What does this git diff-index --quiet HEAD --; mean?

Question

I found the following command in a shell script:

 git diff-index --quiet HEAD --;

What does this command mean/do?

Answer 1

It can be used to help determine whether there are any changes in a working tree by comparing your working tree to the HEAD revision. The --quiet means to silence the output and alter the exit code based on whether the tree is modified (exits with 1) or is not modified (exits with 0). The -- is used to separate paths from the rest of the argument. This helps Git know that HEAD is actually the name of a treeish, rather than confuse it with the name of a file should a file called HEAD exist in your tree.

The short form: if the script is using set -e, then the script will exit with an error if your working tree has changes in it.