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

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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.

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.

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