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Linux Gazette: Introduction to Shell Scripting

Sep 23, 2000, 18:38 (0 Talkback[s])
(Other stories by Ben Okopnik)

"At this point in the series, we're getting pretty close to what I consider the upper limit of basic shell scripting; there are still a few areas I'd like to cover, but most of the issues involved are getting rather, umm, involved. A good example is the `tput' command that I'll be covering this month: in order to really understand what's going on, as opposed to just using it, you'd need to learn all about "termcap/terminfo" controversy (A.K.A. one of the main arguments in the "Why UNIX Sucks" debate) - a deep, involved, ugly issue (for a fairly decent and simple explanation, see Hans de Goede's fixkeys.tgz, which contains a neat little "HOWTO". For a more in-depth study, the Keyboard-and-Console-HOWTO is an awesome reference on the subject). I'll try to make sense despite the confusion, but be warned..."

"The concept of functions is not a difficult one, but is certainly very useful: they are simply blocks of code that you can execute under a single label. Unlike a script, they do not spawn a new subshell but execute within the current one. They can be used within a script, or stand-alone. ... Functions may also be loaded into the environment, and invoked just like shell scripts; we'll talk about sourcing functions later on. For those of you who use Midnight Commander, check out the "mc ()" function described in their man page - it's a very useful one, and is loaded from ".bashrc"."

"Important item: functions are created as "function pour_the_beer () { ... }" or "pour_the_beer () { ... }" (the keyword is optional); they are invoked as "pour_the_beer" (no parentheses). Also, be very careful (as in, _do not_ unless you really mean it) about using an "exit" statement in a function: since you're running the code in the current shell, this will cause you to exit your current (i.e. the "login") shell! Exiting a shell script this way can produce some very ugly results, like a `hung' shell that has to be killed from another VT (yep, I've experimented). The statement that will terminate a function without killing the shell is "return"."

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