8.2.1 Foreground and Background

The built-in command fg brings a background job into the foreground. Normally this means that the job will have control of your terminal or window and therefore will be able to accept your input. In other words, the job will begin to act as if you typed its command without the &.

If you have only one background job running, you can use fg without arguments, and the shell will bring that job into the foreground. But if you have several running in the background, the shell will pick the one that you put into the background most recently. If you want some other job put into the foreground, you need to use the job's command name, preceded by a percent sign (%), or you can use its job number, also preceded by %, or its process ID without a percent sign. If you don't remember which jobs are running, you can use the command jobs to list them.

A few examples should make this clearer. Let's say you created three background jobs as above. Then if you type jobs, you will see this:

[1]   Running                  fred &
[2] - Running                  bob &
[3] + Running                  dave &

jobs has a few interesting options. jobs -l also lists process IDs:

[1]   2349      Running                  fred &
[2] - 2367      Running                  bob &
[3] + 2382      Running                  dave &

The -p option tells jobs to list only process IDs:

2349
2367
2382

This could be useful with command substitution; see Task 8-1 below. Finally, the -n option lists only those jobs whose status has changed since the shell last reported it - whether with a jobs command or otherwise.

If you type fg without an argument, the shell will put dave in the foreground, because it was put in the background most recently. But if you type fg %bob (or fg %2), bob will go in the foreground.

You can also refer to the job most recently put in the background by %+. Similarly, %i- refers to the background job invoked next-most-recently (bob in this case). That explains the plus and minus signs in the above: the plus sign shows the most recently invoked job; the minus sign shows the next-most-recently invoked job. [3]

[3] This is analogous to ~+ and ~- as references to the currently and previous directory; see the footnote in Chapter 7, Input/Output and Command-line Processing. Also: %% is a synonym for %+.

If more than one background job has the same command, then %command will disambiguate by choosing the most recently invoked job (as you'd expect). If this isn't what you want, you need to use the job number instead of the command name. However, if the commands have different arguments, you can use %?string instead of %command. %?string refers to the job whose command contains the string. For example, assume you started these background jobs:

$ bob pete & 
[1]     189
$ bob ralph & 
[2]     190
$

Then you can use %?pete and %?ralph to refer to each of them, although actually %?pe and %?ra are sufficient to disambiguate.

Table 8.1 lists all of the ways to refer to background jobs. We have found that, given how infrequently people use job control commands, job numbers or command names are sufficient, and the other ways are superfluous.

8.2.2 Suspending a Job

Just as you can put background jobs into the foreground with fg, you can also put a foreground job into the background. This involves suspending a job, so that the shell regains control of your terminal.

To suspend a job, type [CTRL-Z] [4] while it is running. This is analogous to typing [CTRL-C] (or whatever your interrupt key is), except that you can resume the job after you have stopped it. When you type [CTRL-Z], the shell responds with a message like this:

[4] This assumes that the [CTRL-Z] key is set up as your suspend key; just as with [CTRL-C] and interrupts, this is conventional but by no means required.

[1] + Stopped                   command 

Then it gives you your prompt back.

To resume a suspended job so that it continues to run in the foreground, just type fg. If, for some reason, you put other jobs in the background after you typed [CTRL-Z], use fg with a job name or number. For example:

fred is running... 
CTRL-Z 
[1] + Stopped                   fred
$ bob & 
[2]     bob &
$ fg %fred 
fred resumes in the foreground... 

The ability to suspend jobs and resume them in the foreground comes in very handy when you have a conventional terminal (as opposed to a windowing workstation) and you are using a text editor like vi on a file that needs to be processed. For example, if you are editing a file for the troff text processor, you can do the following:

$ vi myfile 
edit the file...  CTRL-Z 
Stopped [1] vi
$ troff myfile 
troff reports an error 
$ fg 
vi comes back up in the same place in your file 

Programmers often use the same technique when debugging source code.

You will probably also find it useful to suspend a job and resume it in the background instead of the foreground. You may start a command in the foreground (i.e., normally) and find that it takes much longer than you expected-for example, a grep, sort, or database query. You need the command to finish, but you would also like control of your terminal back so that you can do other work. If you type [CTRL-Z] followed by bg, you will move the job to the background. [5]

[5] Be warned, however, that not all commands are "well-behaved" when you do this. Be especially careful with commands that run over a network on a remote machine; you may end up "confusing" the remote program.