Frerich Raabe Frerich Raabe - 4 months ago 17
Git Question

How can I easily fixup a past commit?

I just read amending a single file in a past commit in git but unfortunately the accepted solution 'reorders' the commits, which is not what I want. So here's my question:

Every now and then, I notice a bug in my code while working on an (unrelated) feature. A quick

git blame
then reveals that the bug has been introduced a few commits ago (I commit quite a lot, so usually it's not the most recent commit which introduced the bug). At this point, I usually do this:

git stash # temporarily put my work aside
git rebase -i <bad_commit>~1 # rebase one step before the bad commit
# mark broken commit for editing
vim <affected_sources> # fix the bug
git add <affected_sources> # stage fixes
git commit -C <bad_commit> # commit fixes using same log message as before
git rebase --continue # base all later changes onto this

However, this happens so often that the above sequence is getting annoying. Especially the 'interactive rebase' is boring. Is there any shortcut to the above sequence, which lets me amend an arbitrary commit in the past with the staged changes? I'm perfectly aware that this changes the history, but I'm doing mistakes so often that I'd really love to have something like

vim <affected_sources> # fix bug
git add -p <affected_sources> # Mark my 'fixup' hungs for staging
git fixup <bad_commit> # amend the specified commit with staged changes,
# rebase any successors of bad commit on rewritten
# commit.

Maybe a smart script which can rewrite commits using plumbing tools or so?



A while ago, a new --fixup argument was added to git commit which can be used to construct a commit with a log message suitable for git rebase --interactive --autosquash. So the simplest way to fixup a past commit is now:

$ git add ...                           # Stage a fix
$ git commit --fixup=a0b1c2d3           # Perform the commit to fix broken a0b1c2d3
$ git rebase -i --autosquash a0b1c2d3~1 # Now merge fixup commit into broken commit


Here's a little Python script I wrote a while ago which implements this git fixup logic I hoped for in my original question. The script assumes that you staged some changes and then applies those changes to the given commit.

NOTE: This script is Windows-specific; it looks for git.exe and sets the GIT_EDITOR environment variable using set. Adjust this as needed for other operating systems.

Using this script I can implement precisely the 'fix broken sources, stage fixes, run git fixup ' workflow I asked for:

#!/usr/bin/env python
from subprocess import call
import sys

# Taken from python
def which(program):
    import os
    def is_exe(fpath):
        return os.path.exists(fpath) and os.access(fpath, os.X_OK)

    fpath, fname = os.path.split(program)
    if fpath:
        if is_exe(program):
            return program
        for path in os.environ["PATH"].split(os.pathsep):
            exe_file = os.path.join(path, program)
            if is_exe(exe_file):
                return exe_file

    return None

if len(sys.argv) != 2:
    print "Usage: git fixup <commit>"

git = which("git.exe")
if not git:
    print "git-fixup: failed to locate git executable"

broken_commit = sys.argv[1]
if call([git, "rev-parse", "--verify", "--quiet", broken_commit]) != 0:
    print "git-fixup: %s is not a valid commit" % broken_commit

if call([git, "diff", "--staged", "--quiet"]) == 0:
    print "git-fixup: cannot fixup past commit; no fix staged."

if call([git, "diff", "--quiet"]) != 0:
    print "git-fixup: cannot fixup past commit; working directory must be clean."

call([git, "commit", "--fixup=" + broken_commit])
call(["set", "GIT_EDITOR=true", "&&", git, "rebase", "-i", "--autosquash", broken_commit + "~1"], shell=True)