Question

I recently asked about keyword expansion in Git and I’m willing to accept the design not to really support this idea in Git.

For better or worse, the project I’m working on at the moment requires SVN keyword expansion like this:

svn propset svn:keywords 'Id' expl3.dtx 

to keep this string up-to-date:

$Id: expl3.dtx 803 2008-09-11 14:01:58Z will $ 

But I would quite like to use Git to do my version control. Unfortunately, git-svn doesn’t support this, according to the docs:

‘We ignore all SVN properties except svn:executable’

But it doesn’t seem too tricky to have this keyword stuff emulated by a couple of pre/post commit hooks. Am I the first person to want this? Does anyone have some code to do this?

Answer 1

What’s going on here: Git is optimized to switch between branches as quickly as possible. In particular, git checkout is designed to not touch any files that are identical in both branches.

Unfortunately, RCS keyword substitution breaks this. For example, using $Date$ would require git checkout to touch every file in the tree when switching branches. For a repository the size of the Linux kernel, this would bring everything to a screeching halt.

In general, your best bet is to tag at least one version:

$ git tag v0.5.whatever 

…and then call the following command from your Makefile:

$ git describe --tags v0.5.15.1-6-g61cde1d 

Here, git is telling me that I’m working on an anonymous version 6 commits past v0.5.15.1, with an SHA1 hash beginning with g61cde1d. If you stick the output of this command into a *.h file somewhere, you’re in business, and will have no problem linking the released software back to the source code. This is the preferred way of doing things.

If you can’t possibly avoid using RCS keywords, you may want to start with this explanation by Lars Hjemli. Basically, $Id$ is pretty easy, and you if you’re using git archive, you can also use $Format$.

But, if you absolutely cannot avoid RCS keywords, the following should get you started:

git config filter.rcs-keyword.clean 'perl -pe 's/\\\$Date[^\\\$]*\\\$/\\\$Date\\\$/'' git config filter.rcs-keyword.smudge 'perl -pe 's/\\\$Date[^\\\$]*\\\$/\\\$Date: `date`\\\$/''  echo '$Date$' > test.html echo 'test.html filter=rcs-keyword' >> .gitattributes git add test.html .gitattributes git commit -m 'Experimental RCS keyword support for git'  rm test.html git checkout test.html cat test.html 

On my system, I get:

$Date: Tue Sep 16 10:15:02 EDT 2008$ 

If you have trouble getting the shell escapes in the smudge and clean commands to work, just write your own Perl scripts for expanding and removing RCS keywords, respectively, and use those scripts as your filter.

Note that you really don’t want to do this for more files than absolutely necessary, or git will lose most of its speed.