Say in --help where the database is and what each report prints (closes #177)

The help of sfdupes and of scan names the database, its default path
and SFDUPES_DATABASE. The help of report and trees lists the columns,
the order of the rows and the four escapes. The help of info lists its
lines. The help of dwim says that it scans /, should run as root and
keeps the scan in the database.

The README's opening drops its sentence on copies deleted after the
scan, an edge case that its report and trees sections already cover.

Model: opus-5-5

README.md

@@ -22,9 +22,8 @@ records the files it finds in an SQLite database. SQLite keeps a whole database
 in one file. The database is kept between runs. A rescan reads only new and
 changed files. That makes `scan` cheap enough to run every day from `cron`, the
 system's job scheduler. `report`, `trees` and `info` work from the database and
-can run at any time. `report` and `trees` leave out a copy that was deleted
+can run at any time. `dwim` scans `/` and then prints the `trees` report, for a
-after the scan. `dwim` scans `/` and then prints the `trees` report, for a first
+first look at a new machine with one command.
-look at a new machine with one command.
 
 ## Getting Started
 
@@ -259,7 +258,7 @@ In order:
   report.
 
 ```
-sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...
+sfdupes scan [--workers N] [-x] [--no-default-excludes] [--exclude PATH]... PATH...
 sfdupes report > dupes.tsv
 sfdupes trees > dupetrees.tsv
 sfdupes info
@@ -304,12 +303,24 @@ The reference below is also the program's specification.
 - `--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").
+- `--exclude PATH` (`scan`) keeps the walk out of `PATH` and everything below
+  it. `PATH` may be a directory or a file. The flag may be given more than once.
+  The walk knows `PATH` by its device ID and inode number, as it knows the
+  database files (see "Rules for the walk"). So the walk leaves `PATH` out
+  whatever path leads to it. That includes a hard link to a file given as
+  `PATH`. A symlink given as `PATH` excludes its target. An empty `PATH`, as in
+  `--exclude ""`, is a usage error, reported in one line on stderr with exit 2
+  before anything is scanned. A `PATH` that does not exist, or whose real path
+  `scan` cannot find, is a fatal error. So is an operand at or below one (see
+  "Operands").
 - `--version`, `-v` prints `sfdupes VERSION` (see "Install") to stdout and
   exits 0. It writes nothing to stderr.
 - `--help`, `-h`, alone or after a subcommand, prints the help text to stderr
-  and exits 0. It writes nothing to stdout. `sfdupes scan --help` also lists the
+  and exits 0. It writes nothing to stdout. The help of `sfdupes` and of `scan`
-  default excludes of the system it runs on, and the filesystem types the walk
+  names the database. The help of `report`, `trees`, `info` and `dwim` says what
-  never records anything on.
+  each prints. `sfdupes scan --help` also lists the default excludes of the
+  system it runs on, and the filesystem types the walk never records anything
+  on.
 
 ### Duplicate detection
 
@@ -362,6 +373,10 @@ default. An operand may be a directory or a regular file.
 - An operand that does not exist is a fatal error, and so is a symlink whose
   target does not exist. `scan` then stops before it walks anything, and exits
   with code 1.
+- An operand at or below an `--exclude` path is a fatal error too. The message
+  names the operand and the `--exclude` path. `scan` compares the operand and
+  each directory above it with the `--exclude` paths by device ID and inode
+  number, not by name.
 - Each operand is resolved to its real path before the walk, as `realpath`
   prints it. A relative operand is joined to the working directory as `pwd -P`
   prints it. Every symlink in the path is then followed. A `..` after a symlink
@@ -437,15 +452,16 @@ operands:
   scanned on different schedules into one database.
 - The scan keeps every record at or below a path that the walk leaves out for
   one of these reasons:
+    - it is an `--exclude` path;
     - it is a default exclude;
     - it is not on a persistent filesystem;
     - with `-x`, it is a directory on another filesystem than its operand.
 
     Such records count as outside the scanned operands. So with `-x`, a
     filesystem mounted under an operand can be scanned on its own schedule into
-    the same database. An operand may itself lie at or inside such a path, as
+    the same database. An operand may itself lie at or inside such a path, other
-    "Operands" describes. Its walk verifies the records below it, so they are
+    than an `--exclude` path (see "Operands"). Its walk verifies the records
-    deleted when their files are gone.
+    below it, so they are deleted when their files are gone.
 
 #### Phases
 
@@ -544,6 +560,17 @@ 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".
 
+- The walk never enters or records an `--exclude` path, nor anything below it.
+  For each one it reaches, `scan` prints a line on stderr naming it:
+
+    ```
+    scan: skipped /srv/cache (--exclude)
+    ```
+
+    The path is not counted as skipped. The scan keeps the records at and below
+    it, as described under "What a scan does to the database".
+    `--no-default-excludes` does not change this rule.
+
 - Below an operand, the walk does not enter a default exclude. On Linux, the
   default excludes hold temporary files and the kernel's view of the running
   system. They depend on the system, and `sfdupes scan --help` lists those of
@@ -618,7 +645,8 @@ from a thread of its own and never blocks the workers.
   it repeats another or lies under another;
 - the progress display of each phase (see "Progress"). `scan` prints each
   warning for a skipped path as it happens. It prints a line the same way for
-  each path left out as a default exclude or as not on a persistent filesystem;
+  each path left out as an `--exclude` path, as a default exclude or as not on a
+  persistent filesystem;
 - the summary line;
 - a warning when a report still has the database open as the scan ends (see
   "Concurrent reads").
@@ -1199,6 +1227,9 @@ showing the count it stopped at.
 - `1`: a fatal error, a `scan` stopped by `SIGINT` or `SIGTERM`, or a `dwim`
   stopped by either during its scan. A fatal error is one of these:
     - an operand that does not exist, or whose real path `scan` cannot find;
+    - an `--exclude` path that does not exist, or whose real path `scan` cannot
+      find;
+    - an operand at or below an `--exclude` path;
     - a list of mounted filesystems that `scan` cannot read;
     - another `scan` holding the lock (see "The scan lock");
     - a database, its directory or its lock file that cannot be created or
@@ -1218,6 +1249,7 @@ showing the count it stopped at.
       `--workers abc`;
     - `scan` with no operand;
     - `--workers` below 1;
+    - an empty `--exclude` value, as in `--exclude ""`;
     - `report`, `trees`, `info` or `dwim` with a positional argument.
 
 A fatal error is reported on stderr, starting with `sfdupes:`. A failed write to

TODO.md

@@ -29,6 +29,16 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- The help of `sfdupes` and of `scan` names the database, its default path and
+  `SFDUPES_DATABASE`. The help of `report` and `trees` lists the columns, the
+  order of the rows and the escapes. The help of `info` lists its lines. The
+  help of `dwim` says that it scans `/`, should run as root and keeps the scan
+  in the database (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/177)
+- New `scan` flag `--exclude PATH`, which may be repeated. The walk neither
+  enters nor records an excluded path, and keeps the records below it. An
+  operand at or below one is a fatal error (2026-11-11,
+  https://git.eeqj.de/sneak/sfdupes/issues/163)
+
 - `report` and `trees` leave out a copy that was deleted after the scan. Each
   checks only the copy itself with `lstat`: the file, or the directory at the
   top of a tree. A group left with one copy is not reported. A missing `scan`

internal/sfdupes/cancel_test.go

@@ -359,7 +359,7 @@ func TestRunScanInterrupted(t *testing.T) {
 	dir := buildSmokeTree(t)
 
 	err := runScan(t.Context(), []string{dir}, walkCancelWorkers, false,
-		false)
+		false, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -383,7 +383,7 @@ func TestRunScanInterrupted(t *testing.T) {
 		defer close(done)
 
 		err = runScan(cancelledContext(t), []string{dir}, walkCancelWorkers,
-			false, false)
+			false, false, nil)
 	}()
 
 	awaitReturn(t, done, "runScan")
@@ -411,7 +411,7 @@ func TestRunScanInterrupted(t *testing.T) {
 	}
 
 	err = runScan(t.Context(), []string{dir}, walkCancelWorkers, false,
-		false)
+		false, nil)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -437,7 +437,7 @@ func TestRunScanInterruptedMidHash(t *testing.T) {
 	dir := buildWalkCancelTree(t)
 
 	err := runScan(newWalkClock(hashCancelAtDone), []string{dir},
-		walkCancelWorkers, false, false)
+		walkCancelWorkers, false, false, nil)
 	if !errors.Is(err, errInterrupted) {
 		t.Fatalf("runScan interrupted mid-hash = %v, want %v",
 			err, errInterrupted)

internal/sfdupes/cli.go

@@ -10,6 +10,7 @@ import (
 	"os"
 	"runtime"
 	"runtime/debug"
+	"slices"
 
 	"github.com/spf13/cobra"
 )
@@ -31,6 +32,69 @@ const (
 	cmdDwim   = "dwim"
 )
 
+// databaseHelp is the paragraph of the root and scan help that names
+// the database (README "Location").
+const databaseHelp = "scan and dwim store what they find in a database " +
+	"file. report, trees\nand info read it. The file is $" + databaseEnv +
+	" when that is set and\nnot empty, and otherwise " +
+	defaultDatabasePath + ", where creating it\nneeds root.\n"
+
+// escapesHelp is the paragraph of the report and trees help on how
+// paths are escaped (README "Report output format").
+const escapesHelp = `Four characters in a path are escaped, so each row stays one line:
+
+  \\  a backslash
+  \t  a tab
+  \n  a newline
+  \r  a carriage return
+`
+
+// reportHelp, treesHelp, infoHelp and dwimHelp follow the one-line
+// summary in the help of their subcommands.
+const (
+	reportHelp = `The report is tab-separated text on stdout: a header line, then one
+row per duplicate file. The columns are:
+
+  first   the path that sorts first, byte by byte, in its group of copies
+  dupe    another copy of that file
+  size    the file's size in bytes
+
+Rows are ordered by size, largest first, then by path.
+
+` + escapesHelp
+
+	treesHelp = `The report is tab-separated text on stdout: a header line, then one
+row per duplicate directory tree. The columns are:
+
+  first      the path that sorts first, byte by byte, in its group of copies
+  dupe       another copy of that tree
+  files      the number of regular files in one copy
+  size       their total size in bytes
+  disk_size  the bytes the dupe copy takes on disk
+
+Rows are ordered by size, largest first, then by path.
+
+` + escapesHelp
+
+	infoHelp = `The details go to stdout, one per line. Each line holds a name and its
+value, separated by a tab:
+
+  database                the path, and where it came from
+  database file size      in bytes, then rounded, as 2147483648 (2.0 GiB)
+  -wal file size          in bytes, then rounded; only if a -wal file exists
+  modified                when the database file last changed, in UTC
+  schema version          the version of the database's layout
+  records                 the number of records
+  size of recorded files  their total size, in bytes, then rounded
+`
+
+	dwimHelp = `dwim runs what "sfdupes scan /" runs, then what "sfdupes trees" runs.
+Run it as root, so that the scan can read every file. The scan is stored
+in the database, as scan stores it, so "sfdupes trees" prints the report
+again later.
+`
+)
+
 // errNoSubcommand is returned by the root command when it is invoked
 // without a subcommand. That is a usage error, and the usage text
 // cobra prints for it is the whole message.
@@ -40,6 +104,11 @@ var errNoSubcommand = errors.New("no subcommand")
 // below 1.
 var errWorkersBelowOne = errors.New("--workers must be at least 1")
 
+// errEmptyExcludeValue is the usage error for an empty scan --exclude
+// value, as from --exclude "$DIR" with DIR unset. Taken as a path, it
+// would name the working directory.
+var errEmptyExcludeValue = errors.New("--exclude must not be empty")
+
 // 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.
@@ -196,41 +265,55 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 		}),
 	}
 
+	root.Long = root.Short + ".\n\n" + databaseHelp
+	reportCmd.Long = reportCmd.Short + ".\n\n" + reportHelp
+	treesCmd.Long = treesCmd.Short + ".\n\n" + treesHelp
+	infoCmd.Long = infoCmd.Short + ".\n\n" + infoHelp
+	dwimCmd.Long = dwimCmd.Short + ".\n\n" + dwimHelp
+
 	root.AddCommand(newScanCommand(), reportCmd, treesCmd, infoCmd, dwimCmd)
 
 	return root
 }
 
-// newScanCommand builds the scan subcommand, with its --workers, -x and
+// newScanCommand builds the scan subcommand, with its --workers, -x,
-// --no-default-excludes flags. Its help lists the default excludes of
+// --no-default-excludes and --exclude flags. Its help names the
-// the system it runs on, and the filesystem types it never records
+// database, and lists the default excludes of the system it runs on and
-// anything on.
+// the filesystem types it never records anything on.
 func newScanCommand() *cobra.Command {
 	var (
 		scanWorkers           int
 		scanOneFS             bool
 		scanNoDefaultExcludes bool
+		scanExclude           []string
 	)
 
 	short := "Walk trees and synchronize the scan database"
 
 	scanCmd := &cobra.Command{
-		Use: cmdScan + " [--workers N] [-x] [--no-default-excludes] PATH...",
+		Use: cmdScan + " [--workers N] [-x] [--no-default-excludes] " +
+			"[--exclude PATH]... PATH...",
 		// The Use line already names every flag, so cobra must not
 		// append its generic "[flags]" to it.
 		DisableFlagsInUseLine: true,
 		Short:                 short,
-		Long:                  short + ".\n\n" + defaultExcludesHelp(runtime.GOOS),
+		Long: short + ".\n\n" + databaseHelp + "\n" +
-		Args:                  cobra.MinimumNArgs(1),
+			defaultExcludesHelp(runtime.GOOS),
+		Args: cobra.MinimumNArgs(1),
 		PreRunE: func(cmd *cobra.Command, _ []string) error {
-			return checkScanWorkers(cmd, scanWorkers)
+			err := checkScanWorkers(cmd, scanWorkers)
+			if err != nil {
+				return err
+			}
+
+			return rejectEmptyExcludeValue(cmd, scanExclude)
 		},
 		RunE: runE(func(ctx context.Context, args []string) error {
 			ctx, stop := interruptContext(ctx)
 			defer stop()
 
 			return runScan(ctx, args, scanWorkers, scanOneFS,
-				scanNoDefaultExcludes)
+				scanNoDefaultExcludes, scanExclude)
 		}),
 	}
 	scanCmd.Flags().IntVar(&scanWorkers, "workers", runtime.NumCPU(),
@@ -239,6 +322,10 @@ func newScanCommand() *cobra.Command {
 		"do not cross filesystem boundaries")
 	scanCmd.Flags().BoolVar(&scanNoDefaultExcludes, "no-default-excludes",
 		false, "enter the default excludes, the paths listed above")
+	// A string array, not a slice, so a comma in a path is not a
+	// separator.
+	scanCmd.Flags().StringArrayVar(&scanExclude, "exclude", nil,
+		"never enter or record `PATH` or anything below it; repeatable")
 
 	return scanCmd
 }
@@ -256,6 +343,18 @@ func checkScanWorkers(cmd *cobra.Command, workers int) error {
 	return fmt.Errorf("%w, got %d", errWorkersBelowOne, workers)
 }
 
+// rejectEmptyExcludeValue rejects an empty scan --exclude value. That
+// is a usage error reported in one line, as for --workers below 1.
+func rejectEmptyExcludeValue(cmd *cobra.Command, excludePaths []string) error {
+	if !slices.Contains(excludePaths, "") {
+		return nil
+	}
+
+	cmd.SilenceUsage = true
+
+	return errEmptyExcludeValue
+}
+
 // 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

@@ -22,6 +22,9 @@ import (
 // error prints and a runtime failure must not.
 const usageMarker = "Usage:"
 
+// helpFlag asks for the help text.
+const helpFlag = "--help"
+
 // TestMain stops the tests at once, saying why, when their temporary
 // directory is not on a persistent filesystem: scan records nothing
 // there (README "Rules for the walk"), so most scan tests would fail
@@ -570,7 +573,8 @@ func TestRunNamesDefaultDatabase(t *testing.T) {
 	}{
 		cmdScan: {
 			run: func() error {
-				return runScan(t.Context(), []string{missing}, 1, false, false)
+				return runScan(t.Context(), []string{missing}, 1, false, false,
+					nil)
 			},
 			want: scanWant,
 		},
@@ -692,7 +696,7 @@ func TestRunHelp(t *testing.T) {
 
 	// README §Settings: help goes to stderr, exits 0, and leaves stdout
 	// empty.
-	cases := [][]string{{"--help"}, {"-h"}, {cmdScan, "--help"}}
+	cases := [][]string{{helpFlag}, {"-h"}, {cmdScan, helpFlag}}
 
 	for _, args := range cases {
 		var stdout, stderr bytes.Buffer
@@ -712,6 +716,16 @@ func TestRunHelp(t *testing.T) {
 				args, got)
 		}
 	}
+
+	// The scan help lists --exclude with its argument.
+	var stdout, stderr bytes.Buffer
+
+	run([]string{cmdScan, "-h"}, &stdout, &stderr)
+
+	if !strings.Contains(stderr.String(), excludeFlag+" PATH") {
+		t.Errorf("scan -h stderr = %q, want %s PATH", stderr.String(),
+			excludeFlag)
+	}
 }
 
 func TestRunScanUsageLine(t *testing.T) {
@@ -719,7 +733,8 @@ func TestRunScanUsageLine(t *testing.T) {
 
 	// README §Subcommands shows this usage line for scan, and the help
 	// and the usage error both print it.
-	const want = "\n  sfdupes scan [--workers N] [-x] [--no-default-excludes] PATH...\n"
+	const want = "\n  sfdupes scan [--workers N] [-x] [--no-default-excludes] " +
+		"[--exclude PATH]... PATH...\n"
 
 	cases := map[string][]string{
 		"help":          {cmdScan, "-h"},
@@ -742,6 +757,73 @@ func TestRunScanUsageLine(t *testing.T) {
 	}
 }
 
+func TestRunHelpDescribesDatabaseAndOutput(t *testing.T) {
+	t.Parallel()
+
+	// README §Settings: the help names the database, and says what
+	// report, trees, info and dwim print.
+	database := []string{
+		"scan and dwim store", databaseEnv, defaultDatabasePath, "needs root",
+	}
+	escapes := []string{`\\`, `\t`, `\n`, `\r`}
+
+	// The subcommand whose help is printed; none for the root.
+	cases := []struct {
+		subcommand []string
+		want       []string
+	}{
+		{nil, database},
+		{[]string{cmdScan}, database},
+		{
+			[]string{cmdReport},
+			append([]string{
+				"tab-separated", "\n  first ", "\n  dupe ", "\n  size ",
+				"largest first",
+			}, escapes...),
+		},
+		{
+			[]string{cmdTrees},
+			append([]string{
+				"tab-separated", "\n  first ", "\n  dupe ", "\n  files ",
+				"\n  size ", "\n  disk_size ", "largest first",
+			}, escapes...),
+		},
+		{
+			[]string{cmdInfo},
+			[]string{
+				"\n  database ", "\n  database file size ",
+				"\n  -wal file size ", "\n  modified ",
+				"\n  schema version ", "\n  records ",
+				"\n  size of recorded files ",
+			},
+		},
+		{
+			[]string{cmdDwim},
+			[]string{
+				"sfdupes scan /", "as root", "sfdupes trees", "database",
+			},
+		},
+	}
+
+	for _, tc := range cases {
+		var stdout, stderr bytes.Buffer
+
+		args := slices.Concat(tc.subcommand, []string{helpFlag})
+
+		code := run(args, &stdout, &stderr)
+		if code != exitOK {
+			t.Fatalf("run(%v) = %d, want %d", args, code, exitOK)
+		}
+
+		for _, want := range tc.want {
+			if !strings.Contains(stderr.String(), want) {
+				t.Errorf("run(%v) help lacks %q:\n%s",
+					args, want, stderr.String())
+			}
+		}
+	}
+}
+
 func TestRunVersion(t *testing.T) {
 	t.Parallel()
 

internal/sfdupes/dwim.go

@@ -17,7 +17,8 @@ import (
 // ends it at once, as it ends trees.
 func runDwim(ctx context.Context, root string, stdout io.Writer) error {
 	scanCtx, stop := interruptContext(ctx)
-	err := runScan(scanCtx, []string{root}, runtime.NumCPU(), false, false)
+	err := runScan(scanCtx, []string{root}, runtime.NumCPU(), false, false,
+		nil)
 
 	stop()
 

internal/sfdupes/exclude.go

@@ -1,8 +1,9 @@
 package sfdupes
 
-// This file holds two rules for the walk (README "Rules for the walk"):
+// This file holds three rules for the walk (README "Rules for the
+// walk"): the paths given with --exclude, which the scan never enters;
 // the default excludes, the paths a scan's walk does not enter below an
-// operand unless scan is given --no-default-excludes, and the
+// operand unless scan is given --no-default-excludes; and the
 // filesystems that are not persistent, on which the scan never records
 // or reads anything.
 
@@ -12,6 +13,7 @@ import (
 	"fmt"
 	"io"
 	"io/fs"
+	"os"
 	"path/filepath"
 	"slices"
 	"strings"
@@ -43,17 +45,25 @@ const typesPerHelpLine = 6
 // readMountInfo reads.
 var errMountInfoLine = errors.New("unexpected line")
 
+// errExcludedOperand reports an operand at or below a path given with
+// --exclude.
+var errExcludedOperand = errors.New("operand excluded by --exclude")
+
 // walkRules holds the rules for the walk that a scan sets up as it
-// starts. excludes are the default excludes, or none with
+// starts. excludePaths are the paths given with --exclude, each with
-// --no-default-excludes. nonPersistent maps the device ID of every
+// the device ID and inode number of the file it names (see
-// mounted filesystem that is not persistent to its type; the scan
+// excludePathInodes), which the walk neither enters nor records,
-// records and reads nothing on one of those, whatever the flags.
+// whatever path leads to them. defaultExcludes are the default
-// databaseFiles are the scan's own database files, which the walk
+// excludes, or none with --no-default-excludes. nonPersistent maps the
-// passes over (see databaseFiles in db.go).
+// device ID of every mounted filesystem that is not persistent to its
+// type; the scan records and reads nothing on one of those, whatever
+// the flags. databaseFiles are the scan's own database files, which the
+// walk passes over (see databaseFiles in db.go).
 type walkRules struct {
-	excludes      []string
+	excludePaths    []fileRec
-	nonPersistent map[uint64]string
+	defaultExcludes []string
-	databaseFiles []fileRec
+	nonPersistent   map[uint64]string
+	databaseFiles   []fileRec
 }
 
 // defaultExcludes returns the default excludes of the system goos
@@ -97,6 +107,88 @@ func isDefaultExclude(path string, excludes []string) bool {
 	return false
 }
 
+// excludePathInodes returns each of paths, the --exclude values, as its
+// real path (see realPaths) with the device ID and inode number of the
+// file there. The walk compares what it reaches with those, as it does
+// with the database files, so it also leaves out an --exclude path
+// spelled another way, such as in another letter case on macOS.
+func excludePathInodes(paths []string) ([]fileRec, error) {
+	resolved, err := realPaths(paths)
+	if err != nil {
+		return nil, err
+	}
+
+	files := make([]fileRec, 0, len(resolved))
+
+	for _, path := range resolved {
+		fi, err := os.Lstat(path)
+		if err != nil {
+			return nil, err
+		}
+
+		dev, ino := inodeOfInfo(fi)
+		files = append(files, fileRec{path: path, dev: dev, ino: ino})
+	}
+
+	return files, nil
+}
+
+// matchingExcludePath returns the path of the one of excludePaths with
+// the device ID and inode number of fi, an lstat result, or "" when
+// there is none.
+func matchingExcludePath(fi fs.FileInfo, excludePaths []fileRec) string {
+	dev, ino := inodeOfInfo(fi)
+	f := fileRec{dev: dev, ino: ino}
+
+	for _, exclude := range excludePaths {
+		if sameInode(exclude, f) {
+			return exclude.path
+		}
+	}
+
+	return ""
+}
+
+// checkOperandsNotExcluded returns an error when one of roots, the
+// operands, or a directory above it is one of excludePaths, the
+// --exclude paths, by device ID and inode number. The error names the
+// operand and the --exclude path. The roots are absolute, so the
+// directories above each one end at /. A path here that lstat cannot
+// read is an error too.
+func checkOperandsNotExcluded(roots []string, excludePaths []fileRec) error {
+	for _, root := range roots {
+		for path := root; ; path = filepath.Dir(path) {
+			fi, err := os.Lstat(path)
+			if err != nil {
+				return fmt.Errorf("scan %s: %w", root, err)
+			}
+
+			exclude := matchingExcludePath(fi, excludePaths)
+			if exclude != "" {
+				return fmt.Errorf("scan %s: %w %s", root, errExcludedOperand,
+					exclude)
+			}
+
+			if path == "/" {
+				break
+			}
+		}
+	}
+
+	return nil
+}
+
+// excludePathLine returns the line the scan prints for path when fi,
+// its lstat, makes it one of rules.excludePaths, and "" when it does
+// not.
+func (rules walkRules) excludePathLine(path string, fi fs.FileInfo) string {
+	if matchingExcludePath(fi, rules.excludePaths) == "" {
+		return ""
+	}
+
+	return fmt.Sprintf("scan: skipped %s (--exclude)", path)
+}
+
 // nonPersistentFilesystemTypes returns the filesystem types that are
 // not persistent: filesystems that the kernel makes up or that are kept
 // in memory. devfs is the macOS filesystem of /dev; the others are

internal/sfdupes/exclude_test.go

@@ -14,6 +14,9 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+// excludeFlag is the scan flag that names a path the walk never enters.
+const excludeFlag = "--exclude"
+
 func TestDefaultExcludes(t *testing.T) {
 	t.Parallel()
 
@@ -188,7 +191,7 @@ func TestScanDoesNotEnterDefaultExclude(t *testing.T) {
 	unseen := writeFile(t, excluded, "new", pattern(2, 10))
 
 	st, err := syncScan(t.Context(), db, []string{dir}, 1, false,
-		walkRules{excludes: []string{excluded}})
+		walkRules{defaultExcludes: []string{excluded}})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -234,7 +237,7 @@ func TestScanWalksOperandInDefaultExclude(t *testing.T) {
 			db := openTestDB(t)
 
 			st, err := syncScan(t.Context(), db, roots, 1, false,
-				walkRules{excludes: []string{excluded}})
+				walkRules{defaultExcludes: []string{excluded}})
 			if err != nil {
 				t.Fatal(err)
 			}
@@ -262,7 +265,7 @@ func TestScanNamesNoDefaultExcludeThatIsAnOperand(t *testing.T) {
 	f := writeFile(t, excluded, "f", pattern(1, 10))
 
 	st, err := syncScan(t.Context(), db, []string{dir, excluded}, 1, false,
-		walkRules{excludes: []string{excluded}})
+		walkRules{defaultExcludes: []string{excluded}})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -356,6 +359,217 @@ func TestRunScanNoDefaultExcludes(t *testing.T) {
 	assertRecorded(t, path, f, true)
 }
 
+// TestRunScanExcludeNested scans a tree through the command with an
+// --exclude path two levels below the operand. The walk records nothing
+// at or below it and says so. The record an earlier scan left there is
+// kept although its file is gone.
+func TestRunScanExcludeNested(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := realTempDir(t)
+	excluded := filepath.Join(dir, "a", "b")
+	kept := writeFile(t, dir, "a/f", pattern(1, 10))
+	gone := writeFile(t, excluded, "gone", pattern(2, 10))
+
+	scanOK(t, dir)
+
+	err := os.Remove(gone)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	unseen := writeFile(t, excluded, "c/new", pattern(3, 10))
+
+	stderr := scanOK(t, excludeFlag, excluded, dir)
+
+	line := "scan: skipped " + excluded + " (--exclude)\n"
+	if !strings.Contains(stderr, line) {
+		t.Errorf("stderr = %q, want %q", stderr, line)
+	}
+
+	assertRecorded(t, path, kept, true)
+	assertRecorded(t, path, gone, true)
+	assertRecorded(t, path, unseen, false)
+}
+
+// TestRunScanExcludeRepeated gives --exclude twice, once for a
+// directory and once for a file, the file through a symlink to it. The
+// walk records neither, and records the file beside them.
+func TestRunScanExcludeRepeated(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := realTempDir(t)
+	inDir := writeFile(t, dir, "skip/f", pattern(1, 10))
+	file := writeFile(t, dir, "skip-file", pattern(2, 10))
+	kept := writeFile(t, dir, "keep", pattern(3, 10))
+
+	link := filepath.Join(t.TempDir(), "link")
+	symlink(t, file, link)
+
+	scanOK(t, excludeFlag, filepath.Dir(inDir), excludeFlag, link, dir)
+
+	assertRecorded(t, path, inDir, false)
+	assertRecorded(t, path, file, false)
+	assertRecorded(t, path, kept, true)
+}
+
+// TestRunScanExcludeReachedByAnotherPath gives --exclude a directory
+// through a symlink and a file through "..", so the walk reaches
+// neither by the path given. The walk also reaches the file through a
+// hard link. It records none of these, prints a line naming each, and
+// records the file beside them.
+func TestRunScanExcludeReachedByAnotherPath(t *testing.T) {
+	path := testDBPath(t)
+	t.Setenv(databaseEnv, path)
+
+	dir := realTempDir(t)
+	inDir := writeFile(t, dir, "a/cache/f", pattern(1, 10))
+	file := writeFile(t, dir, "a/big", pattern(2, 10))
+	kept := writeFile(t, dir, "keep", pattern(3, 10))
+
+	hardLink := filepath.Join(dir, "big-link")
+
+	err := os.Link(file, hardLink)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	link := filepath.Join(t.TempDir(), "link")
+	symlink(t, filepath.Join(dir, "a"), link)
+
+	// Joined as text, since filepath.Join would drop the "..".
+	dotDot := dir + "/a/cache/../big"
+
+	stderr := scanOK(t, excludeFlag, filepath.Join(link, "cache"),
+		excludeFlag, dotDot, dir)
+
+	for _, skipped := range []string{filepath.Dir(inDir), file, hardLink} {
+		line := "scan: skipped " + skipped + " (--exclude)\n"
+		if !strings.Contains(stderr, line) {
+			t.Errorf("stderr = %q, want %q", stderr, line)
+		}
+	}
+
+	assertRecorded(t, path, inDir, false)
+	assertRecorded(t, path, file, false)
+	assertRecorded(t, path, hardLink, false)
+	assertRecorded(t, path, kept, true)
+}
+
+// TestRunScanExcludedOperand checks that an operand at or below an
+// --exclude path is a fatal error that names both, and so is an
+// --exclude path that does not exist. An operand that is a hard link to
+// an --exclude file is at it. Neither scan writes a database.
+func TestRunScanExcludedOperand(t *testing.T) {
+	dir := realTempDir(t)
+	sub := filepath.Join(dir, "sub")
+	f := writeFile(t, sub, "f", pattern(1, 10))
+
+	hardLink := filepath.Join(dir, "f-link")
+
+	err := os.Link(f, hardLink)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	cases := map[string]struct {
+		args []string
+		want string
+	}{
+		"at it": {
+			[]string{excludeFlag, sub, sub},
+			"scan " + sub + ": operand excluded by --exclude " + sub + "\n",
+		},
+		"below it": {
+			[]string{excludeFlag, dir, sub},
+			"scan " + sub + ": operand excluded by --exclude " + dir + "\n",
+		},
+		"after an operand it does not exclude": {
+			[]string{dir, excludeFlag, sub, sub},
+			"scan " + sub + ": operand excluded by --exclude " + sub + "\n",
+		},
+		"a hard link to it": {
+			[]string{excludeFlag, f, hardLink},
+			"scan " + hardLink + ": operand excluded by --exclude " + f + "\n",
+		},
+		"exclude missing": {
+			[]string{excludeFlag, filepath.Join(dir, "nope"), dir},
+			"--exclude: resolve " + filepath.Join(dir, "nope") + ": ",
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			path := testDBPath(t)
+			t.Setenv(databaseEnv, path)
+
+			stdout := captureStdout(t)
+			stderr := captureStderr(t)
+
+			code := run(append([]string{cmdScan}, tc.args...), os.Stdout,
+				os.Stderr)
+			if code != exitFatal {
+				t.Errorf("run(scan %q) = %d, want %d", tc.args, code,
+					exitFatal)
+			}
+
+			assertFatalOutput(t, stderr(), stdout())
+
+			if !strings.Contains(stderr(), "sfdupes: "+tc.want) {
+				t.Errorf("stderr = %q, want %q", stderr(), tc.want)
+			}
+
+			_, err := os.Lstat(path)
+			if !errors.Is(err, os.ErrNotExist) {
+				t.Errorf("lstat %s: %v, want no database", path, err)
+			}
+		})
+	}
+}
+
+// TestRunScanRejectsEmptyExclude checks that an empty --exclude value,
+// given as its own argument, after "=" or after another --exclude, is
+// a usage error reported in one line before anything is scanned
+// (README "Settings").
+func TestRunScanRejectsEmptyExclude(t *testing.T) {
+	dir := t.TempDir()
+
+	cases := map[string][]string{
+		"separate":     {excludeFlag, "", dir},
+		"joined by =":  {excludeFlag + "=", dir},
+		"after a path": {excludeFlag, t.TempDir(), excludeFlag, "", dir},
+	}
+
+	for name, args := range cases {
+		t.Run(name, func(t *testing.T) {
+			path := testDBPath(t)
+			t.Setenv(databaseEnv, path)
+
+			var stdout, stderr bytes.Buffer
+
+			code := run(append([]string{cmdScan}, args...), &stdout,
+				&stderr)
+			if code != exitUsage {
+				t.Errorf("run(scan %q) = %d, want %d", args, code,
+					exitUsage)
+			}
+
+			want := "Error: --exclude must not be empty\n"
+			if stdout.String() != "" || stderr.String() != want {
+				t.Errorf("stdout, stderr = %q, %q, want \"\", %q",
+					stdout.String(), stderr.String(), want)
+			}
+
+			_, err := os.Lstat(path)
+			if !errors.Is(err, os.ErrNotExist) {
+				t.Errorf("lstat %s: %v, want no database", path, err)
+			}
+		})
+	}
+}
+
 // TestRunScanSkipsNonPersistentOperand scans, through the command, a
 // directory on a filesystem that is not persistent, with and without
 // --no-default-excludes: either way it is skipped, with a line saying

internal/sfdupes/scan.go

@@ -102,40 +102,44 @@ func scanPhase(name string) string {
 // scan command rejects anything less. Before the lock is taken, the
 // scan lists the filesystems that are not persistent, on which the walk
 // records nothing; unless noDefaultExcludes is set, the walk does not
-// enter the default excludes either. Once the database is open, the
+// enter the default excludes either. The walk neither enters nor
-// scan notes its files, which the walk passes over.
+// records the paths in excludePaths, the --exclude values; an operand
+// at or below one is a fatal error. Once the database is open, the scan
+// notes its files, which the walk passes over.
 func runScan(ctx context.Context, roots []string, workers int,
-	oneFS, noDefaultExcludes bool,
+	oneFS, noDefaultExcludes bool, excludePaths []string,
 ) error {
 	dbPath, source, err := databasePath()
 	if err != nil {
 		return err
 	}
 
-	// A database file missing now is created by lockScanDatabase below.
+	printDatabasePath(dbPath, source)
-	what := "database"
 
-	_, err = os.Stat(dbPath)
+	roots, err = realPaths(roots)
-	if errors.Is(err, fs.ErrNotExist) {
-		what = "new database"
-	}
-
-	fmt.Fprintf(os.Stderr, "scan: %s %s (%s)\n", what, escapePath(dbPath),
-		source)
-
-	roots, err = resolveRoots(roots)
 	if err != nil {
 		return err
 	}
 
-	nonPersistent, err := nonPersistentFilesystems()
+	var rules walkRules
+
+	rules.excludePaths, err = excludePathInodes(excludePaths)
+	if err != nil {
+		return fmt.Errorf("--exclude: %w", err)
+	}
+
+	err = checkOperandsNotExcluded(roots, rules.excludePaths)
+	if err != nil {
+		return err
+	}
+
+	rules.nonPersistent, err = nonPersistentFilesystems()
 	if err != nil {
 		return fmt.Errorf("list the mounted filesystems: %w", err)
 	}
 
-	rules := walkRules{nonPersistent: nonPersistent}
 	if !noDefaultExcludes {
-		rules.excludes = defaultExcludes(runtime.GOOS)
+		rules.defaultExcludes = defaultExcludes(runtime.GOOS)
 	}
 
 	lock, err := lockScanDatabase(dbPath)
@@ -181,6 +185,21 @@ func runScan(ctx context.Context, roots []string, workers int,
 	return nil
 }
 
+// printDatabasePath prints on stderr the line that names the database
+// at dbPath, with source, saying whether it is new. A database file
+// missing now is new: lockScanDatabase creates it.
+func printDatabasePath(dbPath, source string) {
+	what := "database"
+
+	_, err := os.Stat(dbPath)
+	if errors.Is(err, fs.ErrNotExist) {
+		what = "new database"
+	}
+
+	fmt.Fprintf(os.Stderr, "scan: %s %s (%s)\n", what, escapePath(dbPath),
+		source)
+}
+
 // interruptContext returns a copy of ctx that the first SIGINT or
 // SIGTERM cancels; the scan command runs the scan under it. stop
 // releases the signals.
@@ -209,24 +228,25 @@ func interrupted(walked int) error {
 	return errInterrupted
 }
 
-// resolveRoots converts each PATH operand to its real path: absolute,
+// realPaths returns the real path of each of paths, the PATH operands
-// with every symlink in it followed. Database records are keyed by
+// or the --exclude values: absolute, with every symlink in it followed.
-// that path, so scan results depend neither on the working directory
+// Database records are keyed by that path, so scan results depend
-// nor on the symlinks an operand was given through. An operand that
+// neither on the working directory nor on the symlinks an operand was
-// cannot be resolved, because it or a symlink's target does not exist
+// given through. A path that cannot be resolved, because it or a
-// or cannot be read, is a fatal error before any scanning.
+// symlink's target does not exist or cannot be read, is a fatal error
-func resolveRoots(roots []string) ([]string, error) {
+// before any scanning.
-	resolved := make([]string, 0, len(roots))
+func realPaths(paths []string) ([]string, error) {
+	resolved := make([]string, 0, len(paths))
 
-	for _, root := range roots {
+	for _, path := range paths {
-		a, err := absolutePath(root)
+		a, err := absolutePath(path)
 		if err != nil {
-			return nil, fmt.Errorf("resolve %s: %w", root, err)
+			return nil, fmt.Errorf("resolve %s: %w", path, err)
 		}
 
 		r, err := filepath.EvalSymlinks(a)
 		if err != nil {
-			return nil, fmt.Errorf("resolve %s: %w", root, err)
+			return nil, fmt.Errorf("resolve %s: %w", path, err)
 		}
 
 		resolved = append(resolved, r)
@@ -295,7 +315,7 @@ func walkReaches(above, r string, oneFS bool, rules walkRules,
 	}
 
 	for ; dir != above; dir = filepath.Dir(dir) {
-		if isDefaultExclude(dir, rules.excludes) {
+		if isDefaultExclude(dir, rules.defaultExcludes) {
 			return false
 		}
 
@@ -418,7 +438,7 @@ func (s *scanState) runPhases(ctx context.Context, roots []string,
 
 // walkableRoots returns the operands the walk can start from: regular
 // files, and directories not named .zfs, none of them inside a
-// directory named .zfs. The operands are real paths from resolveRoots,
+// directory named .zfs. The operands are real paths from realPaths,
 // so these checks apply to what a symlink operand points at. Every
 // other operand is warned about, counted as skipped, and dropped. A
 // dropped operand is no longer a root, so the records stored beneath it
@@ -493,10 +513,11 @@ func (s *scanState) loadIndex(ctx context.Context, roots []string) error {
 // as verified. When its disk_size differs from the record's, it is
 // queued for the update phase, which writes the new disk_size. A record
 // at or below a path the walk left out, because -x stopped at it, it is
-// a default exclude, or it is not on a persistent filesystem, counts as
+// an --exclude path or a default exclude, or it is not on a persistent
-// outside the roots: it is not deleted. That does not hold below a root
+// filesystem, counts as outside the roots: it is not deleted. That does
-// at or inside that path, since that root's own walk verifies the
+// not hold below a root at or inside that path, since that root's own
-// records below it; nor is a line printed for such a root.
+// walk verifies the records below it; nor is a line printed for such a
+// root.
 func (s *scanState) walkPhase(events <-chan walkEvent, roots []string) {
 	// The records the last scan left under the roots estimate how many
 	// files the walk will find. A first scan has none, and no estimate.
@@ -834,9 +855,9 @@ type dirJob struct {
 // file's statted record, a warning when fail is set, or a path the walk
 // left out: when otherFSDir is set, a directory -x did not enter
 // because it is on another filesystem than its operand, and when
-// leftOut is set, a default exclude or a directory or file on a
+// leftOut is set, an --exclude path, a default exclude or a directory or
-// filesystem that is not persistent, with warn holding the line that
+// file on a filesystem that is not persistent, with warn holding the
-// says so.
+// line that says so.
 type walkEvent struct {
 	rec        fileRec
 	warn       string
@@ -887,7 +908,7 @@ func sendEvent(ctx context.Context, events chan<- walkEvent,
 // operandWarning returns the one-line warning for an operand the walk
 // does not start from, naming the path and what it is, or "" for one it
 // does: a regular file, or a directory not named .zfs, that is not
-// inside a directory named .zfs. resolveRoots has already followed the
+// inside a directory named .zfs. realPaths has already followed the
 // operand's symlinks, so a symlink here is an operand replaced by one
 // after it was resolved, and is not followed.
 func operandWarning(root string, fi fs.FileInfo) string {
@@ -1118,8 +1139,9 @@ func walkOneDir(ctx context.Context, job dirJob, oneFS bool,
 // no longer exists, and one that has stopped being a regular file is
 // skipped. A file on a filesystem in rules that is not persistent, as
 // through a bind mount of one file, is not recorded: it gets an event
-// naming it instead. One of the scan's own database files in rules gets
+// naming it instead, and so does a file given with --exclude, reached
-// no event at all.
+// by any path, a hard link to it included. One of the scan's own
+// database files in rules gets no event at all.
 func emitFile(ctx context.Context, p string, e fs.DirEntry,
 	rules walkRules, events chan<- walkEvent,
 ) {
@@ -1143,6 +1165,10 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 	}
 
 	line := rules.nonPersistentLine(p, info)
+	if line == "" {
+		line = rules.excludePathLine(p, info)
+	}
+
 	if line != "" {
 		sendEvent(ctx, events, walkEvent{leftOut: p, warn: line})
 
@@ -1167,11 +1193,11 @@ func emitFile(ctx context.Context, p string, e fs.DirEntry,
 // subdirJob applies the descent rules to directory p: never enter
 // .zfs (ZFS snapshot pseudo-dirs would list every file once per
 // snapshot), never enter a directory on a filesystem in rules that is
-// not persistent, nor a default exclude in rules, and with -x never
+// not persistent, nor an --exclude path or a default exclude in rules,
-// enter a directory on a different filesystem than its operand. For the
+// and with -x never enter a directory on a different filesystem than
-// last three it sends an event that names the directory, so the scan
+// its operand. For all but .zfs it sends an event that names the
-// keeps the records below it, except those below an operand at or
+// directory, so the scan keeps the records below it, except those below
-// inside it.
+// an operand at or inside it.
 func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	parent dirJob, oneFS bool, rules walkRules,
 	events chan<- walkEvent,
@@ -1196,13 +1222,17 @@ func subdirJob(ctx context.Context, p string, e fs.DirEntry,
 	}
 
 	line := rules.nonPersistentLine(p, info)
+	if line == "" {
+		line = rules.excludePathLine(p, info)
+	}
+
 	if line != "" {
 		sendEvent(ctx, events, walkEvent{leftOut: p, warn: line})
 
 		return dirJob{}, false
 	}
 
-	if isDefaultExclude(p, rules.excludes) {
+	if isDefaultExclude(p, rules.defaultExcludes) {
 		sendEvent(ctx, events, walkEvent{
 			leftOut: p,
 			warn:    fmt.Sprintf("scan: skipped %s (default exclude)", p),

internal/sfdupes/scan_test.go

@@ -2289,7 +2289,7 @@ func TestPruneRoots(t *testing.T) {
 
 	// The walk of /p does not enter /p/q when it is a default exclude, so
 	// /p/q is kept, without -x too; /p/q/r lies under it and is dropped.
-	got = pruneRoots(nested, false, walkRules{excludes: []string{pqRoot}},
+	got = pruneRoots(nested, false, walkRules{defaultExcludes: []string{pqRoot}},
 		device)
 	if !slices.Equal(got, want) {
 		t.Fatalf("pruneRoots excluding %s = %q, want %q", pqRoot, got, want)