Git sign off previous commits?

These days (starting with Git 2.13) you can generally do something like

git rebase --signoff HEAD~2

to add Signed-off-by footers to the last 2 commits (in this example).

If your range includes the root commit, add the --root option to rebase.

To signoff the previous commit, use amend option:

git commit --amend --signoff

Edit: the amend does signoff only the latest commit. To signoff multiple commits, filter-branch and interpret-trailers as suggested by vonc et. al. should be used. Here is what worked for me.

First, configure git to replace the token sign by Signed-off-by. This has to be done only once and is needed in the next step.

git config trailer.sign.key "Signed-off-by"

The command git filter-branch with the switch --msg-filter will eval the filter once for each commit. The filter can be any shell command that receives the commit message on stdin and outputs on stdout. You can write your own filter, or use git interpret-trailers, which is indepotent. Here is an example that will signoff the latest two commits of the current branch using the current user and email:

export SIGNOFF="sign: $(git config --get user.name) <$(git config --get user.email)>"
git filter-branch -f --msg-filter \
    "git interpret-trailers --trailer \"$SIGNOFF\"" \
     HEAD~2..HEAD

Note 1) Modifying commit messages change the commit id, which means pushing over already published branches will have to be forced either with --force or better --force-with-lease.

Note 2) if you intend to write your custom script, beware that git filter-branch changes the current directory to <repo>/.git-rewrite/t. Using a relative path to the script won't usually work. Instead, the script should be in your $PATH or provided as an absolute path.