Removing a keyword from git history

I recently had to remove a keyword from the git history of a project I was working on. This meant not just removing a file but modifying commits where the keyword was added, commits where the keyword was removed, and even commits with the keyword in the commit message. I eventually came to the right solution through a mix of blog posts and the documentation for git rebase.

For this example, assume the keyword is “matrix”. The example output shown is from the git repo for this website.

1. Use git log -S'matrix' -i --pretty=oneline (see docs for -S and -i) to see the list of commits that add or remove the string “matrix” (ignoring case).

   kyle@leibniz: kylrth.com$ git log -S'matrix' -i --pretty=oneline
   f46eb229ea3c4ba3324338711e75cfb75e9cb64f add Matrix social icon
   c64bb4bb358af98a05baa9b10b7de7abdcca890f update homepage, add posts, revamp theme
   1c5c71aeb429f6bb62c0a6b081b05fea3ca1a0bd start from pickles theme
   e275af820acb771b10ec2cb08c7e283da334af6d chore: import old site to Hugo
   

2. Use git log --grep=matrix -i --notes --pretty=oneline (see docs for --grep and --notes) to see the list of commits whose messages (or notes) contain the string “matrix” (ignoring case).

   kyle@leibniz: kylrth.com$ git log --grep=matrix -i --notes --pretty=oneline
   f46eb229ea3c4ba3324338711e75cfb75e9cb64f add Matrix social icon
   

3. Now determine which of the listed commits is the oldest between the two last commits listed by each command. In my example it’s easy to see that it’s e275af8, but if it’s not clear to you use git merge-base --is-ancestor ${oneHash} ${otherHash} && echo yes || echo no to determine if ${oneHash} is an ancestor of ${otherHash}.

4. Begin the interactive rebase by running the following rebase command, replacing e275af8 with the oldest commit as determined in the previous step. Be sure to include the ^ so that the rebase begins from the commit before your target commit.

   git rebase -i e275af8^ \
       --exec 'grep -qri --exclude-dir=.git matrix . ; if [ $? -ne 1 ]; then exit 1; fi' \
       --exec 'git log --notes | grep -qi matrix ; if [ $? -ne 1 ]; then exit 1; fi'
   

   (See docs for --exec.) The first --exec command runs after each modified commit, and interrupts the rebase if you accidentally commit the keyword. The second --exec command interrupts the rebase if you accidentally leave the keyword in a commit message or note.

Now you can be sure no instances of the keyword will be missed in your rebase.