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 while it is running.[6]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:

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

[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:

alice is running...CTRL-Z[1]+  Stopped                 alice
$ hatter &[2] 145
$ fg %alicealice 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 myfileedit the file... CTRL-ZStopped [1] vi
$ troff myfiletroff reports an error$ fgvi 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.[7]

You can also suspend a job with CTRL-Y. This is slightly different from CTRL-Z in that the process is only stopped when it attempts to read input from the terminal.

 

[3] If you have an older version of UNIX, it is possible that your system does not support job control. This is particularly true for many systems derived from Xenix, System III, or early versions of System V. On such systems, bash does not have the fg and bg commands, job number arguments to kill and wait, typing CTRL-Z to suspend a job, or the TSTP signal.

[4] Options -r and -s are not available in bash prior to version 2.0.

[5] This is analogous to ~+ and ~- as references to the current and previous directory; see the footnote in Chapter 7. Also: %% is a synonym for %+.

[6] 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.

[7] 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.