ifchange   [plain text]

#!/bin/sh
# usage: ifchange target temporary

timestamp=
until [ "$0" = 0 ]; do
    case "$1" in
	--timestamp)
	    timestamp=.
	    ;;
	--timestamp=*)
	    timestamp=`expr \( "$1" : '[^=]*=\(.*\)' \)`
	    ;;
	*)
	    break
	    ;;
    esac
    shift
done

target="$1"
temp="$2"
if [ "$temp" = - ]; then
    temp="tmpdata$$.tmp~"
    cat > "$temp" || exit $?
    trap 'rm -f "$temp"' 0
fi

if cmp "$target" "$temp" >/dev/null 2>&1; then
    echo "$target unchanged"
    rm -f "$temp"
else
    echo "$target updated"
    mv -f "$temp" "$target"
fi

if [ -n "${timestamp}" ]; then
    if [ x"${timestamp}" = x. ]; then
	case "$target" in
	    */*)
		timestamp=`dirname "$target"`/.time.`basename "$target"`
		;;
	    *)
		timestamp=.time."$target"
		;;
	esac
    fi
    : > "$timestamp"
fi