If you're using the Bourne shell, you have to watch out for
putting a series of commands separated by
background. The Bourne shell puts only the last command on the
line into the background, but waits for the first.
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 the Bourne shell, you won't get your prompt back until the
command has finished.
The proper way to put a series of Bourne shell commands into
the background is to group them with parentheses:
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:
you'll get a message like the following:
 + Stopped (tty output) vi
can be active only in the foreground. However, it does make
sense to have vi
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.
is more efficient than using its
shell escape mechanism (30.26
since it lets you go back to your original shell rather than
starting a new one. Simply type
to get back to where you
were in editing.
We have shared a system with new users who were overenthusiastic
users 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. This means that everything takes longer
for everyone. We used to have people who thought that if they ran
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
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
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.