When using `set -eu`, how do you check the number of arguments?:

    if [ -z "$1" ]; then
      usage();
      exit 1;
    fi;

If I'm using `set -eu`, that dies on the `$1` with a nasty error message, rather than printing my usage message. I've resorted to moving `set -eu` to after these kind of checks, but that makes me uneasy.