Executing Commands with bash - 4.4 Running Several Commands All at Once (
Page 4 of 5 )
Problem
You need to run three commands, but they are independent of each other, and don’t need to wait for each other to complete.
Solution
You can run a command in the background by putting an ampersand (&) at the end of the command line. Thus, you could fire off all three jobs in rapid succession as follows:
$ long &
[1] 4592
$ medium
&
[2] 4593
$ short
$
Or better yet, you can do it all on one command line:
$ long & medium & short
[1] 4592
[2] 4593
$
Discussion
When we run a command in the background (there really is no such place in Linux), all that really means is that we disconnect keyboard input from the command and the shell doesn’t wait for the command to complete before it gives another prompt and accepts more command input. Output from the job (unless we take explicit action to do otherwise) will still come to the screen, so all three jobs will be interspersing output to the screen.
The odd bits of numerical output are the job number in square brackets, followed by the process ID of the command that we just started in the background. In our example, job 1 (process 4592) is the long command, and job 2 (process 4593) is medium.
We didn’t put short into the background since we didn’t put an ampersand at the end of the line, so bash will wait for it to complete before giving us the shell prompt (the
$
).
The job number or process ID can be used to provide limited control over the job. You can kill the long job with
kill %1
(since its job number was
1
). Or you could specify the process number (e.g.,
kill 4592
) with the same deadly results.
You can also use the job number to reconnect to a background job. Connect it back to the foreground with
fg %1
. But if you only had one job running in the background, you wouldn’t even need the job number, just
fg
by itself.
If you start a job and then realize it will take longer to complete than you thought, you can pause it using Ctrl-Z, which will return you to a prompt. You can then type
bg
to un-pause the job so it will continue running in the background. This is basically adding a trailing
&
after the fact.
See Also
• Chapter 2 on redirecting output
