git-compare

#!/bin/bash -e

# http://stackoverflow.com/questions/2969214/git-programmatically-know-by-how-much-the-branch-is-ahead-behind-a-remote-branc

usage () {
    # Call as: usage [EXITCODE] [USAGE MESSAGE]
    exit_code=1
    if [[ "$1" == [0-9] ]]; then
        exit_code="$1"
        shift
    fi
    if [ -n "$1" ]; then
        echo >&2 "$*"
        echo
    fi

    me=`basename $0`

    cat <<EOF >&2
Usage: $me [options] LEFT RIGHT

LEFT and RIGHT are "commit-ish" references.

Options:
  -h, --help     Show this help and exit
  -q             Quiet mode
      --sh       Output result as shell code to be source'd
options
EOF
    exit "$exit_code"
}

parse_opts () {
    shmode=

    while [ $# != 0 ]; do
        case "$1" in
            -h|--help)
                usage 0
                ;;
            --sh)
                shmode=yes
                shift
                ;;
            -*)
                usage "Unrecognised option: $1"
                ;;
            *)
                break
                ;;
        esac
    done

    if [ $# != 2 ]; then
        usage
    fi

    left="$1"
    right="$2"
}

main () {
    parse_opts "$@"

    set -- `git rev-list --count --left-right ${left}...${right}`
    behind="$1"
    ahead="$2"

    if [ "$1" = '-q' ]; then
        # quiet mode; just exit successfully iff we're in sync with upstream
        exit $(( behind + ahead ))
    fi

    if [ -n "$shmode" ]; then
        cat <<EOF
behind=$behind
ahead=$ahead
EOF
        exit
    fi

    if [ "$behind" = 0 -a "$ahead" = 0 ]; then
        echo "$right is in sync with $left"
    elif [ "$behind" != 0 -a "$ahead" != 0 ]; then
        echo "$right is behind $left by $behind commits, and ahead by $ahead"
    elif [ "$behind" = 0 ]; then
        echo "$right is ahead of $left by $ahead commits"
    elif [ "$ahead" = 0 ]; then
        echo "$right is behind $left by $behind commits"
    else
        echo "BUG in $0!" >&2
        exit 1
    fi
}

main "$@"