Squash my last X commits together using Git

How can I squash my last X commits together into one commit using Git?

You can do this fairly easily without git rebase or git merge --squash. In this example, we'll squash the last 3 commits.

If you want to write the new commit message from scratch, this suffices:

git reset --soft HEAD~3 &&
git commit

If you want to start editing the new commit message with a concatenation of the existing commit messages (i.e. similar to what a pick/squash/squash/…/squash git rebase -i instruction list would start you with), then you need to extract those messages and pass them to git commit:

git reset --soft HEAD~3 && 
git commit --edit -m"$(git log --format=%B --reverse HEAD..HEAD@{1})"

Both of those methods squash the last three commits into a single new commit in the same way. The soft reset just re-points HEAD to the last commit that you do not want to squash. Neither the index nor the working tree are touched by the soft reset, leaving the index in the desired state for your new commit (i.e. it already has all the changes from the commits that you are about to “throw away”).

Use git rebase -i <after-this-commit> and replace "pick" on the second and subsequent commits with "squash" or "fixup", as described in the manual.

In this example, <after-this-commit> is either the SHA1 hash or the relative location from the HEAD of the current branch from which commits are analyzed for the rebase command. For example, if the user wishes to view 5 commits from the current HEAD in the past the command is git rebase -i HEAD~5.

You can use git merge --squash for this, which is slightly more elegant than git rebase -i. Suppose you're on master and you want to squash the last 12 commits into one.

WARNING: First make sure you commit your work—check that git status is clean (since git reset --hard will throw away staged and unstaged changes)

Then:

# Reset the current branch to the commit just before the last 12:
git reset --hard HEAD~12

# HEAD@{1} is where the branch was just before the previous command.
# This command sets the state of the index to be as it would just
# after a merge from that commit:
git merge --squash HEAD@{1}

# Commit those squashed changes.  The commit message will be helpfully
# prepopulated with the commit messages of all the squashed commits:
git commit

The documentation for git merge describes the --squash option in more detail.

Update: the only real advantage of this method over the simpler git reset --soft HEAD~12 && git commit suggested by Chris Johnsen in his answer is that you get the commit message prepopulated with every commit message that you're squashing.