Commit only part of a file in Git

You can use git add --patch <filename> (or -p for short), and git will begin to break down your file into what it thinks are sensible "hunks" (portions of the file). It will then prompt you with this question:

Stage this hunk [y,n,q,a,d,/,j,J,g,s,e,?]?

Here is a description of each option:

• y stage this hunk for the next commit
• n do not stage this hunk for the next commit
• q quit; do not stage this hunk or any of the remaining hunks
• a stage this hunk and all later hunks in the file
• d do not stage this hunk or any of the later hunks in the file
• g select a hunk to go to
• / search for a hunk matching the given regex
• j leave this hunk undecided, see next undecided hunk
• J leave this hunk undecided, see next hunk
• k leave this hunk undecided, see previous undecided hunk
• K leave this hunk undecided, see previous hunk
• s split the current hunk into smaller hunks
• e manually edit the current hunk
  • You can then edit the hunk manually by replacing +/- by # (thanks veksen)
• ? print hunk help

If the file is not in the repository yet, you can first do git add -N <filename>. Afterwards you can go on with git add -p <filename>.

Afterwards, you can use:

• git diff --staged to check that you staged the correct changes
• git reset -p to unstage mistakenly added hunks
• git commit -v to view your commit while you edit the commit message.

Note this is far different than the git format-patch command, whose purpose is to parse commit data into a .patch files.

Reference for future: Git Tools - Interactive Staging

You can use git add --interactive or git add -p <file>, and then git commit (not git commit -a); see Interactive mode in git-add manpage, or simply follow instructions.

Modern Git has also git commit --interactive (and git commit --patch, which is shortcut to patch option in interactive commit).

If you prefer doing it from GUI, you can use git-gui. You can simply mark chunks which you want to have included in commit. I personally find it easier than using git add -i. Other git GUIs, like QGit or GitX, might also have this functionality as well.

git gui provides this functionality under the diff view. Just right click the line(s) you're interested in and you should see a "stage this line to commit" menu item.