You want to run different commands in your script depending on circumstances. How can you vary which commands run?
There are many solutions to this problem—it’s what scripting is all about.
In coming chapters we’ll discuss various programming logic that can be used to solve this problem, such as if/then/else, case statements, and more.
But here’s a slightly different approach that reveals something about bash.
We can use the contents of a variable (more on those in Chapter 5) not just for parameters, but also for the command itself.
We can assign the program name to a variable (here we use $PROG), and then when we refer to that variable in the place where a command name would be expected, it uses the value of that variable ($PROG) as the command to run.
The bash shell parses the command line, substitutes the values of its variables and takes the result of all the substitutions and then treats that as the command line, as if it had been typed that way verbatim.