diff then cp (bash)

dcp.bash

#!/usr/bin/env bash
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
declare -i VERBOSITY=0
DIFF="${DCP_DIFF:-diff}"
DIFFOPTS="${DCP_DIFFOPTS:--u}"
declare -i ERROR=0
declare -i DEBUG=0
declare -a ARGS=()

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

declare -a PREPARED_ARGS=()

function prepare_args () {
    local arg= char=
    PREPARED_ARGS=()
    while (( $# )); do
        arg="$1"
        shift
        case "$arg" in
            --)
                PREPARED_ARGS+=(-- "$@")
                shift $#
                ;;
            --*)
                # long option
                if [[ $arg =~ ^(--[^=]+)=(.*)$ ]]; then
                    PREPARED_ARGS+=("${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}")
                else
                    PREPARED_ARGS+=("$arg")
                fi
                ;;
            -)
                PREPARED_ARGS+=(-)
                ;;
            -*)
                # short option
                # https://stackoverflow.com/questions/10551981
                while IFS= read -r -d '' -n 1 char; do
                    PREPARED_ARGS+=("-${char}")
                done < <(printf "%s" "${arg#-}")
                ;;
            *)
                PREPARED_ARGS+=("$arg")
                ;;
        esac
    done
}

prepare_args "$@"
parse_opts "${PREPARED_ARGS[@]}"

if (( ${#ARGS[@]} != 2 )); then
    usage
    echo >&2
    echo >&2 "you must specify just 2 arguments"
    exit 1
fi

SRC="${ARGS[0]}"
DEST="${ARGS[1]}"

if (( 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 && ! -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 (( ERROR )); then
        echo >&2 "no such file: ${DEST}"
        exit 10
    fi
elif "$DIFF" $DIFFOPTS -- "$DEST" "$SRC"; then
    if (( 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 >= 3 )); then
    if command -v cowsay >/dev/null 2>&1; then
        cowsay >&2 "completed."
    else
        echo >&2 "completed."
    fi
fi