Start the expression to populate the variable with "cat $1" which is the command line filename input. Check the Usage above

Not able to understand the TODO:

Start the expression to populate the variable with “cat $1” which is the command line filename input. Check the Usage above.

It’s trying tell you to start by filling the given variable’s value using “cat $1” then followed by any other commands you want to append.

If you don’t know what $1 means, refer this link

i understood what cat is.

but i doon’t understand where and why to use it. I am still struck

cat is use to print the content of any txt file or read content into terminal for more reference use google and give it a try into terminal with other files you will get it