#!/bin/bash

$username=$1

function check {
cat /etc/passwd |grep $username
}

if [ -n "$username" ]; then
        if [ -n check=0 ]; then
                echo "User exists"
        else
                echo "User does not exist"
        fi
else
        echo "Username not specified"
fi

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
OUTPUT:

jamie@yourmammasflufferbutter $ sh checkuser.sh adam
checkuser.sh: line 3: =adam: command not found
Username not specified