything on the command line that is not a flag, such
+operands. With none, it uses every record. With one or more, it uses only the
-as the paths `scan` takes. `report` never opens a scanned file. It checks that
+records at or below them (see "Operands of `report` and `trees`"). `report`
-each path in a group still exists, and looks at no other path. The check is
+never opens a scanned file. It checks that each path in a group still exists,
-`lstat`, the system call that looks up a path without following a symlink:
+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.
@@ -722,6 +758,27 @@ 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
@@ -749,7 +806,8 @@ 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.
@@ -760,11 +818,12 @@ 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.
+`trees` reads the same database as `report` and takes the same `PATH` operands
-It reports whole duplicate directory trees. Two directories are duplicate trees
+(see "Operands of `report` and `trees`"). It reports whole duplicate directory
-when they hold the same relative paths, and each path holds a file with the same
+trees. Two directories are duplicate trees when they hold the same relative
-signature in both. It rebuilds the directory tree from the paths in the records,
+paths, and each path holds a file with the same signature in both. It rebuilds
-split on `/`. Like `report`, it never opens a scanned file. It checks only that
+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:
@@ -792,6 +851,11 @@ 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
@@ -845,10 +909,12 @@ 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
@@ -857,8 +923,10 @@ 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. Like `scan`,
+duplicate trees in one command. It takes no positional arguments. A positional
-it should run as root, so that it can read every file.
+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` takes these flags of `scan`, with the same meaning (see "Settings"):
 
@@ -937,7 +1005,22 @@ 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
@@ -977,9 +1060,9 @@ 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
+  database's path, with symlinks resolved, plus `.lock`. For root, by default,
-  `/var/lib/sfdupes/db.sqlite.lock`. A path through a symlink names the same
+  that is `/var/lib/sfdupes/db.sqlite.lock`. A path through a symlink names the
-  lock file.
+  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
@@ -1268,7 +1351,8 @@ showing the count it stopped at.
     - `scan` with no operand;
     - `--workers` below 1;
     - an empty `--exclude` value, as in `--exclude ""`;
-    - `report`, `trees`, `info` or `dwim` with a positional argument.
+    - 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

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
+// database of file signatures that the other subcommands read. The
-// /var/lib/sfdupes/db.sqlite) that the other subcommands read.
+// 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 +3,7 @@ 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

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/cli.go

@@ -34,10 +34,14 @@ const (
 
 // 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 " +
+const databaseHelp = `scan and dwim store what they find in a database file. report,
-	"file. report, trees\nand info read it. The file is $" + databaseEnv +
+trees and info read it. The file is $SFDUPES_DATABASE when that is set
-	" when that is set and\nnot empty, and otherwise " +
+and not empty. Otherwise root uses /var/lib/sfdupes/db.sqlite. Any other
-	defaultDatabasePath + ", where creating it\nneeds root.\n"
+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").
@@ -49,6 +53,13 @@ const escapesHelp = `Four characters in a path are escaped, so each row stays on
   \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 (
@@ -61,7 +72,7 @@ row per duplicate file. The columns are:
 
 Rows are ordered by size, largest first, then by path.
 
-` + escapesHelp
+` + 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:
@@ -74,7 +85,7 @@ row per duplicate directory tree. The columns are:
 
 Rows are ordered by size, largest first, then by path.
 
-` + escapesHelp
+` + escapesHelp + "\n" + operandsHelp
 
 	infoHelp = `The details go to stdout, one per line. Each line holds a name and its
 value, separated by a tab:
@@ -108,6 +119,11 @@ var errWorkersBelowOne = errors.New("--workers must be at least 1")
 // 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.
@@ -229,20 +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,
+		Args:  rejectEmptyOperand,
-		RunE: runE(func(ctx context.Context, _ []string) error {
+		RunE: runE(func(ctx context.Context, args []string) error {
-			return runReport(ctx, stdout)
+			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,
+		Args:  rejectEmptyOperand,
-		RunE: runE(func(ctx context.Context, _ []string) error {
+		RunE: runE(func(ctx context.Context, args []string) error {
-			return runTrees(ctx, stdout)
+			return runTrees(ctx, args, stdout)
 		}),
 	}
 
@@ -387,6 +403,18 @@ func rejectEmptyExcludeValue(cmd *cobra.Command, excludePaths []string) error {
 	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"
 )
 
@@ -559,13 +560,17 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	// line.
 	t.Setenv(databaseEnv, "")
 
-	// Whether scan calls the database new depends on that machine too.
+	dbPath, err := defaultDatabasePath(os.Geteuid())
-	scanWant := "scan: database " + defaultDatabasePath + " (default)\n"
+	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 +
+		scanWant = "scan: new database " + dbPath + " (default)\n"
-			" (default)\n"
 	}
 
 	missing := filepath.Join(t.TempDir(), "nope")
@@ -583,23 +588,23 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 		},
 		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",
 		},
 	}
@@ -626,8 +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},
@@ -718,6 +721,19 @@ 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.
@@ -760,36 +776,37 @@ func TestRunScanUsageLine(t *testing.T) {
 	}
 }
 
-func TestRunHelpDescribesDatabaseAndOutput(t *testing.T) {
+func TestRunHelpDescribesOutput(t *testing.T) {
 	t.Parallel()
 
-	// README §Settings: the help names the database, and says what
+	// README §Settings: the help of report, trees, info and dwim says
-	// report, trees, info and dwim print.
+	// what each prints. The help of report and trees also explains
-	database := []string{
+	// their PATH operands. TestRunHelp checks the database paragraph
-		"scan and dwim store", databaseEnv, defaultDatabasePath, "needs root",
+	// 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; none for the root.
+	// The subcommand whose help is printed.
 	cases := []struct {
 		subcommand []string
 		want       []string
 	}{
-		{nil, database},
-		{[]string{cmdScan}, database},
 		{
 			[]string{cmdReport},
-			append([]string{
+			slices.Concat([]string{
 				"tab-separated", "\n  first ", "\n  dupe ", "\n  size ",
 				"largest first",
-			}, escapes...),
+			}, escapes, operands),
 		},
 		{
 			[]string{cmdTrees},
-			append([]string{
+			slices.Concat([]string{
 				"tab-separated", "\n  first ", "\n  dupe ", "\n  files ",
 				"\n  size ", "\n  disk_size ", "largest first",
-			}, escapes...),
+			}, escapes, operands),
 		},
 		{
 			[]string{cmdInfo},
@@ -996,8 +1013,7 @@ func TestRunScanSucceedsDespiteWarnings(t *testing.T) {
 
 	stderr := scanOK(t, dir)
 
-	warning := "hash " + unreadable + ": open " + unreadable +
+	warning := "scan: skipped " + unreadable + " (permission denied)\n"
-		": permission denied\n"
 	if !strings.Contains(stderr, warning) {
 		t.Errorf("stderr = %q, want %q", stderr, warning)
 	}
@@ -1269,7 +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)
 	}
@@ -1304,7 +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)
 	}
@@ -1338,7 +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
@@ -1390,7 +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"
 
@@ -1609,6 +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.
@@ -1839,8 +1949,12 @@ func TestStdoutWriteErrorPropagates(t *testing.T) {
 	scanFixture(t)
 
 	cases := map[string]func(context.Context, io.Writer) error{
-		cmdReport: runReport,
+		cmdReport: func(ctx context.Context, w io.Writer) error {
-		cmdTrees:  runTrees,
+			return runReport(ctx, nil, w)
+		},
+		cmdTrees: func(ctx context.Context, w io.Writer) error {
+			return runTrees(ctx, nil, w)
+		},
 		cmdInfo: runInfo,
 	}
 

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
+// rootDatabasePath is the database root uses when SFDUPES_DATABASE is
-// SFDUPES_DATABASE is not set.
+// not set.
-const defaultDatabasePath = "/var/lib/sfdupes/db.sqlite"
+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
+// dbDirPerm is the mode for a directory scan creates for root's
-// scan.
+// 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"
+	path, source := os.Getenv(databaseEnv), "from "+databaseEnv
-	if p := os.Getenv(databaseEnv); p != "" {
+	if path == "" {
-		path, source = p, "from "+databaseEnv
+		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
+// databasePath returns it. Each directory gets mode dbDirPerm, or
-// cut off as text, by filepath.Split. filepath.Dir would clean it,
+// stateDirPerm when databasePath uses a user's default path in the XDG
-// dropping each ".." together with the directory name before it; when
+// state directory: the user is not root and SFDUPES_DATABASE is unset
-// that name is a symlink, the cleaned path names another directory than
+// or empty. The directory is path with its last element cut off as
-// the one the system reaches when it opens path.
+// 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

@@ -26,7 +26,7 @@ func runDwim(
 		return err
 	}
 
-	err = runTrees(ctx, stdout)
+	err = runTrees(ctx, nil, stdout)
 	if err != nil {
 		return err
 	}

internal/sfdupes/dwim_test.go

@@ -121,9 +121,9 @@ func TestRunDwimExcludeLeavesTreeOut(t *testing.T) {
 		t.Errorf("group = %q, want it to start %q", lines[1], want)
 	}
 
-	skipped := "scan: skipped " + excluded + " (--exclude)\n"
+	leftOut := "scan: left out " + excluded + " (--exclude)\n"
-	if got := stderr(); !strings.Contains(got, skipped) {
+	if got := stderr(); !strings.Contains(got, leftOut) {
-		t.Errorf("stderr = %q, want it to contain %q", got, skipped)
+		t.Errorf("stderr = %q, want it to contain %q", got, leftOut)
 	}
 }
 

internal/sfdupes/exclude.go

@@ -72,14 +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
@@ -186,7 +203,7 @@ func (rules walkRules) excludePathLine(path string, fi fs.FileInfo) string {
 		return ""
 	}
 
-	return fmt.Sprintf("scan: skipped %s (--exclude)", path)
+	return fmt.Sprintf("scan: left out %s (--exclude)", path)
 }
 
 // nonPersistentFilesystemTypes returns the filesystem types that are
@@ -216,7 +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)
 }
 

internal/sfdupes/exclude_test.go

@@ -24,10 +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",
@@ -63,6 +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) {
@@ -207,7 +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)
 	}
@@ -276,7 +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)
 	}
 }
@@ -290,15 +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") {
+	if strings.Contains(stderr, "scan: left out") {
-		t.Errorf("stderr = %q, want no line saying a path was skipped",
+		t.Errorf("stderr = %q, want no line saying a path was left out",
 			stderr)
 	}
 
@@ -342,7 +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)
@@ -383,7 +418,7 @@ func TestRunScanExcludeNested(t *testing.T) {
 
 	stderr := scanOK(t, excludeFlag, excluded, dir)
 
-	line := "scan: skipped " + excluded + " (--exclude)\n"
+	line := "scan: left out " + excluded + " (--exclude)\n"
 	if !strings.Contains(stderr, line) {
 		t.Errorf("stderr = %q, want %q", stderr, line)
 	}
@@ -445,8 +480,8 @@ func TestRunScanExcludeReachedByAnotherPath(t *testing.T) {
 	stderr := scanOK(t, excludeFlag, filepath.Join(link, "cache"),
 		excludeFlag, dotDot, dir)
 
-	for _, skipped := range []string{filepath.Dir(inDir), file, hardLink} {
+	for _, leftOut := range []string{filepath.Dir(inDir), file, hardLink} {
-		line := "scan: skipped " + skipped + " (--exclude)\n"
+		line := "scan: left out " + leftOut + " (--exclude)\n"
 		if !strings.Contains(stderr, line) {
 			t.Errorf("stderr = %q, want %q", stderr, line)
 		}
@@ -580,7 +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"
 
@@ -634,7 +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)
@@ -667,7 +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)
@@ -711,6 +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/report.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"io/fs"
 	"os"
+	"path/filepath"
 	"slices"
 	"strings"
 	"syscall"
@@ -44,10 +45,12 @@ func reportPhase(name string) string {
 }
 
 // runReport implements the report subcommand: it prints the file-level
-// duplicates report as TSV on stdout. Of the scanned filesystem it
+// duplicates report as TSV on stdout, from the records at or below the
-// only checks, with pathExists, that each path in a duplicate group is
+// operands in args, or from every record when args is empty. Of the
-// still there. It names the database on stderr before it opens it. Any
+// scanned filesystem it only checks, with pathExists, that each path in
-// database problem, including a missing database, is fatal.
+// 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.
 //
 // The work runs in two phases, each under a progress display of its
 // own on stderr: open (open the database) and read (read the number of
@@ -55,7 +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
@@ -87,7 +95,8 @@ func runReport(ctx context.Context, stdout io.Writer) error {
 		label: reportPhase("read"), unit: "records", total: -1,
 		waitingFor: countingRecords, stalledOn: "db/disk/out",
 	})
-	report, err := readReport(ctx, db, dbPath, stdout, pathExists, prog)
+	report, err := readReport(ctx, db, dbPath, operands, stdout, pathExists,
+		prog)
 
 	prog.finish()
 
@@ -108,12 +117,14 @@ 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. A path for which exists returns false is left out of its group.
+// read. Only the records that underOperand finds under operands join a
-// stdout may be the terminal prog is drawn on, so the lines go through
+// group, and a path for which exists returns false is left out of its
-// prog's writer. It returns what it wrote, with the totals for the
+// group. stdout may be the terminal prog is drawn on, so the lines go
-// summary line.
+// 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, exists func(path string) bool, prog *progress,
+	operands []string, stdout io.Writer, exists func(path string) bool,
+	prog *progress,
 ) (*reportWriter, error) {
 	out := prog.writer(stdout)
 	report := &reportWriter{out: out, exists: exists}
@@ -128,6 +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
@@ -159,7 +176,7 @@ 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
 
@@ -179,8 +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)
@@ -273,6 +288,47 @@ func pathExists(path string) bool {
 	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

@@ -102,7 +102,8 @@ func dupeGroups(t *testing.T, db *sql.DB) []dupeGroup {
 
 	var out bytes.Buffer
 
-	_, err := readReport(t.Context(), db, "", &out, everyPathExists, nil)
+	_, err := readReport(t.Context(), db, "", nil, &out, everyPathExists,
+		nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -148,6 +149,136 @@ 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) {
 	dir, recs := recsOnDisk(t, awkwardPairRecs())
 	t.Setenv(databaseEnv, seedDatabase(t, recs))
@@ -195,7 +326,7 @@ func TestReportStdoutFailsWhileReading(t *testing.T) {
 	// Counts the records read, and draws nothing.
 	prog := &progress{now: time.Now}
 
-	_, err = readReport(t.Context(), db, "", failingWriter{},
+	_, err = readReport(t.Context(), db, "", nil, failingWriter{},
 		everyPathExists, prog)
 	if !errors.Is(err, errWriteFailed) ||
 		!strings.HasPrefix(err.Error(), "write stdout: ") {
@@ -292,7 +423,8 @@ func TestReadReportOnTerminal(t *testing.T) {
 		waitingFor: countingRecords,
 	}, terminalWidth, time.Hour, time.Now)
 
-	_, err = readReport(t.Context(), db, "", os.Stderr, everyPathExists, p)
+	_, err = readReport(t.Context(), db, "", nil, os.Stderr,
+		everyPathExists, p)
 
 	p.finish()
 
@@ -514,7 +646,7 @@ func TestReadReportChecksOnlyPossibleDuplicates(t *testing.T) {
 		return true
 	}
 
-	_, err = readReport(t.Context(), db, "", io.Discard, exists, nil)
+	_, err = readReport(t.Context(), db, "", nil, io.Discard, exists, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -548,7 +680,8 @@ func TestReadReportDropsGroupLeftWithOnePath(t *testing.T) {
 
 	exists := func(path string) bool { return path != "/a/gone" }
 
-	report, err := readReport(t.Context(), db, "", io.Discard, exists, nil)
+	report, err := readReport(t.Context(), db, "", nil, io.Discard, exists,
+		nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -581,7 +714,7 @@ func TestReadReportKeepsGroupLeftWithTwoPaths(t *testing.T) {
 
 	var out bytes.Buffer
 
-	report, err := readReport(t.Context(), db, "", &out, exists, nil)
+	report, err := readReport(t.Context(), db, "", nil, &out, exists, nil)
 	if err != nil {
 		t.Fatal(err)
 	}

internal/sfdupes/scan.go

@@ -664,8 +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
+// already recorded). Each path of a run that fails to hash is warned
-// counted as skipped; stale records for its paths, if any, are deleted
+// 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.
@@ -706,9 +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
 		}
@@ -915,8 +918,7 @@ 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",
+		return fmt.Sprintf("scan: skipped %s (inside a .zfs directory)", root)
-			root)
 	}
 
 	var kind string
@@ -942,7 +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
@@ -965,7 +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,
 		})
 
@@ -1094,7 +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,
 		})
 
@@ -1153,7 +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,
 		})
 
@@ -1214,7 +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,
 		})
 
@@ -1235,7 +1251,7 @@ func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	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
+// walk reaches an unreadable directory. It also reaches a file and a
-// can be read but not searched. Another such file is an operand.
+// 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}) {
+	if st != (scanStats{skipped: 4}) {
-		t.Errorf("stats = %+v, want 3 skipped and nothing else", st)
+		t.Errorf("stats = %+v, want 4 skipped and nothing else", st)
 	}
 
 	got := stderr()
 	for _, warning := range []string{
-		"walk " + unreadable + ": open " + unreadable + ": permission denied\n",
+		"scan: skipped " + unreadable + " (permission denied)\n",
-		"stat " + walked + ": lstat " + walked + ": permission denied\n",
+		"scan: skipped " + walked + " (permission denied)\n",
-		"walk " + operand + ": lstat " + operand + ": 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)
@@ -1941,14 +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)
 	}
@@ -1964,6 +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
@@ -2001,7 +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)
 	}

internal/sfdupes/trees.go

@@ -37,12 +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
+// the paths of the records at or below the operands in args (of every
-// prints maximal duplicate-tree groups as TSV on stdout. Of the scanned
+// record when args is empty), computes a Merkle-style digest per
-// filesystem it only checks, with pathExists, that the root of each
+// directory, and prints maximal duplicate-tree groups as TSV on stdout.
-// duplicate tree is still there, never the files below it. It names the
+// Of the scanned filesystem it only checks, with pathExists, that the
-// database on stderr before it opens it. Any database problem,
+// root of each duplicate tree is still there, never the files below it;
-// including a missing database, is fatal.
+// 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
@@ -50,7 +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
@@ -79,7 +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()
 
@@ -101,7 +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()
 
@@ -112,16 +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,
+// reads every record in path order, counting each on prog. It adds the
-// counting each on prog. It returns the number of records read, the
+// records that underOperand finds under operands to the directory
-// super-root and every directory.
+// hierarchy. It returns the number of records read, the super-root and
-func readTree(ctx context.Context, db *sql.DB,
+// 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
@@ -131,7 +145,10 @@ func readTree(ctx context.Context, db *sql.DB,
 		records++
 
 		prog.increment()
+
+		if underOperand(r.path, operands) {
 			tree.add(r)
+		}
 	})
 	if err != nil {
 		return 0, nil, nil, err
@@ -139,29 +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)
+// go through prog's writer. It returns the number of rows (dupe trees).
-// and the bytes they could reclaim.
 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:] {
@@ -169,11 +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()
@@ -181,10 +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

internal/sfdupes/trees_test.go

@@ -214,6 +214,57 @@ func TestRunTreesEscapesPaths(t *testing.T) {
 	}
 }
 
+// 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.
@@ -313,7 +364,7 @@ func TestWriteTreeGroupsOnTerminal(t *testing.T) {
 		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()
 
@@ -587,3 +638,64 @@ func TestCollectTreeGroupsKeepsGroupLeftWithTwoCopies(t *testing.T) {
 		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)
+	}
+}