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Back to the hook: the first problem is how to identify the to-be-committed files? "git diff" lists the files:

git diff --cached --name-only --diff-filter=ACM

The options:

• --staged: lists all files which are stated for the next commit. --staged is an alias for --cached, both versions can be used.
• --name-only: lists only the filenames, not the entire diff.
• --diff-filter: selects what kind of files will be included in the diff.
  • A: Added files
  • C: Copied files
  • M: Modified files

The "git diff" above does not select "Renamed" or "Deleted" files. A renamed file might violate naming rules, in this case you have to include this option as well. And a deleted file is about to be removed from the repository, and most likely does not need to be checked.

Let's put this all together into a small shell script which runs as the git hook. In my example I want to enforce two things:

1. No double spaces in the file
2. No spaces at the end of the line

Here is the ".git/hooks/pre-commit" example:

#!/bin/bash

set -e

echo "pre-flight checks:"

file_list=`git diff --cached --name-only --diff-filter=ACM`

exit_status=0
while IFS= read -r file; do
    if [[ "$file" =~ ^content/post/* ]];
    then
        echo "checking file: $file ..."
        if grep -q '  ' "$file";
        then
            echo "Double spaces in file!"
            exit_status=1
        fi

        if egrep -q ' +$' "$file";
        then
            echo "Spaces at end of line!"
            exit_status=1
        fi
    fi
done <<< "$file_list"

if [ "$exit_status" -eq "0" ];
then
    echo "Pre-flight check passed!"
    exit 0
else
    echo "There are some errors ..."
    exit 1
fi

The first part identifies the changed files. The loop runs over all changed files, and checks them for the two conditons. The $exit_status variable is set by any error. Finally the last if-then-else part returns an error if there is any problem in the files.