How do I enable the ident string for a Git repository?

Question

How do I enable ident $Id$ on files in a Git repository?

Answer 1

Summary: The recommended way of embedding version information in a product is to use the build system for that; see below for details and alternate approaches.

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In Git (and I think usually also in other VCS systems with atomic commits) there is no such thing like version of a single file.

Git does support on-demand expansion of $Id:$ keyword, but:

1. It is done on request only. You have to specify (perhaps using globbing pattern) that a file (or a set of files) has an ident attribute set (in '.gitattributes' file in tree, or in '.git/info/attributes' for local repository settings).
2. It expands to the SHA-1 of file contents (or to be more exact to $Id:<sha-1 of blob>$). The reason for this choice is that Git does not touch files that have not changed during branch switching or rewinding; if '$Id:$' expanded to revision info it would require to update every version-controlled file e.g. when switching branches.

Git supports quite a wide set of $Format:...$ placeholders which expands to commit information (e.g. $Format:%H$ replaced by a commit hash) but:

1. Expansion is done only when running git archive, in its output file.
2. It is done on request, controlled via export-subst attribute.

The recommended way of embedding version information is to do it via the build system (in a build stage); see for example Git Makefile and GIT-VERSION-GEN script used by Makefile in the Git web interface for the git.git repository.

You can however (ab)use a clean/smudge filter driver (via filter attribute) to get CVS-like keyword expansion, expanding keywords on checkout, and cleaning them up on entering contents to the repository.

Answer 2

You can do this by adding a pattern for which files you want this functionality followed by ident in the .gitattributes file. This will replace $Id$ with $Id:<40-digit SHA>$ on checkout of the file. Notice though that it won't give you a revision number of the file as in CVS/SVN.

Example:

$ echo '*.txt ident' >> .gitattributes
$ echo '$Id$' > test.txt
$ git commit -a -m "test"

$ rm test.txt
$ git checkout -- test.txt
$ cat test.txt

Link to gitattributes(5) Manual Page

Answer 3

Git's ident does not do what $Id$ does in other versioning systems. As a kludge, use RCS along with git: RCS for individual file revisions and git to checkpoint the project as a whole. As I said, this is a kludge but it does kinda make sense (sometimes for some things).