What is the difference between merge --squash and rebase?

Question

I'm new to git and I'm trying to understand the difference between a squash and a rebase. As I understand it you perform a squash when doing a rebase.

Answer 1

Merge commits: retains all of the commits in your branch and interleaves them with commits on the base branch

Merge Squash: retains the changes but omits the individual commits from history

Rebase: This moves the entire feature branch to begin on the tip of the master branch, effectively incorporating all of the new commits in master

More on here

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The first two diagrams come from About pull request merges on the GitHub Docs

Answer 2

Both git merge --squash and git rebase --interactive can produce a "squashed" commit. But they serve different purposes.

• git merge --squash abranch

will produce a squashed commit on the destination branch, without marking any merge relationship. (Note: it does not produce a commit right away: you need an additional git commit -m "squash branch")

This is useful if you want to throw away the source branch completely, going from (schema taken from SO question):

git checkout stable            X               stable          / a---b---c---d---e---f---g tmp 

to:

git merge --squash tmp git commit -m "squash tmp"   # In the following graph, G is c--d--e--f--g squashed together            X-------------G stable          / a---b---c---d---e---f---g tmp 

and then deleting tmp branch.

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Note: git merge has a --commit option, but it cannot be used with --squash. It was never possible to use --commit and --squash together. Since Git 2.22.1 (Q3 2019), this incompatibility is made explicit:

See commit 1d14d0c (24 May 2019) by Vishal Verma (reloadbrain). ^{(Merged by Junio C Hamano -- gitster -- in commit 33f2790, 25 Jul 2019)}

merge: refuse --commit with --squash

Previously, when --squash was supplied, 'option_commit' was silently dropped. This could have been surprising to a user who tried to override the no-commit behavior of squash using --commit explicitly.

git/git builtin/merge.c#cmd_merge() now includes:

if (option_commit > 0)     die(_("You cannot combine --squash with --commit.")); 

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• git rebase --interactive

replays some or all of your commits on a new base, allowing you to squash (or more recently "fix up", see this SO question), going directly to:

git checkout tmp git rebase -i stable     stable       X----------------G tmp      / a---b 

If you choose to squash all commits of tmp (but, contrary to merge --squash, you can choose to replay some, and squashing others).

So the differences are:

• squash does not touch your source branch (tmp here) and creates a single commit where you want.
• rebase allows you to go on on the same source branch (still tmp) with:
  • a new base
  • a cleaner history