I guess that line was just a comment for readers marking the end of the script as part of a larger post (not every site has [code][/code] tags like here at LQ). And was mistaken by OP for a part of the script.

Totally unrelated. sshin is your script name which resides in your /home/username/programs directory. Since /home/username/programs is in your environment path and executable (I assume) you can run your script by just typing at the prompt

sshin XX

As an exercise you can add some error checking to your script to verify that you included a number and that it is valid.

You obviously made the script executable and put it into a directory on your PATH.

When you issue the command sshin 12, Bash sees the first word of the command is an executable in one of the directories on the PATH, calls the kernel via execve(2) system call, and passes it the parsed command line.

The kernel, in turn, sees the shebang #!/bin/bash in the first line, understands it is a script, and calls the script interpreter (/bin/bash) with the script name as the first parameter preserving the parameters passed to it by the first instance of bash as the rest of the command line. As if you called it

Thanks for your answers, you have been helpful and explained well. I'm a newcomer at scripting, this is all new to me.

FYI: "I usually never write 'Bash scripts.'" Simply because I know that I can write the first line: (say ...) #!/usr/bin/pick_something and thereafter write my entire script in ("pick_something") a real programming language. And my customers will never know or care.

Two simple characters at the very-beginning of an "executable" file ... "#!" ... ("shebang!") ... never meant so much.

To me they look like, read the next lines in sequence.