#TIL: Using git rebase to combine commits

I normally use git rebase with --fixup commits, and haven't had a need [until now] to take a bunch of commits made at different times and combine them.

Where possible, I prefer commits on the main branch to represent some "unit" of work¹, to prevent a bunch of commits where you might be going back and forth on something from cluttering up the history².

The PRs³ I submit day-to-day are normally isolated features that make this easy to accomplish. I use rebase a lot within a branch for myself before submitting a PR, but the extent is normally:

$ git add -p                           # add the specific thing I want to revise
$ git commit --fixup <hash>            # mark this as a fixup
$ git rebase -i --autosquash <hash>~1  # go to 1 commit _before_ the commit to be fixed

However, in building this blog there's been a few times where I added bits of things at different times, 20+ commits apart, and (for whatever reason⁴) didn't realize I wanted them all combined at the time of committing.

Specifically, I wanted to combine some commits to know "here's everything I did to get <specific feature> working".

Fortunately, git rebase makes this easy and includes helpful instructions in the rebase message:

These lines can be re-ordered; they are executed from top to bottom.

It's clearly laid out if you follow along on the git docs and is as simple as re-ordering the lines so that the commits to be combined are together, and marking them as squash.

For example, given a repo where these are all the commits:

$ git log --oneline
60d10b7 (HEAD -> example) finally figured out A
e68f105 did something for C
f8b495c hack related to A
2ce4970 did something for B
240ffc7 did something for A
ff37364 initial

Using git rebase -i 240ffc7~1 will show you:

pick 240ffc7 did something for A
pick 2ce4970 did something for B
pick f8b495c hack related to A
pick e68f105 did something for C
pick 60d10b7 finally figured out A

# Rebase ff37364..60d10b7 onto ff37364 (5 commands)

If you want to combine everything related to A (the 3 lines above), you can just reorder them like below and mark the two commits to be merged as squash:

pick 240ffc7 did something for A
squash f8b495c hack related to A
squash 60d10b7 finally figured out A
pick 2ce4970 did something for B
pick e68f105 did something for C

# Rebase ff37364..60d10b7 onto ff37364 (5 commands)

Then just save and close, and when you're done ⁵ it will look like:

$ git log --oneline
179c64f (HEAD -> example) did something for C
092aa5f did something for B
79f744b did something for A
ff37364 initial