If you're using Bourne-type
shells, you have to watch out for putting a series of commands
separated by semicolons (Section 28.16) into the background. These shells put only
the last command on the line into the background, but wait for the
An easy way to test this is with the following command line, which
waits for 15 seconds, then does an ls:
$ sleep 15; ls &
In Bourne-like shells, you won't get your prompt
back until the sleep (Section 25.9) command has finished.
With Bourne-type shells, the proper way to put a series of commands
into the background is to group them with parentheses:
( ) Section 43.7
$ (sleep 15; ls)&
This may strike you as a defect, but in fact, it's a
sign of the greater precision of Bourne shell syntax, which makes it
somewhat exasperating for interactive use but much better for
It doesn't make any sense to run an interactive
program such as an editor in the background. For example, if you type
this from the C shell:
% vi &
you'll get a message like the following:
 + Stopped (tty output) vi
vi can be active only in the foreground. However,
it does make sense to have vi
stopped (Section 23.1) in the background.
If you are
running vi or any other interactive program, you
can quickly get back to the shell by typing CTRL-z to stop the
program. The shell will take control of your terminal and print
another shell prompt.
Stopping vi (Section 23.6) is more efficient than using its shell escape mechanism (Section 17.21), since it lets you go back to your original
shell rather than starting a new one. Simply type
fg to get back to where you were in editing.
We have had the
misfortune to share a system with new users who were overenthusiastic
in their use of background processes, rather like the man who loved
loving so much he sought many lovers. Because each background process
is competing for the same resources, running many of them can be a
drain on the system, and everything takes longer for everyone. We
knew people who thought that if they ran three
troff processes at once, they'd
get their three files formatted faster than if they did them one
after another. Boy, were they mistaken.
If you use the Bourne shell, any background processes you have
running will normally be terminated when you log out. To avoid this,
use the nohup (Section 23.10) command.
Not all processes are created equal. Unix maintains a
queue of processes
ordered by priority. Foreground processes, such as a user typing a
command at a prompt, often receive higher priority than background
processes. However, you may want to run background processes at an
even lower priority, by using nice
( Section 26.5). This is
a relatively painless way of being kind to other users -- and
making your foreground job run faster -- though it will make your
background tasks take a little longer.