TIL: In place file updates in Bash

I’ve learned this one several times over the years so I’m finally writing it down. And this time I learned about a GNU utility called sponge that addresses this exact problem. It’s not always available though.

This doesn’t work:

echo test > foo
cat foo > foo

You’ll end up with an empty file. The solution is to either write to a temp file and write over the original, or use sponge:

echo test > foo
cat foo | sponge foo

You can get sponge (and some other goodies) on a Mac with:

brew install moreutils