Rewrite Git history to prefix a range of commit messages —15 August 2016

Imagine that you cloned an open source project to contribute something. You implemented a bugfix through a series of atomic commits on a private branch. Just when you’re about to create a Pull Request to submit your changes, you discover in the contributor’s guide that you’re supposed to prefix each commit with the bug tracking number.

Rewriting the commit message of the last commit is easy:

git commit --amend

Rewriting a range of commits is easy too, once you know how. That’s a job for the filter-branch command, like this:

git filter-branch --msg-filter 'printf "THE_PREFIX " && cat' master..

The --msg-filter flag is to rewrite the commit messages. The argument within the '...' is the shell commands to execute to manipulate the message, received from stdin. In this example we use the printf command to print the prefix and a space character, and then the cat command to print the rest of the commit message. (Note that printf is like the less portable echo -n.)

The last argument is the range of commits, master.. is equivalent to master..HEAD. This is short and sweet and perfect in this use case of working on a feature branch that started from master.