Git pre-commit hook to check for AWS keys

pre-commit

#!/usr/bin/env bash

# Install globally using https://coderwall.com/p/jp7d5q/create-a-global-git-commit-hook		
# The checks are simple and can give false positives. Amend the hook in the specific repository.

if git rev-parse --verify HEAD >/dev/null 2>&1
then
    against=HEAD
else
    # Initial commit: diff against an empty tree object
    EMPTY_TREE=$(git hash-object -t tree /dev/null)
    against=$EMPTY_TREE
fi

# Redirect output to stderr.
exec 1>&2
 
# Check changed files for an AWS keys
FILES=$(git diff --cached --name-only $against)

if [ -n "$FILES" ]; then
    KEY_ID=$(grep -rE --line-number '(^|[^A-Za-z0-9/+=])AKIA[A-Z0-9]{16}($|[^A-Za-z0-9/+=])' $FILES)
    KEY=$(grep -rE --line-number '(^|[^A-Za-z0-9/+=])[A-Za-z0-9/+=]{40}($|[^A-Za-z0-9/+=])' $FILES)

    if [ -n "$KEY_ID" ] || [ -n "$KEY" ]; then
        exec < /dev/tty # Capture input
        echo "=========== Possible AWS Access Key IDs ==========="
        echo "${KEY_ID}"
        echo ""

        echo "=========== Possible AWS Secret Access Keys ==========="
        echo "${KEY}"
        echo ""

        while true; do
            read -p "[AWS Key Check] Possible AWS keys found. Commit files anyway? (y/N) " yn
            if [ "$yn" = "" ]; then
                yn='N'
            fi
            case $yn in
                [Yy] ) exit 0;;
                [Nn] ) exit 1;;
                * ) echo "Please answer y or n for yes or no.";;
            esac
        done
        exec <&- # Release input
    fi
fi

# Normal exit
exit 0