Forgot to sign-off commits?

README.md

No problem,

run

git filter-branch --msg-filter "cat - && echo && echo 'Signed-off-by: Your Name <Your.Name@example.com>'" HEAD~2..HEAD

To sign-off the last two commits.

Then force to push them to the remote repo with the -f option:

git push -f origin

(Source: http://stackoverflow.com/questions/13043357/git-sign-off-previous-commits)

How to apply to specific commit
When I run
git filter-branch --msg-filter "cat - && echo && echo 'Signed-off-by: Ganesh Satpute <name@gmail.com>'" HEAD~2..HEAD~1
I get
Which ref do you want to rewrite?

For signing off a specific commit you could do git commit --amend -s -c <SHA>

This approach works better https://confluence.atlassian.com/stashkb/how-do-you-make-changes-on-a-specific-commit-747831891.html. It uses interactive rebase

the command of jan25 works but with an uppercase S : git commit --amend -S -c <SHA>

if you also want the commit with the SHA itself: git commit --amend -S -c <SHA>^

git commit --amend --no-edit -S [SHA]
git push --force-with-lease [origin branch_name]