Post-receive git hook to check if file changed

post-receive

#!/bin/bash
WATCH_FILE="abc.123"
while read oldrev newrev refname; do
    if [ "$refname" = "refs/heads/master" ]; then
        if git diff-tree --name-only -r -z $oldrev $newrev | grep --quiet $WATCH_FILE ; then
            # WATCH_FILE changed...
        fi
    fi
done

1. git diff-tree --name-only -r -z $oldrev $newrev | grep --quiet $WATCH_FILE
   will search any substring which may be part of filename, soo it return true for 'foo${WATCH_FILE}bar'

! git diff-tree --exit-code --name-only -r -z $oldrev $newrev -- "$WATCH_FILE"
note: exit code will be 1 if $WATCH_FILE changed

2. $oldrev is 0000000000000000000000000000000000000000 for new branch, so git diff-tree will fail for each new branch. As long as you are limited only by existing branches, then everything is correct. But in general, this does not work.