cat-file: add --batch-check=<format>

The `cat-file --batch-check` command can be used to quickly
get information about a large number of objects. However, it
provides a fixed set of information.

This patch adds an optional <format> option to --batch-check
to allow a caller to specify which items they are interested
in, and in which order to output them. This is not very
exciting for now, since we provide the same limited set that
you could already get. However, it opens the door to adding
new format items in the future without breaking backwards
compatibility (or forcing callers to pay the cost to
calculate uninteresting items).

Since the --batch option shares code with --batch-check, it
receives the same feature, though it is less likely to be of
interest there.

The format atom names are chosen to match their counterparts
in for-each-ref. Though we do not (yet) share any code with
for-each-ref's formatter, this keeps the interface as
consistent as possible, and may help later on if the
implementations are unified.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 44 insertions, 11 deletions

diff --git a/Documentation/git-cat-file.txt b/Documentation/git-cat-file.txt
index 30d585af5d..7c57eca9b0 100644
--- a/Documentation/git-cat-file.txt
+++ b/Documentation/git-cat-file.txt
@@ -58,12 +58,16 @@ OPTIONS
 	to apply the filter to the content recorded in the index at <path>.
 
 --batch::
-	Print the SHA-1, type, size, and contents of each object provided on
-	stdin. May not be combined with any other options or arguments.
+--batch=<format>::
+	Print object information and contents for each object provided
+	on stdin.  May not be combined with any other options or arguments.
+	See the section `BATCH OUTPUT` below for details.
 
 --batch-check::
-	Print the SHA-1, type, and size of each object provided on stdin. May not
-	be combined with any other options or arguments.
+--batch-check=<format>::
+	Print object information for each object provided on stdin.  May
+	not be combined with any other options or arguments.  See the
+	section `BATCH OUTPUT` below for details.
 
 OUTPUT
 ------
@@ -78,23 +82,52 @@ If '-p' is specified, the contents of <object> are pretty-printed.
 If <type> is specified, the raw (though uncompressed) contents of the <object>
 will be returned.
 
-If '--batch' is specified, output of the following form is printed for each
-object specified on stdin:
+BATCH OUTPUT
+------------
+
+If `--batch` or `--batch-check` is given, `cat-file` will read objects
+from stdin, one per line, and print information about them.
+
+Each line is considered as a whole object name, and is parsed as if
+given to linkgit:git-rev-parse[1].
+
+You can specify the information shown for each object by using a custom
+`<format>`. The `<format>` is copied literally to stdout for each
+object, with placeholders of the form `%(atom)` expanded, followed by a
+newline. The available atoms are:
+
+`objectname`::
+	The 40-hex object name of the object.
+
+`objecttype`::
+	The type of of the object (the same as `cat-file -t` reports).
+
+`objectsize`::
+	The size, in bytes, of the object (the same as `cat-file -s`
+	reports).
+
+If no format is specified, the default format is `%(objectname)
+%(objecttype) %(objectsize)`.
+
+If `--batch` is specified, the object information is followed by the
+object contents (consisting of `%(objectsize)` bytes), followed by a
+newline.
+
+For example, `--batch` without a custom format would produce:
 
 ------------
 <sha1> SP <type> SP <size> LF
 <contents> LF
 ------------
 
-If '--batch-check' is specified, output of the following form is printed for
-each object specified on stdin:
+Whereas `--batch-check='%(objectname) %(objecttype)'` would produce:
 
 ------------
-<sha1> SP <type> SP <size> LF
+<sha1> SP <type> LF
 ------------
 
-For both '--batch' and '--batch-check', output of the following form is printed
-for each object specified on stdin that does not exist in the repository:
+If a name is specified on stdin that cannot be resolved to an object in
+the repository, then `cat-file` will ignore any custom format and print:
 
 ------------
 <object> SP missing LF