git rebase provides a simple way of combining multiple commits into a single one. However using rebase to squash an entire branch down to a single commit is not completely straightforward.

Squashing normal commits

Using the following repository:

$ git log --oneline  
c172641 Fix second file  
24f5ad2 Another file  
97c9d7d Add first file

we can combine the last two commits (c172641 and 24f5ad2) by rebasing up to the first commit:

$ git rebase -i 97c9d7d

and specify the following commands in the interactive rebase screen:

pick 24f5ad2 Another file  
squash c172641 Fix second file

which will rewrite the history into this:

$ git log --oneline  
1a9d5e4 Another file  
97c9d7d Add first file

Rebasing the initial commit

Trying to include the initial commit in the interactive rebase screen will return this error:

$ git rebase -i 97c9d7d^  
fatal: Needed a single revision  
Invalid base

and squashing the top commit in the interactive rebase screen:

$ git rebase -i 97c9d7d  

squash 24f5ad2 Another file  
squash c172641 Fix second file

will return this error:

Cannot 'squash' without a previous commit

So we need to use a different approach to deal with the initial commit.

Amending the initial commit

Here is an alternative to rebase which will work on commits that don't have a parent.

Taking the previously rebased branch:

$ git log --oneline  
1a9d5e4 Another file  
97c9d7d Add first file

we can rewind the branch to the initial commit:

$ git reset 97c9d7d  
$ git log --oneline  
97c9d7d Add first file

without losing any of the changes introduced in 1a9d5e4 (shown here as uncommitted changes):

$ git status  
# On branch master  
# Changed but not updated:  
# (use "git add ..." to update what will be committed)  
# (use "git checkout -- ..." to discard changes in working directory)  
#  
# modified: file1  
#  
# Untracked files:  
# (use "git add ..." to include in what will be committed)  
#  
# file2  
no changes added to commit (use "git add" and/or "git commit -a")

Then we can reopen commit 97c9d7d and add the changes present in the working directory:

$ git add .  
$ git commit -a --amend -m "Initial version"

which will finally give us a fully squashed branch:

$ git log --oneline  
fcb85fb Initial version  

$ git status  
# On branch master  
nothing to commit (working directory clean)