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A job is a program which has been started from the command
line of a Unix shell. Since Unix is a multi-tasking system,
there may be multiple jobs running at the same time. When the
shell starts a program it adds it to the job list, where it
maintains the list of currently running jobs and their state
(which is either running or suspended), and a number that is
used to identify the job.
In addition, there are two different modes in which a job can
be running:
- In the foreground
- In the background.
The
default behavior is to run in the foreground where the
program is in control of the terminal. A job can also be run
in the background, where the user does not have any
interaction with it, and the shell retains control of the
terminal. A job is put in the background on startup by
following the command, arguments, and any input/output
redirection with an ampersand (&). If a background job
requires any input and there is no input redirection, the job
will suspend itself until it is brought to the foreground.
Output of a job in the background will go to the screen if
not redirected, however full screen applications, such as
emacs, vi, trn, and the like will be suspended, until brought
to the foreground.
To view the list of current jobs, the shell provides the
jobs command. The jobs command lists the job number,
the state, and the command as executed (after variable and
alias substitution, and any i/o redirection). The shell has
several methods to allow you to changes the running mode of
the job:
-
If the program is a foreground job, you can suspend the job
by pressing control-Z
-
If the program is a foreground job, you can terminate the
job by hitting control-C, however if the job is blocking
control-C it will not work.
-
A suspended job (which is always in the background, but not
running) can be continued in the foreground by the fg
command.
-
A suspended job can be told to run in the background with
the bg command.
-
A background command can be terminated with the kill
command. (This is a shell command, and differs from the
KILL signal discussed in the processes section)
Arguments for each of the above commands, except the kill
command, are optional, and apply to the current job, which is
marked in the job list with a '+' next to the state. The
current job, is the job which most recently changed mode
(i.e. was suspended or put in the background). However, a job
launched in the background will not be the current job if a
suspended job appears on the list. The shell indicates a
previous job by a '-' next to the state in the jobs list.
There are several ways to specify a job for the above
commands. The most common way is to use the job's number
preceeded by a percent sign. You can also refer to a job by a
sub-string of the command line by typing %?substring. Both
the prefix and sub-string must not be ambiguous; that is, in
some way the string must be unique to a command on the list.
Finally you can indicate the current and previous jobs by %+
and %-.
Given the following jobs list:
[1] Suspended trn alt.politics.usa.republican
[2] - Suspended ftp sunsite.unc.edu
[3] + Suspended cc foobar.c >& foobar.compile
[4] Running du -s ~ > usage
bg %3
would continue the compiling job in the background,
%3& would be the same as bg %3, fg
%1 would continue the trn job in the foreground,
%1 would be the same as fg %1, kill
%du would terminate the du job, and fg %?suns
would continue the ftp job in the foreground.
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