A script to determine whether one git commit is the ancestor of another

git-is-ancestor

#!/bin/bash
# 
# git-is-ancestor, by Simon Whitaker
# 
# Suggested usage
# 
# Store this file somewhere, make it executable, then alias
# it to git is-ancestor by putting this in your $HOME/.gitconfig:
# 
# [alias]
#   is-ancestor = !/path/to/git-is-ancestor
# 
# Then use thus:
#
# $ git is-ancestor rev1 rev2

script_name=$(basename $0)

usage()
{
    cat << EOF >&2
usage: ${script_name} <REV1> <REV2>
EOF
}

if [ $# -ne 2 ]; then
    usage
    exit 2
fi

if $( git merge-base --is-ancestor $1 $2 ); then
    echo "$1 is an ancestor of $2"
    exit 0
elif $( git merge-base --is-ancestor $2 $1 ); then
    echo "$2 is an ancestor of $1"
    exit 0
else
    echo "$1 and $2 are not related"
    exit 1
fi

if $( git merge-base --is-ancestor $1 $2 ); then

if git merge-base --is-ancestor "$1" "$2"; then

You definitely don't want run the stdout of the command (if any, fortunately none for now.)

#!/bin/bash

Looks completely POSIX to me, so why not /bin/sh?