Check if git branch could be successfully merged via fast-forward.

ff-merge-check.sh

#!/bin/sh
set -e

# Check if HEAD and REF can be related by a fast-forward merge.
# Requires that either HEAD or REF be the merge-base between the two branches
usage () {
    cat << EOM
    $0 -v -h REF
    Checks if HEAD and REF can be fast-forwared to each other.

    exits with non-zero status (2) if a --ff-only merge would fail.
    
      -q:   be quiet
      -v:   be verbose
      -h:   show this help
EOM
    exit 0
}

while getopts ":qvh" opt; do
    case $opt in
        q)
            QUIET=1
            ;;
        v)
            VERBOSE=1
            ;;
        h)
            usage
            ;;
        \?)
            echo "Invalid option: -$OPTARG" >&2
            exit 1
            ;;
    esac
done
shift $(($OPTIND - 1))

if [ $# -ne 1 ]; then
    usage
fi

verbose () {
    if [ ${VERBOSE:0} > 0 ]; then
        echo $1
    fi
}

quiet () {
    if [ -z "$QUIET" ]; then
        echo $1
    fi
}

OTHER_REF=$1

OTHER_SHA=$(git rev-parse --verify $OTHER_REF^{object})
HEAD_SHA=$(git rev-parse HEAD)
MERGE_BASE=$(git merge-base HEAD $OTHER_REF)

verbose "HEAD_SHA: $HEAD_SHA"
verbose "OTHER_SHA: $OTHER_SHA"
verbose "MERGE_BASE: $MERGE_BASE"

if [ "$HEAD_SHA" = "$OTHER_SHA" ]; then
    verbose "Already up-to-date."
elif [ "$HEAD_SHA" = "$MERGE_BASE" -o "$OTHER_SHA" = "$MERGE_BASE" ]; then
    verbose "Clean to fast-forward merge"
else
    quiet "Needs to be rebased"
    exit 2
fi