git-mergeable

#!/bin/bash -e

# Prints appropriate usage
display_usage()
{
    echo "Usage: $0 [-h | --help] [-v | --verbose] BRANCH_TO BRANCH_FROM"
    HELP_TEXT="Runs git merge-base --is-ancestor to determine if BRANCH_TO can "
    HELP_TEXT+="be fast-forwarded onto BRANCH_FROM."
    echo "${HELP_TEXT}" | fold -s

    echo "  BRANCH_TO         Merging to this branch"
    echo "  BRANCH_FROM       Merging from this branch"
    echo "  -h | --help       Displays this help text"
}

BRANCH_TO=
BRANCH_FROM=
POSITIONAL_ARG_COUNT=0

while [ -n "${1}" ]; do

    case "${1}" in

        -h | --help)
            display_usage
            exit 0
            ;;

        *)
            # These are the positional arguments so we have to count.
            POSITIONAL_ARG_COUNT=$((POSITIONAL_ARG_COUNT + 1))

            case "${POSITIONAL_ARG_COUNT}" in
                1)
                    BRANCH_TO="${1}"
                    ;;
                2)
                    BRANCH_FROM="${1}"
                    ;;
                *)
                    echo "Too many positional arguments" 2>&1
                    exit 1
                    ;;
            esac
            ;;
    esac

    # We're done with the current argument, move to the next.
    shift

done

# Actually do the check
if ! git merge-base --is-ancestor "${BRANCH_TO}" "${BRANCH_FROM}"; then
    echo "Not an ancestor"
    exit 1
fi