Example
Given the following history, imagine you make a change that you want to squash into the commit bbb2222 A second commit:
$ git log --oneline --decorate
ccc3333 (HEAD -> master) A third commit
bbb2222 A second commit
aaa1111 A first commit
9999999 Initial commit
Once you've made your changes, you can add them to the index as usual, then commit them using the --fixup argument with a reference to the commit you want to squash into:
$ git add .
$ git commit --fixup bbb2222
[my-feature-branch ddd4444] fixup! A second commit
This will create a new commit with a commit message that Git can recognize during an interactive rebase:
$ git log --oneline --decorate
ddd4444 (HEAD -> master) fixup! A second commit
ccc3333 A third commit
bbb2222 A second commit
aaa1111 A first commit
9999999 Initial commit
Next, do an interactive rebase with the --autosquash argument:
$ git rebase --autosquash --interactive HEAD~4
Git will propose you to squash the commit you made with the commit --fixup into the correct position:
pick aaa1111 A first commit
pick bbb2222 A second commit
fixup ddd4444 fixup! A second commit
pick ccc3333 A third commit
To avoid having to type --autosquash on every rebase, you can enable this option by default:
$ git config --global rebase.autosquash true
