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Shell Programming,continued...

What Is Shell Programming?

 If you have been following the earlier Guides to (mostly) Harmless Hacking (GTMHH), you are already familiar with many fun Unix commands. Shell programming is writing a file that holds a sequence of Unix commands, which you can run in your shell account by typing in only one line.

Newbie note: Don't know what a shell account is? Unix leaves you scratching your head? You *must* have a shell account to learn shell programming. You can get one for free at http://sdf.lonestar.org. Just set up a PPP connection and telnet into Lonestar for your Unix fun! However, Lonestar
doesn't allow you to telnet out. For a full service shell account, check out http://rt66.com. Yes! They have ssh logins! Note -- Linux, FreeBSD, and OpenBSD operating systems give you shell accounts on your home computer.

 If you are familiar with DOS, you may have already done something similar to shell programming: DOS batch files. The basic idea is that you write a series of DOS commands and save them with a file that ends with the extension "bat."

 For example, you might name your batch file "myfile.bat." Then any time you want to run it, you just type "myfile" and it runs all the commands inside that file. (Note: if you are in a different directory from myfile.bat, you either have to tell your computer where to look for it with a "path" command, or by typing in the entire path, for example "c:\myprograms\myfile.")

 Unix -- an operating system that was created long before DOS -- can do something very similar to a DOS batch file. Instead of typing Unix commands one by one every time you need them, you can write a shell script that automatically executes that sequence. Then you save it as a file with permissions that make it executable.

Newbie note: "Executable" doesn't mean the computer goes out and murders your poor file. It means that when you type the name of that file, the computer looks inside and does what your file tells it to do. "Permissions" mean what can be done by who with a file. For example, you could set the permissions on your shell account file so that only someone in your account could execute it. Or you could make it so anyone in the world could run (execute) it -- something you usually do with the files in your Web site, so that anyone who surfs in may read them.

 But there is one huge difference between DOS and Unix commands. In DOS, the commands "mkdir" and "MKDIR" do exactly the same thing. In Unix, they would be two totally different commands. Be absolutely careful in this lesson to type all commands in lower case (small) letters, or this stuff will not work.

More shell programming --->>

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