IDL sucks, that is the first thing you need to know.
IDL is the illegitemate offspring of FORTRAN, and some close cousin of MATLAB. It is a commercial product tangled up with licenses and such.

An IDL program has the .pro extension. I have yet to just find a way to make such a file into an executable script, the usual unix trick using the "shebang" line to specify the interpreter by putting #!/usr/local/rsi/bin/idl as the first line of the file does not work (for one thing the IDL comment character is the ; not #. So, to run an idl program you must start idl (I have a bash shell alias alias idl='/usr/local/rsi/bin/idl' so that I can just type "idl" and get the idl prompt.)
Then to run a file whammo.pro, you just type whammo at the IDL prompt.

IDL statements have commas after the keyword (I guess the language has some kind of stone age parser), so you type:

print, var

print var

As near as I can tell, the language is case-insensitive, so you can type PRO, pro, or pRo with equal effect.

The manual set is gigantic, and for all its size, not all that helpful. The set I have apparently once included a set of three introductory books, of which only one, Using IDL remains. There are lots of online copies of the manuals, and tutorials and such, so liberal use of google can lead to a lot of information. There is one manual that it would probably be nice to have entitled Building Applications with IDL.

An outfit called Fanning Consulting his written quite a nice set of IDL notes, and you will likely find your google searches often bring you here, and you should be pleased when this happens. The provide a set of notes entitled Creating a Run-Time IDL Application that I have found helpful, although when I follow them exactly (as near as I can tell), and then use idl -rt=dingus.sav to try to run the result, I get:

% Attempt to call undefined procedure/function: 'IDLRTMAIN'.