Limit report and trees to given paths (closes #174)

report and trees take optional PATH operands and use only the records
at or below them. Operands are made absolute and cleaned as text, never
looked up on the filesystem; each reported copy is still checked on
disk. With operands, trees reports only directories at or below an
operand: any other directory lies above an operand and can hold unused
files, so its digest neither gets reported nor suppresses its
children's group. The summary still counts every record read.

Model: opus-5-5

README.md

@@ -44,7 +44,9 @@ as opposed to a directory, a symlink or a device node. A symlink is a file that
 holds the path of another file and stands in for it. A device node is a file
 that stands for a piece of hardware, such as a disk or a terminal. `report`
 prints the file-level duplicates. `trees` prints the duplicate directory trees.
-`info` prints the database's path, size and record count.
+Both cover the whole database, or only the paths given after them, as in
+`sfdupes report /srv/photos`. `info` prints the database's path, size and record
+count.
 
 The database is `/var/lib/sfdupes/db.sqlite` unless `SFDUPES_DATABASE` names
 another file. The first `scan` creates it.
@@ -260,8 +262,8 @@ 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
 sfdupes --version
@@ -686,11 +688,12 @@ 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 +725,27 @@ 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. The report then has no
+  rows.
+
 #### Report output format
 
 The report is TSV (tab-separated values) on stdout. It holds a header line, then
@@ -749,7 +773,8 @@ 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 +785,12 @@ 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 +818,11 @@ 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
@@ -845,7 +876,8 @@ 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.
@@ -858,7 +890,9 @@ trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimab
 
 `dwim`, short for "do what I mean", scans a new machine and reports its
 duplicate trees in one command. It takes no positional arguments and no flags of
-its own. Like `scan`, it should run as root, so that it can read every file.
+its own. A positional argument is anything on the command line that is not a
+flag, such as the paths `scan` takes. Like `scan`, it should run as root, so
+that it can read every file.
 
 `dwim` runs what `sfdupes scan /` runs with no flags. The walk skips the default
 excludes and crosses into other filesystems. `--workers` keeps its default. The
@@ -1249,7 +1283,7 @@ 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.
+    - `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

TODO.md

@@ -29,6 +29,11 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- `report` and `trees` take optional `PATH` operands. With operands, they use
+  only the records at or below them. `trees` then reports only directories at or
+  below an operand. Operands are made absolute and cleaned as text, never looked
+  up on the filesystem (2026-11-11,
+  https://git.eeqj.de/sneak/sfdupes/issues/174)
 - 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,

internal/sfdupes/cli.go

@@ -32,6 +32,13 @@ const (
 	cmdDwim   = "dwim"
 )
 
+// operandsHelp explains the PATH operands of report and trees in their
+// help.
+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."
+
 // 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.
@@ -167,20 +174,24 @@ 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,
+		Long: "Read the scan database and print the file-level duplicates " +
-		RunE: runE(func(ctx context.Context, _ []string) error {
+			"report.\n\n" + operandsHelp,
-			return runReport(ctx, stdout)
+		Args: cobra.ArbitraryArgs,
+		RunE: runE(func(ctx context.Context, args []string) error {
+			return runReport(ctx, args, stdout)
 		}),
 	}
 
 	treesCmd := &cobra.Command{
-		Use:   cmdTrees,
+		Use:   cmdTrees + " [PATH...]",
 		Short: "Read the scan database and print the duplicate-tree report",
-		Args:  cobra.NoArgs,
+		Long: "Read the scan database and print the duplicate-tree " +
-		RunE: runE(func(ctx context.Context, _ []string) error {
+			"report.\n\n" + operandsHelp,
-			return runTrees(ctx, stdout)
+		Args: cobra.ArbitraryArgs,
+		RunE: runE(func(ctx context.Context, args []string) error {
+			return runTrees(ctx, args, stdout)
 		}),
 	}
 

internal/sfdupes/cli_test.go

@@ -577,14 +577,14 @@ 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" +
 				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" +
 				treesOpenLine + "\n" + treesOpenLine + "\n",
@@ -620,8 +620,6 @@ 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},
@@ -1766,8 +1764,12 @@ 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/dwim.go

@@ -26,7 +26,7 @@ func runDwim(ctx context.Context, root string, stdout io.Writer) error {
 		return err
 	}
 
-	err = runTrees(ctx, stdout)
+	err = runTrees(ctx, nil, stdout)
 	if err != nil {
 		return err
 	}

internal/sfdupes/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 +288,46 @@ 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.
+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 +149,103 @@ 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)
+	}
+}
+
 func TestRunReportEscapesPaths(t *testing.T) {
 	dir, recs := recsOnDisk(t, awkwardPairRecs())
 	t.Setenv(databaseEnv, seedDatabase(t, recs))
@@ -195,7 +293,7 @@ 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 +390,8 @@ 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 +613,7 @@ 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 +647,8 @@ 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 +681,7 @@ 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/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()
 
@@ -118,10 +125,17 @@ func runTrees(ctx context.Context, stdout io.Writer) error {
 }
 
 // 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
@@ -139,7 +156,21 @@ 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

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.