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Chapter 11 Process Control
Every program that is running is called a process. These processes range from things like the X Window System to system programs (daemons) that are started when the computer boots. Every process runs as a particular user. Processes that are started at boot time usually run as root or nobody. Processes that you start will run as you. Processes started as other users will run as those users.
You have control over all the processes that you start. Additionally, root has control over all processes on the system, including those started by other users. Processes can be controlled and monitored through several programs, as well as some shell commands.
Programs started from the command line start up in the foreground. This allows you to see all the output of the program and interact with it. However, there are several occasions when you'd like the program to run without taking up your terminal. This is called running the program in the background, and there are a few ways to do it.
The first way to background a process is by adding an ampersand to the command line when you start the program. For example, assume you wanted to use the command line mp3 player amp to play a directory full of mp3s, but you needed to do something else on the same terminal. The following command line would start up amp in the background:
The program will run as normal, and you are returned to a prompt.
The other way to background a process is to do so while it is running. First, start up a program. While it is running, hit Control+z. This suspends the process. A suspended process is basically paused. It momentarily stops running, but can be started up again at any time. Once you have suspended a process, you are returned to a prompt. You can background the process by typing:
Now the suspended process is running in the background.
If you need to interact with a backgrounded process, you can bring it back into the foreground. If you've only got one backgrounded process, you can bring it back by typing:
If the program is not done running, the program will take control over you terminal and you will not be returned to a prompt. Sometimes, the program will finish running while backgrounded. In this instance, you'll get a message like this:
[1]+ Done /bin/ls $LS_OPTIONS
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That tells you that the backgrounded process (in this case ls - not terribly interesting) has completed.
It is possible to have several processes backgrounded at once. When this happens, you'll need to know which process you want to bring back to the foreground. Just typing fg will foreground the process that was last backgrounded. What if you had a whole list of processes in the background? Luckily, bash includes a command to list all the processes. It's called jobs and gives output like so:
% jobs
[1] Stopped vim
[2]- Stopped amp
[3]+ Stopped man ps
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This shows you a list of all the processes that are backgrounded. As you can see, they are all stopped. This means that the processes are suspended. The number is a sort of ID for all the backgrounded processes. The ID with a plus sign beside it (man ps) is the process that will be foregrounded if you just type fg.
If you wanted to foreground vim, you would type:
and vim would spring back up to the console. Backgrounding processes can be very useful if you only have one terminal open over a dialup connection. You can have several programs running on that one terminal, periodically switching back and forth between them.
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