Formatting commit hooks done right

post-commit.sh

#!/bin/bash
set -e -x

if [ -a .commit ]
then
    rm .commit

    # We format partially-staged files in the pre-commit hook, as that's the only safe way.
    # If a partially-staged file needed reformatting, it's been re-formatted in pre-commit,
    # and the commit has been cancelled so that the user can re-add the re-formmatted hunks.
    # Including partially-staged files below would include the user's unstaged changes in the reformatting commit.
    FULLY_STAGED=$( comm -23 <(git diff --name-only HEAD^..HEAD | sort) <(git diff --name-only | sort) )
    FULLY_STAGED_SCALA=$( echo "${FULLY_STAGED[*]}" | grep -E '\.scala$|\.sbt$' || true )
    if [ -n "$FULLY_STAGED_SCALA" ]
    then
        echo "$FULLY_STAGED_SCALA" | xargs -d '\n' time scalafmt --non-interactive || true
        AUTHOR="Scalafmt / $( git config user.name ) <$( git config user.email )>"
        git diff --exit-code --quiet || \
          echo "$FULLY_STAGED_SCALA" | xargs -d '\n' git commit --fixup=HEAD --no-verify --author "$AUTHOR" --
    fi
fi
exit

pre-commit.sh

#!/bin/bash
set -e -x

#TODO add scalafmt version check to ensure fast execution

# If a partially-staged file gets reformatted, there's no way to tell what part
# of the reformatted file should make it into the commit. In that case, we ask the user
# to re-do the commit using the reformatted file.
PARTIALLY_STAGED=$( comm -12 <(git diff --name-only | sort) <(git diff --name-only --cached | sort) )
if [ -n "$PARTIALLY_STAGED" ]
then
    PATIALLY_STAGED_SCALA=$(echo "${PARTIALLY_STAGED[*]}" | grep -E '\.scala$|\.sbt$' || true)
    if [ -n "$PATIALLY_STAGED_SCALA" ]
    then
        PRE_DIFF=$( echo "$PATIALLY_STAGED_SCALA" | xargs -d '\n' git diff HEAD -- )
        echo "$PATIALLY_STAGED_SCALA" | xargs -d '\n' scalafmt --non-interactive > /dev/null || true
        POST_DIFF=$( echo "$PATIALLY_STAGED_SCALA" | xargs -d '\n' git diff HEAD -- )
        if [ "$PRE_DIFF" != "$POST_DIFF" ]
        then
            echo "One of partially-staged files was re-formatted."
            echo "You'll need to do the commit again :("
            exit 1
        fi
    fi
fi
touch .commit
exit