git diff

git diff can be used to check if the working directory is dirty (assuming you don’t care about untracked files):

$ git diff HEAD

If files are modified, then it will output text. If the working directory is clean, then it will output nothing.

Using the conditional if statement:

#!/bin/bash
if [[ $(git diff --stat) != '' ]]; then
  echo 'dirty'
else
  echo 'clean'
fi

Or a logical operator:

$ git diff --quiet || echo 'dirty'

git status

But to check for the presence of untracked files, you’ll need git status:

$ git status --short

For example, if LICENSE is created and README.md is modified:

$ git status -s
M  README.md
?? LICENSE

Using a logical operator to check if working tree has modified/untracked files (-z tests if the string is null or empty):

$ [[ -z $(git status -s) ]] || echo 'modified/untracked'

And using a logical operator to check if working tree is clean (-n tests if the string is not empty):

$ [[ -n $(git status -s) ]] || echo 'clean'

Also, there’s the option --porcelain which formats the output like --short. But because it’s a high level command, it’s known to be slow for larger repositories.