If the shell script is readable and executable then to execute it just type the name of the shell script file. Otherwise, you can explicity call a shell interpreter to run the file as a shell script, i.e.,

ksh myfile

No, the shell needs both execute and read permissions to run the script.

You don't need a shell script for that; use either 'whoami' or 'id'

The 'exit' command allows you to stop a running shell script at any point and to return a "status" value back to whomever called the shell script. This is a very common practice with shell scripts; sometimes you want to stop the script before it gets to the end of the shell script (for various logic reasons).

The 'exit' command also allows you to give a status that any other calling process can use to determine if the shell script ended successfully or not.