Shell Script backup a file ?

Shell Programming and Scripting
1

could you please find a solution for this

script that backup a file. The file name to backup should be provided as input parameter, the backup file should have the same file name with the extension ".bak". If the user provides no input parameter, the script should display an error message. If there is an input file name, but it does not exist, the script should display an error message. If the input file exists, the script should create the backup file and overwrite any existing backup file with the same name.

2 
#! /bin/sh

if [ "$#" -eq 0 ]
then 
        exit 1;
fi

cp ${1} "${1}.back"
exit 0