Interactive Rebase and Squash

Overview

Questions

• Why would I rebase a branch?
• When would I squash commits during a rebase?

Objectives

• Understand the reasons for interactive rebases.
• Remove amending commits from the history.

The process of getting a pull request or merge request accepted and merged into the upstream repository can require several updates to the original proposed changes. These changes may be functional fixes or just changes due to coding policies required for the project. This often results in multiple changes concerning the same piece of the code or document to be both spatially and temporally apart from each other in the Git history. For some projects a cluttered history is not acceptable and you may need to clean up the history.

Git provides a command to modify the history of a branch called an interactive rebase.

We’ve set up a branch that contains multiple commits modifying the same file to demonstrate how to use an interactive rebase to clean up the history.

Let’s first look at the history of our branch

BASH

git switch pie-recipes
git log --oneline pie-recipes

OUTPUT

7c77bba (HEAD -> pie-recipes, origin/pie-recipes) Complete pecan pie recipe instructions
f203cce Additional instructions to pecan pie recipe
c65036e Fix typo in ingredients
8613dde Add recipe for Pecan Pie with ingredients
3f40052 Add Apple Pie recipe
9761864 Initial commit with recipe files

For cleaning up the history we focus on the first 4 commit of the branch.

BASH

git rebase -i HEAD~4

Git will open an editor with a list of the requested commits in the format

OUTPUT

pick 5df0b61 Add recipe for Pecan Pie with ingredients
pick f34d3e0 Fix typo in ingredients
pick 80e1e0b Additional instructions to pecan pie recipe
pick 376a80c Complete pecan pie recipe instructions

# Rebase deeb7a6..376a80c onto deeb7a6 (4 commands)
#
# Commands:
# p, pick <commit> = use commit
# r, reword <commit> = use commit, but edit the commit message
# e, edit <commit> = use commit, but stop for amending
# s, squash <commit> = use commit, but meld into previous commit
# f, fixup [-C | -c] <commit> = like "squash" but keep only the previous
#                    commit's log message, unless -C is used, in which case
#                    keep only this commit's message; -c is same as -C but
#                    opens the editor
# x, exec <command> = run command (the rest of the line) using shell
# b, break = stop here (continue rebase later with 'git rebase --continue')
# d, drop <commit> = remove commit
# l, label <label> = label current HEAD with a name
# t, reset <label> = reset HEAD to a label
# m, merge [-C <commit> | -c <commit>] <label> [# <oneline>]
# .       create a merge commit using the original merge commit's
# .       message (or the oneline, if no original merge commit was
# .       specified); use -c <commit> to reword the commit message
#
# 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.
#

Git provides different options for <cmd> to modify the specific commit. Below you find a selection of commands that are most often used in basic interactive rebasing.

• pick: Use this commit
• reword: Use this commit, but adapt the commit message
• edit: Use commit, but pause to shell before committing
• squash: Use commit, but as part of the previous commit combining commit messages
• fixup: Like squash, but keep only a single commit message
• drop: Remove commit

Initially all commits are listed with pick, as this would recreate the same state as before the interactive rebase was started. You are now free to change the order of those commits as long as dependencies are retained.

In our example before, we could reorder and squash to keep two commits regarding Pecan Pie.

OUTPUT

pick 8613dde Add recipe for Pecan Pie with ingredients
fixup c65036e Fix typo in ingredients
pick f203cce Additional instructions to pecan pie recipe
pick 7c77bba Complete pecan pie recipe instructions

Now save this list and let Git apply the desired changes. After the rebase is complete, you can look at the rewritten history.

BASH

git log --oneline pie-recipes

OUTPUT

a03a64c (HEAD -> pie-recipes) Complete pecan pie recipe instructions
6c4a44a Additional instructions to pecan pie recipe
cf155bb Add recipe for Pecan Pie with ingredients
deeb7a6 Add Apple Pie recipe
e4b2098 Initial commit with recipe files

As we have changed the history, the branch at origin has diverged from the local branch. This will be noted in the current status of repository.

git status

OUTPUT

On branch pie-recipes
Your branch and 'origin/pie-recipes' have diverged,
and have 3 and 4 different commits each, respectively.
  (use "git pull" to merge the remote branch into yours)

nothing to commit, working tree clean

As this is the last step before final check and merge, it should be safe to force-push the changes to origin.

BASH

git push --force origin

OUTPUT

Enumerating objects: 15, done.
Counting objects: 100% (15/15), done.
Delta compression using up to 2 threads
Compressing objects: 100% (12/12), done.
Writing objects: 100% (12/12), 1.29 KiB | 1.29 MiB/s, done.
Total 12 (delta 5), reused 0 (delta 0), pack-reused 0
remote:
remote: To create a merge request for pie-recipes, visit:
remote:   https://gitlab.git.nrw/hartman/git-workshop-practice/-/merge_requests/new?merge_request%5Bsource_branch%5D=pie-recipes
remote:
To https://gitlab.git.nrw/hartman/git-workshop-practice.git
 + 7c77bba...062addc pie-recipes -> pie-recipes (forced update)

Challenge

Exercise 1: Cleaning up history in a feature branch

Clean up the history of your feature branch before the pull request/merge request is merged.

Show me the solution

tbd.

Key Points

• Use an interactive rebase to clean up merge requests before the merge.
• Rebased branches need to be force-pushed due to history changes.
• Squashing can be used to combine multiple commits
• Depending on the project policy merge requests may need to be cleaned up before they are allowed upstream.

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