How can I easily fixup a past commit?

Question

UPDATED ANSWER

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

ORIGINAL ANSWER

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 http://stackoverflow.com/questions/377017/test-if-executable-exists-in 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
    else:
        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>"
    sys.exit(1)

git = which("git.exe")
if not git:
    print "git-fixup: failed to locate git executable"
    sys.exit(2)

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
    sys.exit(3)

if call([git, "diff", "--staged", "--quiet"]) == 0:
    print "git-fixup: cannot fixup past commit; no fix staged."
    sys.exit(4)

if call([git, "diff", "--quiet"]) != 0:
    print "git-fixup: cannot fixup past commit; working directory must be clean."
    sys.exit(5)

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

What I do is:

git add ...           # Add the fix.
git commit            # Committed, but in the wrong place.
git rebase -i HEAD~5  # Examine the last 5 commits for rebasing.

Your editor will open with a list of the last 5 commits, ready to be meddled with. Change:

pick 08e833c Good change 1.
pick 9134ac9 Good change 2.
pick 5adda55 Bad change!
pick 400bce4 Good change 3.
pick 2bc82n1 Fix of bad change.

...to:

pick 08e833c Good change 1.
pick 9134ac9 Good change 2.
pick 5adda55 Bad change!
f 2bc82n1 Fix of bad change. # Move up, and change 'pick' to 'f' for 'fixup'.
pick 400bce4 Good change 3.

Save & exit your editor, and the fix will be squished back into the commit it belongs with.

After you've done that a few times, you'll do it in seconds in your sleep. Interactive rebasing is the feature that really sold me on git. It's incredibly useful for this and more...

A bit late to the party, but here is a solution that works as the author imagined.

Add this to your .gitconfig:

[alias]
    fixup = "!sh -c '(git diff-files --quiet || (echo Unstaged changes, please commit or stash with --keep-index; exit 1)) && COMMIT=$(git rev-parse $1) && git commit --fixup=$COMMIT && git rebase -i --autosquash $COMMIT~1' -"

Example usage:

git add -p
git fixup HEAD~5

However if you have unstaged changes, you must stash them before the rebase.

git add -p
git stash --keep-index
git fixup HEAD~5
git stash pop

You could modify the alias to stash automatically, instead of giving a warning. However, if the fixup does not apply cleanly you will need pop the stash manually after fixing the conflicts. Doing both the saving and popping manually seems more consistent and less confusing.

To fixup one commit :

git commit --fixup a0b1c2d3 .
git rebase --autosquash -i HEAD~2

where a0b1c2d3 is commit that you want fixup and where 2 is the number of commits +1 pasted that you want to change.

Note: git rebase --autosquash without -i doesn't worked but with -i worked, which is strange.

UPDATE: A cleaner version of the script can now be found here: https://github.com/deiwin/git-dotfiles/blob/docs/bin/git-fixup.

I've been looking for something similar. This Python script seems too complicated, though, therefore I've hammered together my own solution:

First, my git aliases look like that (borrowed from here):

[alias]
  fixup = !sh -c 'git commit --fixup=$1' -
  squash = !sh -c 'git commit --squash=$1' -
  ri = rebase --interactive --autosquash

Now the bash function becomes quite simple:

function gf {
  if [ $# -eq 1 ]
  then
    if [[ "$1" == HEAD* ]]
    then
      git add -A; git fixup $1; git ri $1~2
    else
      git add -A; git fixup $1; git ri $1~1
    fi
  else
    echo "Usage: gf <commit-ref> "
  fi
}

This code first stages all current changes(you can remove this part, if you wish to stage the files yourself). Then creates the fixup(squash can also be used, if that's what you need) commit. After that it starts an interactive rebase with the --autosquash flag on the parent of the commit you give as the argument. That will open your configured text editor, so you could verify that everything is as you expect and simply closing the editor will finish the process.

The if [[ "$1" == HEAD* ]] part (borrowed from here) is used, because if you use, for example, HEAD~2 as your commit(the commit you want to fix current changes up with) reference then the HEAD will be displaced after the fixup commit has been created and you would need to use HEAD~3 to refer to the same commit.

What really bothered me about the fixup workflow was that I had to figure out myself which commit I wanted to squash the change into every time. I created a "git fixup" command that helps with this.

This command creates fixup commits, with the added magic that it uses git-deps to automatically find the relevant commit, so the workflow often comes down to:

# discover and fix typo in a previously committed change
git add -p # stage only typo fix
git fixup

# at some later point squash all the fixup commits that came up
git rebase --autosquash master

This only works if the staged changes can be unambiguously attributed to a particular commit on the working tree (between master and HEAD). I find that is the case very often for the type of small changes I use this for, e.g. typos in comments or names of newly introduced (or renamed) methods. If this is not the case, it will at least display a list of candidate commits.

I use this a lot in my daily workflow, to quickly integrate small changes to previously changed lines into commits on my working branch. The script is not as beautiful as it could be, and it's written in zsh, but it has been doing the job for me well enough for a good while now that I never felt the need to rewrite it:

https://github.com/Valodim/git-fixup

You can avoid the interactive stage by using a "null" editor:

$ EDITOR=true git rebase --autosquash -i ...

This will use /bin/true as the editor, instead of /usr/bin/vim. It always accepts whatever git suggests, without prompting.