Moving a process around

Juggling processes between the foreground and the background can save you lots of typing and time since you don't have to restart one program or command every time you want to work with another program or command.

Placing a process in the background with "&"

To place a process in the background immediately, type the ampersand symbol (&) at the end of the command. The process will start and run in the background.

For example, if you type

xclock &

you will see a line showing some numbers, and then the system prompt will reappear. A clock will eventually appear on your screen (if you're on a workstation). The sequence should look similar to the following

unity% xclock &
[1] 13673
unity%

The number in brackets is the job number. The second number is called the process ID (PID). Each process has a unique PID which you can use to reference that particular process.

If you don't type the & to put the process in the background, the clock will appear, but you will not get the system prompt back and you will not be able to issue any other commands until you kill or suspend the xclock process.

Placing a job in the background with bg

To put a process in the background, even after it is running, you must suspend it and then background it. To do this, type

Control-z (hold down the Control key and press z.)

You will get a message that the process has been suspended. Now type

bg

and press the Return key. The process will be moved to the background.

If you have more than two processes running, it's a good idea to use the job number or PID to reference which job you want placed in the background.

The syntax for the bg command is

bg %job_number or PID

That is, you have to type bg % and then the job number or PID. For example

unity% bg %2
[2]    oclock &
unity%

Placing a job in the foreground

When a job is suspended or in the background, it can be brought to the foreground with the fg command. Note that even if a process is suspended in some way, the act of bringing it to the foreground will restart it.

The syntax for the fg command is

fg % job_number or PID

That is, you have to type fg % and then the job number or PID. For example

unity% fg %2
xclock

You'll notice that the prompt does not reappear after foregrounding the clock. Since the clock is busy ticking the seconds away in the foreground, you're unable to use the workstation until you either kill or suspend the process.

Last modified July 20, 2004 by cawalker