Things you MUST know about the shell

Note: I've tried to create diagrams in this page using HTML tables rather than graphics. They look right on my browsers, but they may not on yours.

What is this "shell"?

The "shell" is another name for the command shell or command interpreter. This is the program that gives you a command prompt, accepts the commands you type there, and basically makes the computer do what you tell it to. In DOS the program that did this was command.com (unless you were a real technogeek and used 4dos). In Linux, the shell is the first program that starts when you log in, and it keeps running until you log out, waiting to do your bidding. Linux is able to use any of several different shells, but the default Linux shell is called bash and is the only one I will discuss.

Why It Is Called a Shell

The shell's job is to interpret your commands and run the programs you request. Linux was designed to be a multitasking operating system, which means you can run more than one program at one time. Linux was also designed as a multi-user OS, which means that you can have more than one shell running at the same time. (Each user gets his own shell at login.) As a user, you have access only to the programs you are running, not the ones other users are running (though you can run your own copy of the same program). The programs are kept separate because they are "enclosed" in a "shell".

In the diagram, my programs are kept separate from Fred's programs by our shells, (represented here by table borders and color changes in the cells to show separation). It is even possible to have a shell running inside another shell. This is common when using X11.

Cool Command Line Tricks

Here is the stuff that you MUST know. The command line interface would be completely unbearable if not for a few simple tricks that make life okay.

Command History

The shell stores a list of the commands that you have issued. You can navigate up and down through this list using the Up and Down Arrow keys. To repeat the last command is just Up Arrow, Enter.

Command Completion

Linux can have some outrageously long file names, which is great when you're reading them in a list, but it sucks when you have to type it on the command line. At least it would suck if not for this nifty little feature. Type the first letter or two and then press the Tab key. The shell will try to complete your command. If you typed enough letters to uniquely identify the file name, the shell fills the rest in for you. If the letters match multiple file names, the shell will fill in everything up to the point where they differ and wait. Hit Tab again and it will give you a list of possibilities. The shell will also help you complete path names. Try it out. You will love this.

Multitasking

I'll try to expand this later, but here's the one minute version. To run a job (program) in the background, type an ampersand (&) at the end of the command line. To suspend the currently running job, Ctrl-Z. To force a suspended job to run in the background, type bg <job>. To bring a background job into the foreground, type fg <job>. To see a list of jobs currently running (or suspended) type jobs. If you exclude the <job> argument on these commands, it defaults to whatever job was last running in the foreground (not the one currently in the foreground).