Limit report and trees to given paths (closes #174) (#191)

README.md

@@ -43,7 +43,9 @@ 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.
@@ -263,8 +265,8 @@ In order:
 
 ```
 sfdupes scan [--workers N] [-x] [--no-default-excludes] [--exclude PATH]... PATH...
-sfdupes report > dupes.tsv
-sfdupes trees > dupetrees.tsv
+sfdupes report [PATH...] > dupes.tsv
+sfdupes trees [PATH...] > dupetrees.tsv
 sfdupes info
 sfdupes dwim [--workers N] [-x] [--no-default-excludes] [--exclude PATH]...
 sfdupes --version
@@ -691,11 +693,12 @@ and `scan` leaves it ignored.
 
 ### `report` mode
 
-`report` reads every record from the database. It takes no positional arguments.
-A positional argument is anything on the command line that is not a flag, such
-as the paths `scan` takes. `report` never opens a scanned file. It checks that
-each path in a group still exists, and looks at no other path. The check is
-`lstat`, the system call that looks up a path without following a symlink:
+`report` reads every record from the database. It takes optional `PATH`
+operands. With none, it uses every record. With one or more, it uses only the
+records at or below them (see "Operands of `report` and `trees`"). `report`
+never opens a scanned file. It checks that each path in a group still exists,
+and looks at no other path. It never looks up an operand. The check is `lstat`,
+the system call that looks up a path without following a symlink:
 
 - A path is gone when `lstat` finds that it, or a directory above it, does not
   exist.
@@ -727,6 +730,27 @@ with one signature come together, and so do the records of one size. So `report`
 holds only the duplicate groups of the size it is reading. Once it reaches a
 smaller size, it writes them, ordered by `first` path.
 
+#### Operands of `report` and `trees`
+
+`report` and `trees` take the same `PATH` operands:
+
+- A record is used when its path is an operand or lies below one. `/srv/photos`
+  covers `/srv/photos/x` but not `/srv/photos2/x`.
+- The records used are grouped, ordered and totalled as they are without
+  operands. A copy outside the operands is not reported. With copies `/a/f`,
+  `/srv/photos/x` and `/srv/photos/y`, `sfdupes report /srv/photos` prints one
+  row, `/srv/photos/x` and `/srv/photos/y`.
+- A relative operand is joined to the working directory as `pwd -P` prints it.
+- Each operand is then cleaned as text, as `realpath -ms` prints it. Its `.`
+  elements and repeated or trailing slashes are dropped. Each `..` drops the
+  element before it.
+- No operand is looked up on the filesystem. A symlink in an operand is not
+  followed, and an operand need not exist. The records hold real paths (see
+  "Operands" under "`scan` mode"), so an operand that passes through a symlink
+  matches no record.
+- An operand that matches no record is not an error; it adds nothing to the
+  report.
+
 #### Report output format
 
 The report is TSV (tab-separated values) on stdout. It holds a header line, then
@@ -754,7 +778,8 @@ 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.
@@ -765,11 +790,12 @@ 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.
-It reports whole duplicate directory trees. Two directories are duplicate trees
-when they hold the same relative paths, and each path holds a file with the same
-signature in both. It rebuilds the directory tree from the paths in the records,
-split on `/`. Like `report`, it never opens a scanned file. It checks only that
+`trees` reads the same database as `report` and takes the same `PATH` operands
+(see "Operands of `report` and `trees`"). It reports whole duplicate directory
+trees. Two directories are duplicate trees when they hold the same relative
+paths, and each path holds a file with the same signature in both. It rebuilds
+the directory tree from the paths in the records, split on `/`. Like `report`,
+it never opens a scanned file and never looks up an operand. It checks only that
 the directory at the top of each copy still exists, as `report` checks a path.
 
 Definitions:
@@ -797,6 +823,11 @@ 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
@@ -850,7 +881,8 @@ 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.
@@ -862,8 +894,10 @@ trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimab
 ### `dwim` mode
 
 `dwim`, short for "do what I mean", scans a new machine and reports its
-duplicate trees in one command. It takes no positional arguments. Like `scan`,
-it should run as root, so that it can read every file.
+duplicate trees in one command. It takes no positional arguments. A positional
+argument is anything on the command line that is not a flag, such as the paths
+`scan` takes. Like `scan`, it should run as root, so that it can read every
+file.
 
 `dwim` takes these flags of `scan`, with the same meaning (see "Settings"):
 
@@ -1265,7 +1299,8 @@ showing the count it stopped at.
     - `scan` with no operand;
     - `--workers` below 1;
     - an empty `--exclude` value, as in `--exclude ""`;
-    - `report`, `trees`, `info` or `dwim` with a positional argument.
+    - an empty `PATH` for `report` or `trees`, as in `sfdupes report ""`;
+    - `info` or `dwim` with a positional argument.
 
 A fatal error is reported on stderr, starting with `sfdupes:`. A failed write to
 stdout, such as a full disk, is reported the same way. Two cases never reach