Question

I’m trying to understand the difference between a squash and a rebase. As I understand it, one performs a squash when doing a rebase.

Answer 1

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 d--e--f--g squashed together

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

and then deleting tmp branch.

---

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."));

---

• 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