# usage: atomicwrite_ifchanged output.txt
#   - overwrite atomically (mv)
#   - only writes to the file if new content is different
# by @nil0x42
function atomicwrite_ifchanged() {
    test "$#" -eq 1 # ARGC == 1
    test ! -t 0 # STDIN not a TTY
    local file="$1"
    local tmp_file="$(mktemp "${file}.XXXXXX.atomicwrite_ifchanged.part")"
    cat - >| "$tmp_file"
    if cmp -s "$file" "$tmp_file"; then
        rm "$tmp_file"
    else
        mv "$tmp_file" "$file"
    fi
}