-z

[[ -z $1 ]] && { [[usage]]; }
[ -z $1 ] && { usage; }

Related

function usage() {
    echo "Usage: $(basename $0) Explain options here"
    exit 0
}

• If
• binary operator expected
• Square bracket
• Single bracket
• Double bracket

See also

• if, man test, -z, elif
• Command-line argument parsing, argparse, optparse, click, inquirer, docopt, help, usage, minimist