Whitespace matters.

Git has support for checking whitespace in patches. git apply and git am have the option --whitespace, which can be used to warn or error about whitespace errors in the patches about to be applied. git diff has the option --check to check a change for whitespace errors.

But all this assumes that your existing code is cool, and only new changes are candidates for problems. Curiously, it is a bit hard to use those same tools for going back and checking whether an existing tree satisfies the whitespace rules applied to new patches.

The core of the whitespace checking is in git diff-tree. With the --check option, you can check the whitespace in the diff between two objects.

But how do you check the whitespace of a tree rather than a diff? Basically, you want

git diff-tree --check EMPTY HEAD

except there is no EMPTY. But you can compute the hash of an empty Git tree:

git hash-object -t tree /dev/null

So the full command is

git diff-tree --check $(git hash-object -t tree /dev/null) HEAD

If have this as an alias in my ~/.gitconfig:

[alias]
    check-whitespace = !git diff-tree --check $(git hash-object -t tree /dev/null) HEAD

Then running

git check-whitespace

can be as easy as running make or git commit.