Rewrite README.md sentence by sentence for a newcomer

The old README was one dense specification: long sentences, lists
chained with commas, exceptions ahead of the rules, and release
history in the user text. This rewrite leads with what the program is
and how to use it, then gives a reference for the settings, each
subcommand, the database, the output formats, the progress display
and the exit codes. Non-goals come before the reference; the
constraints, the entrypoints and the build come last. Every statement
was checked against the code on next and against what the built
binary prints on small trees. The section names the code comments
cite are kept. Five comments in main_test.go cite Settings, and one
each in scan_test.go, trees_test.go and exclude_test.go cites what
the README now says. TODO.md records the step.

Model: fable-5-1

README.md

@@ -38,11 +38,9 @@ export SFDUPES_DATABASE="$HOME/.local/share/sfdupes/db.sqlite"
 `scan` walks the trees named by its operands, the paths given after `scan` on
 the command line. The walk is the listing of every directory under them. `scan`
 keeps one database record per regular file. A regular file is an ordinary file,
-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.
+as opposed to a directory, a symlink or a device. `report` prints the file-level
+duplicates. `trees` prints the duplicate directory trees. `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.
@@ -69,12 +67,11 @@ The binary goes to `$(go env GOPATH)/bin`, or to `$GOBIN` when that is set.
 
 `sfdupes --version` prints `sfdupes VERSION` and exits. For a binary from
 `go install`, `VERSION` is the version Go recorded in it. For `@latest` that is
-a release tag such as `v0.0.3`. For `@main` it is the commit's tag when it has
-one. Otherwise it is a pseudo-version, Go's version string for an untagged
-commit. That string ends in the commit's hash. A binary built from a clone
-carries the output of `git describe --tags --always --dirty` in that clone, such
-as `v0.0.3` on a tagged commit. `VERSION` is `dev` when no version is known. To
-see everything Go recorded in an installed binary:
+a release tag such as `v0.0.3`. For `@main` it is a pseudo-version, Go's version
+string for an untagged commit. That string ends in the commit's hash. A binary
+built from a clone carries the version `git describe --tags --always --dirty`
+prints in that clone, such as `v0.0.3` on a tagged commit. `VERSION` is `dev`
+when no version is known. To see everything Go recorded in an installed binary:
 
 ```sh
 go version -m "$(command -v sfdupes)"
@@ -108,10 +105,9 @@ docker run --rm -v /var/lib/sfdupes:/var/lib/sfdupes sfdupes report > dupes.tsv
 
 ### Daily scan from `cron`
 
-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`:
+Run `scan` as root, so that it can read every file. A path it cannot read is
+skipped with a warning and loses its database record. 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
@@ -127,9 +123,8 @@ walk"). As the file `/etc/cron.d/sfdupes`:
     - 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 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`;
+    - a line for each path left out because its filesystem is one the kernel
+      makes up or keeps in memory, such as `tmpfs`;
     - the summary line;
     - a warning when a report still has the database open as the scan ends (see
       "Concurrent reads").
@@ -139,8 +134,8 @@ walk"). As the file `/etc/cron.d/sfdupes`:
 - A scan that fails exits 1, with the reason among the last lines of the log.
   `tail` prints those lines, and `cron` mails them to root if the host can send
   mail.
-- A running scan carries on when the next one starts. The new one fails at once,
-  and the lines that `cron` mails include
+- A scan still running when the next one starts carries on. The new one fails at
+  once, and the lines `cron` mails include
   `sfdupes: another scan is running (lock held on /var/lib/sfdupes/db.sqlite.lock)`.
 - `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.
@@ -158,8 +153,8 @@ 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 the files as they were at the last scan. A file may have
+  changed or gone since.
 - 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
@@ -206,9 +201,9 @@ two ends and at 1 MiB per gigabyte. A full read of a small file is cheap, and it
 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.
+The hashes live in a database that is kept between runs. A rescan reads only the
+files that are new or whose size or modification time (mtime) changed. All
+analysis then works from the database alone.
 
 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
@@ -229,9 +224,10 @@ In order:
    its ends and at 1 MiB per gigabyte. No file of 100 MiB or more is ever read
    in full. The target is a filesystem of about ten million files and 150 TB, on
    disks that may be slow or busy. During a scan, one small record per file is
-   held in memory. The hashes stay in the database. `report` holds the duplicate
-   paths of one file size at a time. `trees` holds one entry per directory, plus
-   the files of the directories it is reading.
+   held in memory. It holds the path and the two values the walk compares, the
+   size and the mtime. The hashes stay in the database. `report` holds the
+   duplicate paths of one file size at a time. `trees` holds one entry per
+   directory, plus 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
@@ -265,14 +261,11 @@ The reference below is also the program's specification.
 
 ## Non-goals
 
-- No byte-for-byte compare.
-- No deleting of duplicates.
-- No replacing a copy with a hard link to another copy. The reports are
-  advisory. Acting on them is your job.
+- No byte-for-byte compare, and no deleting or linking of duplicates. The
+  reports are advisory. Acting on them is your job.
 - The SQLite database is the only stored data. There are no export or import
   formats.
-- `sfdupes` does not run in the background or watch the filesystem for changes.
-  Scheduling rescans is `cron`'s job.
+- No daemon or filesystem watcher. Scheduling rescans is `cron`'s job.
 
 ## Reference
 
@@ -287,9 +280,9 @@ The reference below is also the program's specification.
   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.
+- `-x`, `--one-file-system` (`scan`) keeps the walk of each operand on that
+  operand's filesystem, 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").
@@ -350,15 +343,14 @@ default. An operand may be a directory or a regular file.
   the exit code is 2.
 - An operand that does not exist is a fatal error. `scan` then stops before it
   walks anything, and exits 1.
-- Each operand is made absolute before the walk. A relative operand is joined to
-  the working directory as `pwd` prints it, with its symlinks kept. A `.` or
-  `..` in it is resolved as text, and symlinks are not followed. Each record is
-  stored under that absolute path.
+- Each operand is made absolute before the walk. A `.` or `..` in it is resolved
+  as text, and symlinks are not followed. Each record is stored under that
+  absolute path, so results do not depend on the working directory.
 - All operands belong to one scan, and one walk covers them all at once (see
   "Concurrency").
-- An operand that repeats another is dropped before the walk.
+- An operand that repeats another is dropped before the walk, without a line.
 - An operand under another operand is dropped the same way, unless one of the
-  cases below applies. No path is then walked twice.
+  cases below applies. No file is then reached twice.
 - An operand under another is kept, and walked on its own, when the inner
   operand or a directory between the two is one of these:
     - a default exclude (see "Rules for the walk");
@@ -367,21 +359,19 @@ default. An operand may be a directory or a regular file.
     It is also kept when a directory between the two is on a filesystem that is
     not persistent. Such a filesystem is one the kernel makes up or keeps in
     memory (see "Rules for the walk"). The outer operand's walk does not enter
-    such a directory, so the inner operand is walked on its own.
+    such a directory, so the inner operand is walked on its own. For an inner
+    operand that is a regular file, the check starts at its directory.
 
 - No other case keeps an operand under another. An operand reached through a
-  symlinked directory below the outer operand is dropped too. The outer
-  operand's walk does not follow the symlink, so no file under the inner operand
-  is scanned. The records that an earlier scan left under it are deleted. The
-  `-x` case above also covers an operand reached that way.
+  symlinked directory below the outer operand is dropped too, without a line.
+  The outer operand's walk does not follow the symlink, so no file under the
+  inner operand is scanned. The records an earlier scan left under it are
+  deleted.
 - An operand that is neither a regular file nor a directory is not scanned.
   `scan` prints a warning naming the path and what it is. It counts the operand
-  as skipped and drops it. An operand beneath a symlink operand is scanned all
-  the same. The scan keeps the records beneath a dropped operand, unless another
-  operand covers them. After a `cd` through a symlink, `scan .` names the
-  symlink and is skipped as a symlink operand. `scan sub` in that directory
-  records its files under the symlink's path. `report` can then list each as a
-  duplicate of the same file recorded under its real path.
+  as skipped and drops it. An operand beneath a dropped one is scanned all the
+  same. The scan keeps the records beneath a dropped operand, unless another
+  operand covers them.
 - An operand at or inside a directory named `.zfs` is never scanned, whether
   given alone or under another operand. The check looks at the path as given and
   at the path with its symlinks resolved. `scan` prints a warning naming the
@@ -391,9 +381,6 @@ default. An operand may be a directory or a regular file.
 - An operand on a filesystem that is not persistent is dropped too. `scan`
   prints a line naming it and its filesystem type, and does not count it as
   skipped.
-- `scan` prints these warnings, and the line for an operand that is not on a
-  persistent filesystem, even when the operand repeats another or lies under
-  another.
 - A scan whose every operand is dropped walks nothing and exits 0.
 
 #### What a scan does to the database
@@ -405,16 +392,15 @@ operands:
 - 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.
-- `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
+- A file whose size differs, or whose mtime is newer, is hashed again. 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.
 - 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
-  opening it. So after a complete scan, the database holds only what that scan
-  could verify under its operands. A later successful scan adds such files
-  again.
+  opening it. So the database holds only what the latest scan over each path
+  could verify. A later successful scan adds such files again.
 - Records outside the scanned operands are left alone, so separate trees can be
   scanned on different schedules into one database.
 - The scan keeps every record at or below a path that the walk leaves out for
@@ -425,8 +411,9 @@ operands:
 
     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
+    the same database. An operand may lie at or inside a default exclude, or,
+    with `-x`, at or inside a directory on another filesystem. Such an operand
+    is walked on its own. Its walk verifies the records below it, so they are
     deleted when their files are gone.
 
 #### Phases
@@ -437,13 +424,12 @@ phases.
 1. `load` reads the existing records. Each record under the operands is kept in
    memory by path, with its size and mtime, for the walk to compare against.
 2. `walk` lists the trees under all operands. Several workers share one queue of
-   directories to read. A worker is one thread of the program. A thread is one
-   of several parts of a program that run at the same time and share its memory.
-   "Concurrency" says how many workers there are. Each worker reads one
-   directory at a time. It hands the subdirectories it finds back to the queue,
-   and it stats each regular file as it finds it. The walk compares each file
-   with its record. An unchanged file needs nothing more. Every new or changed
-   file is passed to the hash phase.
+   directories to read. A worker is one thread of the program, and "Concurrency"
+   says how many there are. Each worker reads one directory at a time. It hands
+   the subdirectories it finds back to the queue, and it stats each regular file
+   as it finds it. The walk compares each file with its record. An unchanged
+   file needs nothing more. Every new or changed file is passed to the hash
+   phase.
 3. `hash` reads every new or changed file and computes its three hashes. Files
    are read in inode order. An inode is the filesystem's own record of a file.
    Files with nearby inode numbers usually lie near each other on disk. Reading
@@ -462,7 +448,7 @@ walk practical on large or busy storage pools.
 #### Rules for the walk
 
 - Only regular files are recorded. Every other kind of file, such as a symlink
-  or a device node, is passed over. The walk follows no symlink it finds.
+  or a device node, is passed over. Symlinks are never followed.
 - The walk never enters a directory named `.zfs`. Such a directory holds the
   filesystem's ZFS snapshots, which are its saved earlier states. Walking it
   would list every file once per snapshot. `scan` never starts a walk at or
@@ -480,14 +466,14 @@ walk practical on large or busy storage pools.
     operand at the path given, even when that path runs through a symlinked
     directory. Such an operand can lead to the database files, and the walk
     passes over them there as well. The walk prints no line for the database
-    files and does not count them as skipped. A record that an earlier scan left
-    for one of them is deleted.
+    files and does not count them as skipped. A record an earlier scan left for
+    one of them is deleted.
 
 - By default the walk enters directories on other filesystems than its
   operand's. With `-x`, it never enters a directory on a different filesystem
-  than its operand. `scan` prints no line for a directory that only `-x` leaves
-  out, and does not count it as skipped. The records below it are kept, as
-  described under "What a scan does to the database".
+  than its operand. `scan` prints no line for that directory and does not count
+  it as skipped. The records below it are kept, as described under "What a scan
+  does to the database".
 - The walk never records anything on a filesystem that is not persistent. A
   filesystem is not persistent when the kernel makes it up or keeps it in
   memory. These are the types, as `sfdupes scan --help` lists them on Linux and
@@ -523,10 +509,11 @@ walk practical on large or busy storage pools.
     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:
+- Below an operand, the walk does not enter a default exclude. The default
+  excludes hold the kernel's view of the running system and temporary files, so
+  that `sfdupes scan /` walks only the files worth comparing. They depend on the
+  system, and `sfdupes scan --help` lists those of the system it runs on. On
+  Linux they are:
 
     ```
     /proc, /sys, /dev, /run, /tmp, /var/tmp
@@ -555,27 +542,21 @@ walk practical on large or busy storage pools.
     scan: skipped /var/tmp (default exclude)
     ```
 
-    The directory is not counted as skipped. `scan` prints the filesystem line
-    instead for a default exclude that is not on a persistent filesystem, such
-    as `/proc`. One that is itself an operand gets no line, because it is walked
-    as that operand. The scan keeps the records below a default exclude that the
+    The directory is not counted as skipped. A default exclude on a filesystem
+    that is not persistent, such as `/proc`, gets the line for its filesystem
+    instead. One that is itself an operand gets no line, because it is walked as
+    that operand. The scan keeps the records below a default exclude that the
     walk did not enter, as described under "What a scan does to the database".
 
-- `scan` skips each of these, with a one-line warning on stderr:
-    - a directory it cannot read;
-    - a path it cannot stat;
-    - a new or changed file it cannot read.
-
-    An unchanged file is not opened, so it keeps its record even when it has
-    become unreadable. The scan continues, and the summary line says how many
-    paths were skipped. A skipped path that has a record from an earlier scan
-    loses that record, and so does every path under an unreadable directory.
-
+- A path that cannot be read, for lack of permission or any other error, gets a
+  one-line warning on stderr and is skipped. The scan continues, and the summary
+  line says how many paths were skipped. A skipped path that has a record from
+  an earlier scan loses that record, and so does every path under an unreadable
+  directory. The database mirrors what the latest scan could verify.
 - A file or directory that disappears while the scan runs is not an error. The
   scan prints no line for it and does not count it as skipped. A path gone when
   the walk reaches it gets no record, and so does a file gone when the hash
-  phase opens it. The records that an earlier scan left at or below it are
-  deleted.
+  phase opens it. The records an earlier scan left at or below it are deleted.
 
 #### Concurrency
 
@@ -593,8 +574,8 @@ from a thread of its own and never blocks the workers.
 
 - one line naming the database (see "Database");
 - for each dropped operand, in operand order, the warning or the line described
-  under "Operands". `scan` prints neither for an operand it drops only because
-  it repeats another or lies under another;
+  under "Operands". An operand dropped as a repeat of another, or as lying under
+  another, gets neither;
 - 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;
@@ -621,14 +602,13 @@ signal `kill` sends by default:
 - The records hashed but not yet written are written, so the next scan does not
   hash them again.
 - Nothing else is written or deleted. Records are deleted only after a complete
-  walk. So the records under paths that an interrupted walk never reached are
-  kept.
-- The database is closed and the lock released.
+  walk. So the records under paths an interrupted walk never reached are kept.
+- The database is closed and the lock released, as on any other exit.
 - The line `scan: interrupted after N files` goes to stderr. `N` is the number
   of files the walk reached. The exit code is 1.
 
-The next scan reads none of the unchanged files already recorded, and brings the
-database up to date as usual.
+The next scan reads none of the files already recorded and brings the database
+up to date as usual.
 
 After the first signal `scan` stops catching signals. A second one ends it at
 once, as any uncaught signal does. The records not yet written are lost, and the
@@ -687,8 +667,8 @@ Four bytes are written as escapes:
 
 Every other byte is written unchanged, including bytes that are not valid UTF-8,
 the usual text encoding. Undoing the four escapes gives back the stored path.
-Grouping and ordering use the stored path. The warnings about skipped paths that
-`scan` prints are escaped the same way, so each stays one line.
+Grouping and ordering use the stored path. The warnings `scan` prints are
+escaped the same way, so each warning stays one line.
 
 The summary line on stderr gives:
 
@@ -726,11 +706,11 @@ Processing:
 - `trees` builds the directory tree and computes every directory's digest. It
   then groups the directories by digest. Every group with two or more
   directories is a duplicate-tree group.
-- `trees` leaves out a group that a group of its members' parents already
-  implies. A group is left out when its members' parents are distinct
-  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.
+- Only the outermost duplicate trees are reported. A group is left out when its
+  members' parents are distinct directories that all have the same digest. The
+  parents' group, or an ancestor's, already implies it. A group holding sibling
+  directories, or members whose parents have different digests, is always
+  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.
@@ -742,8 +722,8 @@ Two limitations are accepted:
   They share storage, so removing one frees nothing. The scan uses the inode to
   avoid reading a file twice, and does not store it.
 - Only the regular files in the database define a tree. Empty directories are
-  invisible. When the scan skips a file in one copy but not in the other, the
-  two trees differ.
+  invisible. A file skipped during the scan in one copy but not in the other
+  makes two otherwise identical trees differ.
 
 `trees` has four phases, each with its own progress display on stderr (see
 "Progress"). `open` opens the database. `read` counts the records, then reads
@@ -820,7 +800,7 @@ All four subcommands work on one SQLite database file.
 
 #### Location
 
-The database is the file named by `SFDUPES_DATABASE`, when that is set and not
+The database is the file `SFDUPES_DATABASE` names, when that is set and not
 empty, and otherwise `/var/lib/sfdupes/db.sqlite`.
 
 Each subcommand prints one line on stderr naming the database it uses, before
@@ -854,21 +834,20 @@ the error for a missing database comes after it.
 
 #### The scan lock
 
-Only one `scan` at a time can hold the lock.
+Only one `scan` runs against a database at a time.
 
 - 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.
+  lock is taken with the `flock(2)` system call, 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`. Every path to
+  one database therefore 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
   at the database path is followed, and its target is created.
-- A second `scan` that finds the lock held does not wait. It fails at once with
-  an error naming the lock file, and exits 1. It has walked nothing and has not
-  opened the database. The running scan carries on.
+- A second `scan` against the same database does not wait. It fails at once with
+  a one-line error naming the lock file, and exits 1. It has walked nothing and
+  opened nothing. The running scan carries on.
 - The lock file is created on first use and never deleted. Only its owner may
   open it. A leftover file blocks nothing. The lock ends with the process that
   holds it, however that process ends. Deleting the file while a scan runs would
@@ -881,9 +860,8 @@ Only one `scan` at a time can hold the lock.
   SQLite appends changes to a `-wal` file beside the database, and keeps a
   lookup table for that file in a `-shm` file. Readers do not block the writer,
   so running a report during a scan is safe.
-- Every subcommand opens the database with a busy timeout of 10 seconds. That is
-  how long it waits for a database another process has locked before it gives
-  up.
+- Every connection has a busy timeout of 10 seconds. That is how long it waits
+  for a database another connection has locked before it gives up.
 - The filesystem is authoritative, and the database follows it. Hashed records
   are written 10,000 at a time while the scan runs, so a report may see a scan's
   changes partly applied. A scan that dies partway leaves a valid database
@@ -921,15 +899,13 @@ Only one `scan` at a time can hold the lock.
 #### Schema
 
 The schema is the database's layout, its table and its index.
-`PRAGMA user_version` holds the schema version, currently 1. An empty database
-is at version 0, and `scan` sets it up. Any other version is a fatal error.
-`report`, `trees` and `info` set nothing up, so for them version 0 is a fatal
-error too. `scan` creates the table and the index and sets the version in one
-transaction (see "`info` mode"). So the three land together or not at all. A
-first scan stopped while doing so leaves an empty database, which the next scan
-sets up. A database at version 0 that already has a `files` table was not made
-by `sfdupes`. Every subcommand refuses it, with an error telling you to remove
-the file and rescan.
+`PRAGMA user_version` holds the schema version, currently 1. A database with any
+other version is a fatal error. `scan` creates the table and the index and sets
+the version in one transaction (see "`info` mode"). So the three land together
+or not at all. A first scan stopped while doing so leaves an empty database,
+which the next scan sets up. A database at version 0 that already has a `files`
+table was not made by `sfdupes`. Every subcommand refuses it, with an error
+telling you to remove the file and rescan.
 
 ```sql
 CREATE TABLE files (
@@ -959,9 +935,9 @@ CREATE INDEX files_signature ON files (size, head, tail, content);
 
 ### Progress
 
-Each phase of every subcommand gets a display of its own on stderr from the
-moment it starts. The display names the phase and shows its progress. The parts
-of its line are listed below.
+Every subcommand always shows that it is working. Each phase gets a display of
+its own on stderr from the moment it starts. The display names the phase and
+shows its progress. The parts of its line are listed below.
 
 The phases, and what each display counts:
 
@@ -984,7 +960,7 @@ The phases, and what each display counts:
 - `trees` has four phases:
     - `[1/4] open` and `[2/4] read` are as for `report`, except that `read`
       writes nothing.
-    - `[3/4] group` has the directories that `read` found as its total.
+    - `[3/4] group` has the directories `read` found as its total.
     - `[4/4] write` has the duplicate-tree groups as its total.
 - `info` has two phases:
     - `[1/2] open` is as for `report`.
@@ -1022,7 +998,7 @@ phase waits, in place of the rate. That happens while one large file is read, or
 while a disk or the database is slow. Each phase names the places it may wait
 in, and a phase that may wait in two places names both, as `disk/db`:
 
-- `walk` says `disk`, because it reads directories and stats files.
+- `walk` says `disk`, for files.
 - `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`.
@@ -1050,18 +1026,19 @@ On a terminal:
     ⠹ [3/4] hash: [12345/98765], 92 files/s, elapsed 2m32s, remain 17m44s            12% |█         |
     ```
 
-- 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
-  the rest does not fit on its own. The rest is:
+- No line reaches the terminal's last column, however narrow the terminal is. 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 the rest does not fit on its own. The
+  rest is:
     - the phase and its place;
     - the progress, when the phase shows one;
     - the elapsed time;
     - the time remaining.
 
-    On a terminal 80 columns wide, no line is cut short, even hours into a scan
-    of millions of files. A terminal made narrower during a phase is fitted at
-    the next draw.
+    That happens hours into a scan of millions of files, on a terminal narrower
+    than about 75 columns. A terminal made narrower while a phase runs is fitted
+    at the next draw.
 
 - A warning printed during a phase lands on a line of its own, never inside the
   display. So does every line written to stdout when stdout and stderr are the
@@ -1077,8 +1054,8 @@ Each phase prints its line:
 - every 5 seconds while it runs, whether or not an item has completed;
 - once more as it ends.
 
-When a phase stops short of its total, as an interrupted one does, it keeps
-showing the count it stopped at.
+A phase that stops short of its total, as an interrupted one does, keeps showing
+the count it stopped at.
 
 ### Error handling and exit codes
 
@@ -1087,14 +1064,12 @@ showing the count it stopped at.
   error is one of these:
     - an operand that does not exist, or whose details `scan` cannot read;
     - 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
-      opened;
-    - a read or write of the database that fails;
+    - another `scan` already running against the same database;
+    - a database that cannot be created or opened, or whose reads or writes
+      fail, its directory and its lock file included;
     - a relative database path when the working directory cannot be read;
-    - a database at a schema version other than 0 or 1;
-    - a database at version 0 read by `report`, `trees` or `info`;
-    - a database at version 0 that already has a `files` table (see "Schema");
+    - a database with another schema version, or one at version 0 that already
+      has a `files` table;
     - a record with a hash that is not 32 bytes, read by `report` or `trees`;
     - a missing database for `report`, `trees` or `info`;
     - a failed write to stdout.
@@ -1107,9 +1082,9 @@ showing the count it stopped at.
     - `--workers` below 1;
     - `report`, `trees` or `info` 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
-`sfdupes` as a failed write:
+A fatal error is reported in one line on stderr that starts with `sfdupes:`. A
+failed write to stdout, such as a full disk, is reported the same way. Two cases
+never reach `sfdupes` as a failed write:
 
 - The reader of a stdout pipe may exit early, as `head` does in
   `sfdupes report | head`. The next write then ends `sfdupes` with `SIGPIPE`,
@@ -1143,8 +1118,8 @@ the repository root itself, so it can be run from any directory.
     - An installed `node` is used when its major version is 22. A `yarn` already
       on `PATH` is then used as it is. When `yarn` is missing, `yarn` 1.22.22 is
       activated through `corepack`, the `yarn` installer that ships with `node`.
-      When there is no `corepack` either, the script installs `yarn` with `npm`,
-      the package manager that ships with `node`.
+      When there is no `corepack` either, `npm`, the package manager that ships
+      with `node`, installs it.
     - Otherwise the script installs `node` 22.17.0 through `nvm`, a `node`
       version manager, and activates `yarn` 1.22.22 under that `node` through
       `corepack`. When `nvm` is missing, the script first downloads it and
@@ -1174,9 +1149,9 @@ the repository root itself, so it can be run from any directory.
 - `script/lint` runs the linter, `golangci-lint`, which checks the Go sources
   for common mistakes. It builds only the `lint` stage of the `Dockerfile` and
   writes no image. The stage starts from the `golangci/golangci-lint` image,
-  pinned by its digest, and runs these checks as build steps:
-    - the `gofmt` check, which fails when `gofmt`, Go's own formatter, would
-      change a Go source;
+  pinned by its digest, and runs three build steps:
+    - the `gofmt` check, which fails on a Go source that `gofmt`, Go's own
+      formatter, would change;
     - `golangci-lint config verify`, which checks the linter's settings file
       `.golangci.yml`;
     - `golangci-lint run`, which lints the Go sources.
@@ -1188,8 +1163,8 @@ the repository root itself, so it can be run from any directory.
   with `prettier`, in place. `prettier` indents by 4 spaces and wraps prose, as
   `.prettierrc` sets and as the script also passes on the command line. It wraps
   at 80 columns, its default width. Both run on the host. `prettier` runs
-  through `yarn` at the version that `yarn.lock` pins. When the `node` on `PATH`
-  is not `node` 22, the script loads the `node` 22.17.0 that `script/bootstrap`
+  through `yarn` at the version `yarn.lock` pins. When the `node` on `PATH` is
+  not `node` 22, the script loads the `node` 22.17.0 that `script/bootstrap`
   installed.
 - `script/fmt-check` is the read-only form of `script/fmt`. It names any
   unformatted file and exits non-zero instead of writing. `gofmt` and `prettier`
@@ -1210,8 +1185,8 @@ the repository root itself, so it can be run from any directory.
   every check passed. The tests and the linter run twice, once in `script/check`
   and again as stages of the image build.
 - `script/precommit` runs from the `git` pre-commit hook, a script that `git`
-  runs before each commit. It runs `go mod tidy`, and fails the commit when that
-  changes `go.mod` or `go.sum`. Then it runs `script/check`.
+  runs before each commit. It fails the commit when `go mod tidy` would change
+  anything. Then it runs `script/check`.
 - `script/install-precommit` installs `.git/hooks/pre-commit`, which runs
   `script/precommit`.
 
@@ -1221,9 +1196,9 @@ install system packages and download the Go modules. Every step that checks or
 compiles the code comes after it. Every `RUN` step after it runs again on each
 build. Without that, Docker would serve the test and lint steps from its cache
 on an unchanged tree, and the build would pass without running them. The steps
-above `ARG CACHEBUST` stay cached until a change to something they depend on,
-such as `go.mod`. Go's build cache is kept between builds, so only what changed
-is compiled again. The tests run with `-count=1`, so no test reports a stored
+above `ARG CACHEBUST` stay cached until something they depend on changes, such
+as `go.mod`. Go's build cache is kept between builds, so only what changed is
+compiled again. The tests run with `-count=1`, so no test reports a stored
 result. `golangci-lint run` keeps the linter's own cache as well, so an
 unchanged package is not linted again.
 
@@ -1283,7 +1258,6 @@ All of the following, run in this directory, must pass:
    data.
 
     ```sh
-    make build
     d=$(mktemp -d /var/tmp/sfdupes.XXXXXX)
     export SFDUPES_DATABASE="$(mktemp -d /var/tmp/sfdupes.XXXXXX)/db.sqlite"
     mkdir -p "$d/a" "$d/b"

main_test.go

@@ -773,8 +773,8 @@ func TestBuildVersion(t *testing.T) {
 func TestRunVersionWriteFailureIsFatal(t *testing.T) {
 	t.Parallel()
 
-	// README §Error handling: a failed write to stdout is a fatal error,
-	// exit 1, reported on stderr starting with "sfdupes:".
+	// README §Error handling: a stdout write failure exits 1, reported
+	// in one line on stderr.
 	var stderr bytes.Buffer
 
 	code := run([]string{"--version"}, failingWriter{}, &stderr)
@@ -1279,8 +1279,9 @@ func holdScanLock(t *testing.T, path string) {
 }
 
 func TestRunSecondScanFails(t *testing.T) {
-	// README §Database: a second scan that finds the lock held fails
-	// at once, naming the lock file, without opening the database.
+	// README §Database: while one scan holds the lock, a second scan
+	// fails at once, naming the lock file, without opening the
+	// database.
 	path := testDBPath(t)
 	t.Setenv(databaseEnv, path)
 
@@ -1339,7 +1340,7 @@ func assertSecondScanFails(t *testing.T, path string) {
 }
 
 func TestRunSecondScanThroughSymlinkFails(t *testing.T) {
-	// README §Database: a path through a symlink names the same lock
+	// README §Database: every path to one database names the same lock
 	// file, so while a scan holds it through one path, a scan through
 	// another fails, naming the lock file beside the database's real
 	// path.
@@ -1444,8 +1445,8 @@ func TestRunReportsDuringScan(t *testing.T) {
 }
 
 func TestRunStdoutClosedIsFatal(t *testing.T) {
-	// README §Error handling: a failed write to stdout is a fatal error,
-	// exit 1, reported on stderr starting with "sfdupes:".
+	// README §Error handling: a stdout write failure exits 1, reported
+	// in one line on stderr.
 	for _, name := range []string{cmdReport, cmdTrees, cmdInfo} {
 		t.Run(name, func(t *testing.T) {
 			t.Setenv(databaseEnv, testDBPath(t))