ruby/tool/git-refresh

#!/bin/sh
set -e

if (cd -P .) 2>/dev/null; then
    CHDIR='cd -P'
else
    CHDIR='cd'
fi

quiet=
branch=

until [ $# = 0 ]; do
    case "$1" in
	--) shift; break;;
	-C|--directory) shift; $CHDIR "$1";;
	-C*) $CHDIR `expr "$1" : '-C\(.*\)'`;;
	--directory=*) $CHDIR `expr "$1" : '[^=]*=\(.*\)'`;;
	-q) quiet=1;;
	-b|--branch) shift; branch="$1";;
	-b*) branch=`expr "$1" : '-b\(.*\)'`;;
	--branch=*) branch=`expr "$1" : '[^=]*=\(.*\)'`;;
	-*) echo "unknown option: $1" 1>&2; exit 1;;
	*) break;;
    esac
    shift
done

url="$1"
dir="$2"
shift 2
[ x"$branch" = x ] && unset branch || :
if [ -d "$dir" ]; then
    if [ x"$(git -C "$dir" describe --tags)" = x"$branch" ]; then
        exit 0 # already up-to-date
    fi
    echo updating `expr "/$dir/" : '.*/\([^/][^/]*\)/'` ...
    [ $quiet ] || set -x
    $CHDIR "$dir"
    ${branch+git} ${branch+fetch} ${branch+"$@"}
    exec git ${branch+checkout} "${branch-pull}" "$@"
else
    echo retrieving `expr "/$dir/" : '.*/\([^/][^/]*\)/'` ...
    [ $quiet ] || set -x
    exec git clone ${branch+--branch} ${branch+"$branch"} "$url" "$dir" "$@"
fi