Squash commits with git rebase

That’s one thing I’ve just recently learned, and I’m writing down here for future reference.

Git’s rebase command permits to change the history of a git repository.
Note that this is safe to do only for commits that haven’t already been shared to the team you are working with.

Why is it useful?

Consider the following scenario:

bruno ~/git/hello-git
$ git hist
# git hist is a custom alias for "git log --pretty=format:'%h %ad | %s%d [%an]' --graph --date=short"
* 8a3a015 2016-10-26 | The most perfect commit in the history of the Universe (HEAD -> master) [brunoscopelliti]
* a48ca6d 2016-10-26 | Init repo [brunoscopelliti]

I’m so proud of myself… I’ve just pushed the best commit in the whole history of the Universe!

I should absolutely show someone else my greatness, so it’s time for a code review… and it turns out that the commit was not so perfect as I thought. However the fix is easy, and so we arrive here:

bruno ~/git/hello-git
$ git hist
* bd2b8f4 2016-10-26 | Ops, I've forgot something... Fixed now (HEAD -> master) [brunoscopelliti]
* 8a3a015 2016-10-26 | The most perfect commit in the history of the Universe [brunoscopelliti]
* a48ca6d 2016-10-26 | Init repo [brunoscopelliti]

It works, but the history is a bit messy, and going down this path, it won’t get better!

Git gives the option to squash two, or more commits into a single one using the rebase command.

How to squash commits?

Let’s try to squash those commits into a single one.

bruno ~/git/hello-git
$ git rebase -i HEAD~2

This cause your default Git editor - on my macbook pro, it is nano - to show up with the following content:

pick 5e170e0 The most perfect commit in the history of the Universe
pick bd2b8f4 Ops, I've forgot something... Fixed now
# Rebase a48ca6d..bd2b8f4 onto a48ca6d (2 commands)
#
# Commands:
# p, pick = use commit
# r, reword = use commit, but edit the commit message
# e, edit = use commit, but stop for amending
# s, squash = use commit, but meld into previous commit
# f, fixup = like "squash", but discard this commit log message
# x, exec = run command (the rest of the line) using shell
# d, drop = remove commit
#
# These lines can be re-ordered; they are executed from top to bottom.
#
# If you remove a line here THAT COMMIT WILL BE LOST.
#
# However, if you remove everything, the rebase will be aborted.
#
# Note that empty commits are commented out

We’re seeing only the last two commits cause I expressly requested to rebase only the last two (remember the HEAD~2).

The comments make pretty clear how to proceed; so I change the content.

pick 5e170e0 The most perfect commit in the history of the Universe
squash bd2b8f4 Ops, I've forgot something... Fixed now

Save, exit. Git will give you a reassuring successfull message at the end of the process.

bruno ~/git/hello-git
$ git rebase -i HEAD~2
[detached HEAD 7c442c5] The most perfect commit in the history of the Universe
 Date: Wed Oct 26 18:31:15 2016 +0200
 1 file changed, 10 insertions(+), 1 deletion(-)
Successfully rebased and updated refs/heads/master.

However let’s check.

bruno ~/git/hello-git
$ git hist
* 7c442c5 2016-10-26 | The most perfect commit in the history of the Universe (HEAD -> master) [brunoscopelliti]
* a48ca6d 2016-10-26 | Init repo [brunoscopelliti]

You may have noted that in the process the hash of the commit with the message The most perfect commit in the history of the Universe has changed. That’s because this process actually generates brand new commits with completely different IDs than the old commits, and leaves the old commits where they were.

bruno ~/git/hello-git
$ git cat-file -t bd2b8f4
commit

Common pitfalls

It’s not possible to squash a commit without a previous commit. So if you try this one

squash 5e170e0 The most perfect commit in the history of the Universe

pick bd2b8f4 Ops, I've forgot something... Fixed now

Git will throw an error at you.

bruno ~/git/hello-git
$ git rebase -i HEAD~2
error: Cannot 'squash' without a previous commit
You can fix this with 'git rebase --edit-todo' and then run 'git rebase --continue'.
Or you can abort the rebase with 'git rebase --abort'.

However Git is also so kind that suggest how to solve the issue:
git rebase --edit-todo will reopen the editor and give you a chance to review, then git rebase --continue will restart the rebase process.