Prepend string to messages of a range of git commits

Question

I have a range of commits in my local git repository with the following messages:

Added source.h
Broken link fixed
Corrected unit test
Deleted deprecated algorithm
...

Before pushing them to the server, I would like to prepend ticket information to every commit message:

Ticket #9999 (1): Added source.h
Ticket #9999 (2): Broken link fixed
Ticket #9999 (3): Corrected unit test
Ticket #9999 (4): Deleted deprecated algorithm
...

Of course I could git rebase and reword every sigle commit, but I would like to automate the process as it's so repetitive.

I think it should somehow be possible with git filter-branch, but the environment variables only provide GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL, GIT_AUTHOR_DATE, GIT_COMMITTER_NAME, GIT_COMMITTER_EMAIL, and GIT_COMMITTER_DATE, but not the message.

How can I access and automatically manipulate the commit messages?

EDIT: Note that the ticket number is always the same, but the number in the brackets is incremented successively.

Answer 1

Following the hint from Mark to use the --msg-filter option of filter-branch, the full solution is:

If the commit range is XXX..HEAD, the bracketed number can be expressed with git rev-list --count XXX..$GIT_COMIIT. Thus the messages can be transformed with sed:

git filter-branch --msg-filter '
    i=`git rev-list --count XXX..$GIT_COMIIT`
    sed "1s/^/Ticket #9999 ($i): /"
    ' XXX..HEAD