Typically, as in many "Learning" books, they are building
on past examples and it saves alot of paper if you don't keep
printing the "whole" program/script over and over. So...
you must call functions by name for them to be executed.
Just FYI, the "exit 0" statement is not necessary.
I just like including an exit code in case this shell script
is executed by some other shell script or program. This way,
you can determine the success or failure of the script by
its' exit code. For instance...
exit 0 - All is well
exit 255 - Huston... we've had a problem
The actual values are up to you but normally a "0" exit
code says everything is AOK.