diffcp

#!/bin/bash

usage() {
    cat >&2 <<EOM
usage: $(basename "$0") SOURCE DEST

Show a diff of source and dest, and prompt before overwriting.

options:
    -h   show this help message
    -m   use mv rather than cp
    -q   be more quiet, less verbose
    -b   make a backup of the destination file
EOM
}

operation=cp
verbose=1
backup=

while getopts bhmv OPT; do
    case $OPT in
        b)  backup=1 ;;
        m)  operation=mv ;;
        q)  verbose='' ;;
        h)
            usage
            exit 0
            ;;
        \?)
            usage
            exit 1
            ;;
    esac
done

shift $((OPTIND-1))


if [ -z "$1" -o -z "$2" ]; then
    usage
    exit 1
fi

set -o nounset

src="$1"
dst="$2"

diff="diff"
if which colordiff > /dev/null; then
    diff="colordiff"
fi

"$diff" -u -- "$src" "$dst"

ret=$?

if [ $ret -eq 0 ]; then
    # no changes

    if [ -n "$verbose" ]; then
        echo >&2 "\`$src' == \`$dst'"
    fi

    exit 0

elif [ $ret -eq 1 ]; then
    # changes

    opts=()

    if [ -n "$verbose" ]; then
        opts+=("-v")
    fi

    if [ -n "$backup" ]; then
        opts+=("-b")
    fi

    if [ "$operation" = "cp" ]; then
        opts+=("-a")
    fi

    echo

    "$operation" -i "${opts[@]}" "$src" "$dst"

else
    # error
    exit $ret
fi