diff then cp

dcp.sh

set -eu

PROGRAM="$(basename "$0")"

usage () {
    cat <<EOF
${PROGRAM}: diff then cp

Usage:
    ${PROGRAM} [-f|-i] SRC DEST

Options:
    -f, --force
    -i, --interactive
        --diff DIFF
        --diffopts DIFFOPTS
    -e, --error
    -v, --verbose
    -g, --debug
    -h, --help
EOF
}

MODE=interactive
VERBOSITY=0
DIFF="${DCP_DIFF:-diff}"
DIFFOPTS="${DCP_DIFFOPTS:--u}"
ERROR=
OPTIND=1
DEBUG=

parse_opts () {
    OPTIND=1
    while [ $# -ne 0 ]; do
        case "$1" in
            -f|--force)
                MODE=force
                ;;
            -i|--interactive)
                MODE=interactive
                ;;
            --diff)
                DIFF="$2"
                OPTIND="$(expr "$OPTIND" + 1)"
                shift
                ;;
            --diffopts)
                DIFFOPTS="$2"
                OPTIND="$(expr "$OPTIND" + 1)"
                shift
                ;;
            -e|--error)
                ERROR=1
                ;;
            -v|--verbose)
                VERBOSITY="$(expr "$VERBOSITY" + 1)"
                ;;
            -g|--debug)
                DEBUG=1
                ;;
            -h|--help)
                usage
                exit 0
                ;;
            -)
                return
                ;;
            -*)
                usage
                echo >&2
                echo >&2 "invalid option: $1"
                exit 1
                ;;
            *)
                return
                ;;
        esac
        OPTIND="$(expr "$OPTIND" + 1)"
        shift
    done
}

parse_opts "$@"
shift "$(expr "$OPTIND" - 1)"

if [ $# -ne 2 ]; then
    usage
    echo >&2
    echo >&2 "you must specify just 2 arguments"
    exit 1
fi

SRC="$1"
DEST="$2"
shift 2

if [ -n "$DEBUG" ]; then
    cat >&2 <<EOF
MODE=${MODE}
VERBOSITY=${VERBOSITY}
DIFF=${DIFF}
DIFFOPTS=${DIFFOPTS}
ERROR=${ERROR}
SRC=${SRC}
DEST=${DEST}
EOF
fi

if [ "$DEST" = - ]; then
    echo >&2 "destination must not be -"
    exit 1
fi

if [ "$MODE" = interactive -a ! -t 0 ]; then
    echo >&2 "stdin is not a tty. use -f (--force) option"
    exit 1
fi

rm_tmp_src () {
    [ -e "$SRC" ] && rm -f "$SRC" || :
}

if [ "$SRC" = - ]; then
    SRC="$(mktemp "${PROGRAM}-XXXXXX")"
    trap rm_tmp_src EXIT HUP INT TERM
    cat >"$SRC"
fi

if [ ! -e "$DEST" ]; then
    if [ -n "$ERROR" ]; then
        echo >&2 "no such file: ${DEST}"
        exit 10
    fi
elif "$DIFF" $DIFFOPTS "$DEST" "$SRC"; then
    if [ -n "$ERROR" ]; then
        echo >&2 'no difference'
        exit 11
    else
        exit 0
    fi
fi

# has difference
case "$MODE" in
    interactive)
        cp -iT "$SRC" "$DEST"
        ;;
    force)
        cp -fT "$SRC" "$DEST"
        ;;
    *)
        echo >&2 "invalid mode: ${MODE}"
        exit 1
        ;;
esac

if [ "$VERBOSITY" -ge 3 ]; then
    if command -v cowsay >/dev/null 2>&1; then
        cowsay >&2 "completed."
    else
        echo >&2 "completed."
    fi
fi