Git hook to prepend the issue number to the commit message

prepare-commit-msg.sh

#!/bin/sh

exec < /dev/tty

detect_issue() {
    local branch="$(git branch | grep '^\*' | sed 's/^\* //')"
    local issue=$(echo "$branch" | grep -o '[0-9][0-9]*' | head -n 1)
    if [ -z "$issue" ]; then
        return 1
    fi
    echo "$issue"
}

check_commit_msg() {
    local issue=$(detect_issue)
    if [ -z "$issue" ]; then
        return 0
    fi

    local detected_issue=$(head -n 1 "$1" | \
        grep -o '^[0-9][0-9]*:' | \
        grep -o '^[0-9][0-9]*' )
    if [ -n "$detected_issue" ]; then
        if [ "$detected_issue" != "$issue" ]; then
            echo "$detect_issue"
            return 1
        else
            return 0
        fi
    fi

    return 1
}

case "$2,$3" in
    message,)
        check_commit_msg "$1" > /dev/null
        if [ $? -ne 0 ]; then
            printf "No issue number at head of commit. Want to prepend? [Y/n] "
            read choice
            if [ -z "$choice" -o \
                 "$choice" = "Y" -o \
                 "$choice" = "y" -o \
                 "$choice" = "yes" ]; then
                
                printf "$(detect_issue ): " | cat - "$1" > "$1.$$"
                mv "$1.$$" "$1"
            elif [ "$choice" = "n" -o \
                   "$choice" = "N" -o \
                   "$choice" = "no" ]; then
                :
            else
                echo "Invalid choice. Aborting..." >&2
                exit 1
            fi
        fi
        ;;
    ,)
        issue=$(detect_issue)
        if [ $? -eq 0 ]; then
            printf "$issue: " | cat - "$1" >> "$1.$$"
            mv "$1.$$" "$1"
        fi
        ;;
esac

# SOB=$(git var GIT_AUTHOR_IDENT | sed -n 's/^\(.*>\).*$/Signed-off-by: \1/p')
# grep -qs "^$SOB" "$1" || echo "$SOB" >> "$1"