How would I extract a single file (or changes to a file) from a git stash?

On the git stash manpage you can read (in the "Discussion" section, just after "Options" description) that:

A stash is represented as a commit whose tree records the state of the working directory, and its first parent is the commit at HEAD when the stash was created.

So you can treat stash (e.g. stash@{0} is first / topmost stash) as a merge commit, and use:

$ git diff stash@{0}^1 stash@{0} -- <filename>

Explanation: stash@{0}^1 means the first parent of the given stash, which as stated in the explanation above is the commit at which changes were stashed away. We use this form of "git diff" (with two commits) because stash@{0} / refs/stash is a merge commit, and we have to tell git which parent we want to diff against. More cryptic:

$ git diff stash@{0}^! -- <filename>

should also work (see git rev-parse manpage for explanation of rev^! syntax, in "Specifying ranges" section).

Likewise, you can use git checkout to check a single file out of the stash:

$ git checkout stash@{0} -- <filename>

or to save it under another filename:

$ git show stash@{0}:<full filename>  >  <newfile>

or

$ git show stash@{0}:./<relative filename> > <newfile>

(note that here <full filename> is full pathname of a file relative to top directory of a project (think: relative to stash@{0})).

You might need to protect stash@{0} from shell expansion, i.e. use "stash@{0}" or 'stash@{0}'.

If you use git stash apply rather than git stash pop, it will apply the stash to your working tree but still keep the stash.

With this done, you can add/commit the file that you want and then reset the remaining changes.

Edit: See cambunctious's answer, which is basically what I now prefer because it only uses the changes in the stash, rather than comparing them to your current state. This makes the operation additive, with much less chance of undoing work done since the stash was created.

To do it interactively, you would first do

git diff stash^! -- path/to/relevant/file/in/stash.ext perhaps/another/file.ext > my.patch

...then open the patch file in a text editor, alter as required, then do

git apply < my.patch

cambunctious's answer bypasses the interactivity by piping one command directly to the other, which is fine if you know you want all changes from the stash. You can edit the stash^! to be any commit range that has the cumulative changes you want (but check over the output of the diff first).

If applying the patch/diff fails, you can change the last command to git apply --reject which makes all the changes it can, and leaves .rej files where there are conflicts it can't resolve. The .rej files can then be applied using wiggle, like so:

wiggle --replace path/to/relevant/file/in/stash.ext{,.rej}

This will either resolve the conflict, or give you conflict markers that you'd get from a merge.

If your distro doesn't have wiggle, you can just build it:

cd /usr/local/src/git clone git://git.neil.brown.name/wigglecd wiggle/git checkout v1.3make install

Previous solution: There is an easy way to get changes from any branch, including stashes:

$ git checkout --patch stash@{0} path/to/file

You may omit the file spec if you want to patch in many parts. Or omit patch (but not the path) to get all changes to a single file. Replace 0 with the stash number from git stash list, if you have more than one. Note that this is like diff, and offers to apply all differences between the branches. To get changes from only a single commit/stash, have a look at git cherry-pick --no-commit.