nsid is a program that checks against NSIDs provided in either standard input, passed in as arguments to the program, or added as input file. It prints the NSID given and whether it is valid or not.
When the program is run without arguments, it gets the input from standard input, with the help of cat, and it loops through the values and evaluates them one by one, printing the answer on the stdout.
When the program is run with arguments, it loops through the arguments and evaluates each argument and prints the answer on stdout.
When a file is provided to the command through file substitution, it gets the values and loops through the values and evaluates each one, printing the answer on stdout.
This program uses is_nsid function that takes in as parameter potential NSID and returns 0 if it is a valid NSID and 1 otherwise. Then there is a wrapper script that gets input from standard input, using cat command, if no arguments are provided and from arguments if they are provided. For each of the value that it receives, regardless of how they are obtained, it evaluates and prints out appropriate message.
The program assumes that the correct NSID is either ABC123 (Upper case letters) abc123 (Lower case letters), and AbC123 (Mixed case letters).