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git/Documentation/git-cat-file.txt
Patrick Steinhardt f79e18849b cat-file: add option '-Z' that delimits input and output with NUL 
In db9d67f2e9 (builtin/cat-file.c: support NUL-delimited input with
`-z`, 2022-07-22), we have introduced a new mode to read the input via
NUL-delimited records instead of newline-delimited records. This allows
the user to query for revisions that have newlines in their path
component. While unusual, such queries are perfectly valid and thus it
is clear that we should be able to support them properly.

Unfortunately, the commit only changed the input to be NUL-delimited,
but didn't change the output at the same time. While this is fine for
queries that are processed successfully, it is less so for queries that
aren't. In the case of missing commits for example the result can become
entirely unparsable:

```
$ printf "7ce4f05bae8120d9fa258e854a8669f6ea9cb7b1 blob 10\n1234567890\n\n\commit000" |
    git cat-file --batch -z
7ce4f05bae blob 10
1234567890

commit missing
```

This is of course a crafted query that is intentionally gaming the
deficiency, but more benign queries that contain newlines would have
similar problems.

Ideally, we should have also changed the output to be NUL-delimited when
`-z` is specified to avoid this problem. As the input is NUL-delimited,
it is clear that the output in this case cannot ever contain NUL
characters by itself. Furthermore, Git does not allow NUL characters in
revisions anyway, further stressing the point that using NUL-delimited
output is safe. The only exception is of course the object data itself,
but as git-cat-file(1) prints the size of the object data clients should
read until that specified size has been consumed.

But even though `-z` has only been introduced a few releases ago in Git
v2.38.0, changing the output format retroactively to also NUL-delimit
output would be a backwards incompatible change. And while one could
make the argument that the output is inherently broken already, we need
to assume that there are existing users out there that use it just fine
given that revisions containing newlines are quite exotic.

Instead, introduce a new option `-Z` that switches to NUL-delimited
input and output. While this new option could arguably only switch the
output format to be NUL-delimited, the consequence would be that users
have to always specify both `-z` and `-Z` when the input may contain
newlines. On the other hand, if the user knows that there never will be
newlines in the input, they don't have to use either of those options.
There is thus no usecase that would warrant treating input and output
format separately, which is why we instead opt to "do the right thing"
and have `-Z` mean to NUL-terminate both formats.

The old `-z` option is marked as deprecated with a hint that its output
may become unparsable. It is thus hidden both from the synopsis as well
as the command's help output.

Co-authored-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2023-06-12 13:23:46 -07:00

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git-cat-file(1)
===============
NAME
----
git-cat-file - Provide content or type and size information for repository objects
SYNOPSIS
--------
[verse]
'git cat-file' <type> <object>
'git cat-file' (-e | -p) <object>
'git cat-file' (-t | -s) [--allow-unknown-type] <object>
'git cat-file' (--batch | --batch-check | --batch-command) [--batch-all-objects]
[--buffer] [--follow-symlinks] [--unordered]
[--textconv | --filters] [-Z]
'git cat-file' (--textconv | --filters)
[<rev>:<path|tree-ish> | --path=<path|tree-ish> <rev>]
DESCRIPTION
-----------
In its first form, the command provides the content or the type of an object in
the repository. The type is required unless `-t` or `-p` is used to find the
object type, or `-s` is used to find the object size, or `--textconv` or
`--filters` is used (which imply type "blob").
In the second form, a list of objects (separated by linefeeds) is provided on
stdin, and the SHA-1, type, and size of each object is printed on stdout. The
output format can be overridden using the optional `<format>` argument. If
either `--textconv` or `--filters` was specified, the input is expected to
list the object names followed by the path name, separated by a single
whitespace, so that the appropriate drivers can be determined.
OPTIONS
-------
<object>::
The name of the object to show.
For a more complete list of ways to spell object names, see
the "SPECIFYING REVISIONS" section in linkgit:gitrevisions[7].
-t::
Instead of the content, show the object type identified by
`<object>`.
-s::
Instead of the content, show the object size identified by
`<object>`. If used with `--use-mailmap` option, will show
the size of updated object after replacing idents using the
mailmap mechanism.
-e::
Exit with zero status if `<object>` exists and is a valid
object. If `<object>` is of an invalid format exit with non-zero and
emits an error on stderr.
-p::
Pretty-print the contents of `<object>` based on its type.
<type>::
Typically this matches the real type of `<object>` but asking
for a type that can trivially be dereferenced from the given
`<object>` is also permitted. An example is to ask for a
"tree" with `<object>` being a commit object that contains it,
or to ask for a "blob" with `<object>` being a tag object that
points at it.
--[no-]mailmap::
--[no-]use-mailmap::
Use mailmap file to map author, committer and tagger names
and email addresses to canonical real names and email addresses.
See linkgit:git-shortlog[1].
--textconv::
Show the content as transformed by a textconv filter. In this case,
`<object>` has to be of the form `<tree-ish>:<path>`, or `:<path>` in
order to apply the filter to the content recorded in the index at
`<path>`.
--filters::
Show the content as converted by the filters configured in
the current working tree for the given `<path>` (i.e. smudge filters,
end-of-line conversion, etc). In this case, `<object>` has to be of
the form `<tree-ish>:<path>`, or `:<path>`.
--path=<path>::
For use with `--textconv` or `--filters`, to allow specifying an object
name and a path separately, e.g. when it is difficult to figure out
the revision from which the blob came.
--batch::
--batch=<format>::
Print object information and contents for each object provided
on stdin. May not be combined with any other options or arguments
except `--textconv`, `--filters`, or `--use-mailmap`.
+
--
* When used with `--textconv` or `--filters`, the input lines
must specify the path, separated by whitespace. See the section
`BATCH OUTPUT` below for details.
* When used with `--use-mailmap`, for commit and tag objects, the
contents part of the output shows the identities replaced using the
mailmap mechanism, while the information part of the output shows
the size of the object as if it actually recorded the replacement
identities.
--
--batch-check::
--batch-check=<format>::
Print object information for each object provided on stdin. May not be
combined with any other options or arguments except `--textconv`, `--filters`
or `--use-mailmap`.
+
--
* When used with `--textconv` or `--filters`, the input lines must
specify the path, separated by whitespace. See the section
`BATCH OUTPUT` below for details.
* When used with `--use-mailmap`, for commit and tag objects, the
printed object information shows the size of the object as if the
identities recorded in it were replaced by the mailmap mechanism.
--
--batch-command::
--batch-command=<format>::
Enter a command mode that reads commands and arguments from stdin. May
only be combined with `--buffer`, `--textconv`, `--use-mailmap` or
`--filters`.
+
--
* When used with `--textconv` or `--filters`, the input lines must
specify the path, separated by whitespace. See the section
`BATCH OUTPUT` below for details.
* When used with `--use-mailmap`, for commit and tag objects, the
`contents` command shows the identities replaced using the
mailmap mechanism, while the `info` command shows the size
of the object as if it actually recorded the replacement
identities.
--
+
`--batch-command` recognizes the following commands:
+
--
contents <object>::
Print object contents for object reference `<object>`. This corresponds to
the output of `--batch`.
info <object>::
Print object info for object reference `<object>`. This corresponds to the
output of `--batch-check`.
flush::
Used with `--buffer` to execute all preceding commands that were issued
since the beginning or since the last flush was issued. When `--buffer`
is used, no output will come until a `flush` is issued. When `--buffer`
is not used, commands are flushed each time without issuing `flush`.
--
+
--batch-all-objects::
Instead of reading a list of objects on stdin, perform the
requested batch operation on all objects in the repository and
any alternate object stores (not just reachable objects).
Requires `--batch` or `--batch-check` be specified. By default,
the objects are visited in order sorted by their hashes; see
also `--unordered` below. Objects are presented as-is, without
respecting the "replace" mechanism of linkgit:git-replace[1].
--buffer::
Normally batch output is flushed after each object is output, so
that a process can interactively read and write from
`cat-file`. With this option, the output uses normal stdio
buffering; this is much more efficient when invoking
`--batch-check` or `--batch-command` on a large number of objects.
--unordered::
When `--batch-all-objects` is in use, visit objects in an
order which may be more efficient for accessing the object
contents than hash order. The exact details of the order are
unspecified, but if you do not require a specific order, this
should generally result in faster output, especially with
`--batch`. Note that `cat-file` will still show each object
only once, even if it is stored multiple times in the
repository.
--allow-unknown-type::
Allow `-s` or `-t` to query broken/corrupt objects of unknown type.
--follow-symlinks::
With `--batch` or `--batch-check`, follow symlinks inside the
repository when requesting objects with extended SHA-1
expressions of the form tree-ish:path-in-tree. Instead of
providing output about the link itself, provide output about
the linked-to object. If a symlink points outside the
tree-ish (e.g. a link to `/foo` or a root-level link to `../foo`),
the portion of the link which is outside the tree will be
printed.
+
This option does not (currently) work correctly when an object in the
index is specified (e.g. `:link` instead of `HEAD:link`) rather than
one in the tree.
+
This option cannot (currently) be used unless `--batch` or
`--batch-check` is used.
+
For example, consider a git repository containing:
+
--
f: a file containing "hello\n"
link: a symlink to f
dir/link: a symlink to ../f
plink: a symlink to ../f
alink: a symlink to /etc/passwd
--
+
For a regular file `f`, `echo HEAD:f | git cat-file --batch` would print
+
--
ce013625030ba8dba906f756967f9e9ca394464a blob 6
--
+
And `echo HEAD:link | git cat-file --batch --follow-symlinks` would
print the same thing, as would `HEAD:dir/link`, as they both point at
`HEAD:f`.
+
Without `--follow-symlinks`, these would print data about the symlink
itself. In the case of `HEAD:link`, you would see
+
--
4d1ae35ba2c8ec712fa2a379db44ad639ca277bd blob 1
--
+
Both `plink` and `alink` point outside the tree, so they would
respectively print:
+
--
symlink 4
../f
symlink 11
/etc/passwd
--
-Z::
Only meaningful with `--batch`, `--batch-check`, or
`--batch-command`; input and output is NUL-delimited instead of
newline-delimited.
-z::
Only meaningful with `--batch`, `--batch-check`, or
`--batch-command`; input is NUL-delimited instead of
newline-delimited. This option is deprecated in favor of
`-Z` as the output can otherwise be ambiguous.
OUTPUT
------
If `-t` is specified, one of the `<type>`.
If `-s` is specified, the size of the `<object>` in bytes.
If `-e` is specified, no output, unless the `<object>` is malformed.
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.
BATCH OUTPUT
------------
If `--batch` or `--batch-check` is given, `cat-file` will read objects
from stdin, one per line, and print information about them. By default,
the whole line is considered as an object, as if it were fed to
linkgit:git-rev-parse[1].
When `--batch-command` is given, `cat-file` will read commands from stdin,
one per line, and print information based on the command given. With
`--batch-command`, the `info` command followed by an object will print
information about the object the same way `--batch-check` would, and the
`contents` command followed by an object prints contents in the same way
`--batch` would.
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 full hex representation of the object name.
`objecttype`::
The type 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).
`objectsize:disk`::
The size, in bytes, that the object takes up on disk. See the
note about on-disk sizes in the `CAVEATS` section below.
`deltabase`::
If the object is stored as a delta on-disk, this expands to the
full hex representation of the delta base object name.
Otherwise, expands to the null OID (all zeroes). See `CAVEATS`
below.
`rest`::
If this atom is used in the output string, input lines are split
at the first whitespace boundary. All characters before that
whitespace are considered to be the object name; characters
after that first run of whitespace (i.e., the "rest" of the
line) are output in place of the `%(rest)` atom.
If no format is specified, the default format is `%(objectname)
%(objecttype) %(objectsize)`.
If `--batch` is specified, or if `--batch-command` is used with the `contents`
command, 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:
------------
<oid> SP <type> SP <size> LF
<contents> LF
------------
Whereas `--batch-check='%(objectname) %(objecttype)'` would produce:
------------
<oid> SP <type> LF
------------
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
------------
If a name is specified that might refer to more than one object (an ambiguous short sha), then `cat-file` will ignore any custom format and print:
------------
<object> SP ambiguous LF
------------
If `--follow-symlinks` is used, and a symlink in the repository points
outside the repository, then `cat-file` will ignore any custom format
and print:
------------
symlink SP <size> LF
<symlink> LF
------------
The symlink will either be absolute (beginning with a `/`), or relative
to the tree root. For instance, if dir/link points to `../../foo`, then
`<symlink>` will be `../foo`. `<size>` is the size of the symlink in bytes.
If `--follow-symlinks` is used, the following error messages will be
displayed:
------------
<object> SP missing LF
------------
is printed when the initial symlink requested does not exist.
------------
dangling SP <size> LF
<object> LF
------------
is printed when the initial symlink exists, but something that
it (transitive-of) points to does not.
------------
loop SP <size> LF
<object> LF
------------
is printed for symlink loops (or any symlinks that
require more than 40 link resolutions to resolve).
------------
notdir SP <size> LF
<object> LF
------------
is printed when, during symlink resolution, a file is used as a
directory name.
Alternatively, when `-Z` is passed, the line feeds in any of the above examples
are replaced with NUL terminators. This ensures that output will be parsable if
the output itself would contain a linefeed and is thus recommended for
scripting purposes.
CAVEATS
-------
Note that the sizes of objects on disk are reported accurately, but care
should be taken in drawing conclusions about which refs or objects are
responsible for disk usage. The size of a packed non-delta object may be
much larger than the size of objects which delta against it, but the
choice of which object is the base and which is the delta is arbitrary
and is subject to change during a repack.
Note also that multiple copies of an object may be present in the object
database; in this case, it is undefined which copy's size or delta base
will be reported.
GIT
---
Part of the linkgit:git[1] suite
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