Background Jobs

Pipes are actually a special case of a more general feature: doing more than one thing at a time. This is a capability that many other commercial operating systems don't have, because of the rigid limits that they tend to impose upon users. UNIX, on the other hand, was developed in a research lab and meant for internal use, so it does relatively little to impose limits on the resources available to users on a computer—as usual, leaning towards uncluttered simplicity rather than overcomplexity.

"Doing more than one thing at a time" means running more than one program at the same time. You do this when you invoke a pipeline; you can also do it by logging on to a UNIX system as many times simultaneously as you wish. (If you try that on an IBM's VM/CMS system, for example, you will get an obnoxious "already logged in" message.)

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The shell also lets you run more than one command at a time during a single login session. Normally, when you type a command and hit RETURN, the shell will let the command have control of your terminal until it is done; you can't type in further commands until the first one is done. But if you want to run a command that does not require user input and you want to do other things while the command is running, put an ampersand (&) after the command.

This is called running the command in the background, and a command that runs in this way is called a background job; by contrast, a job run the normal way is called a foreground job. When you start a background job, you get your shell prompt back immediately, enabling you to enter other commands.

The most obvious use for background jobs is programs that take a long time to run, such as sort or uncompress on large files. For example, assume you just got an enormous compressed file loaded into your directory from magnetic tape.[14] Let's say the file is gcc.tar.Z, which is a compressed archive file that contains well over 10 MB of source code files.

Type uncompress gcc.tar & (you can omit the .Z), and the system will start a job in the background that uncompresses the data "in place" and ends up with the file gcc.tar. Right after you type the command, you will see a line like this:

[1] 175

followed by your shell prompt, meaning that you can enter other commands. Those numbers give you ways of referring to your background job; Chapter 8 explains them in detail.

You can check on background jobs with the command jobs. For each background job, jobs prints a line similar to the above but with an indication of the job's status:

[1]+ Running uncompress gcc.tar &

When the job finishes, you will see a message like this right before your shell prompt:

[1]+ Done uncompress gcc.tar

The message changes if your background job terminated with an error; again, see Chapter 8 for details.