Usage Help If Script Has Arguments

#!/bin/bash
# A shell script to lookup usernames in /etc/passwd file
# Written by: Vivek Gite
# Last updated on: Sep/10/2003
# -------------------------------------------------------
# Set vars
user=$1 # first command line argument
passwddb=/etc/passwd
# Verify the type of input and number of values
# Display an error message if the username (input) is not correct
# Exit the shell script with a status of 1 using exit 1 command.
[ $# -eq 0 ] && { echo "Usage: $0 username"; exit 1; }
grep "^$user" $passwddb >/dev/null
retval=$? # store exit status of grep
# If grep found username, it sets exit status to zero
# Use exit status to make the decision
[ $retval -eq 0 ] && echo "$user found" || echo "$user not found"