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<a name="Jobs-_0026-Signals"></a>
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<a name="Jobs-_0026-Signals-1"></a>
<h1 class="chapter">10 Jobs & Signals</h1>
<hr>
<a name="Jobs"></a>
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<h2 class="section">10.1 Jobs</h2>
<a name="index-jobs"></a>
<a name="index-MONITOR_002c-use-of"></a>
<p>If the <tt>MONITOR</tt> option is set,
an interactive shell associates a <em>job</em> with each pipeline.
It keeps a table of current jobs, printed by the <tt>jobs</tt>
command, and assigns them small integer numbers.
When a job is started asynchronously with ‘<tt>&</tt>’,
the shell prints a line to standard error which looks like:
</p>
<div class="example">
<pre class="example">[1] 1234
</pre></div>
<p>indicating that the job which was started asynchronously was job number
1 and had one (top-level) process, whose process ID was 1234.
</p>
<p>If a job is started with ‘<tt>&|</tt>’ or ‘<tt>&!</tt>’,
then that job is immediately disowned. After startup, it
does not have a place in the job table, and is not subject
to the job control features described here.
</p>
<p>If you are running a job and wish to do something else you may hit the key
^Z (control-Z) which sends a <tt>TSTP</tt> signal to the current job: this key
may be redefined by the <tt>susp</tt> option of the external <tt>stty</tt> command.
<a name="index-jobs_002c-suspending"></a>
<a name="index-suspending-jobs"></a>
The shell will then normally indicate that the job has been ‘suspended’,
and print another prompt. You can then manipulate the state of this job,
<a name="index-bg_002c-use-of"></a>
putting it in the background with the <tt>bg</tt> command, or run some other
commands and then eventually bring the job back into the foreground with
<a name="index-fg_002c-use-of"></a>
the foreground command <tt>fg</tt>. A ^Z takes effect immediately and
is like an interrupt in that pending output and unread input are discarded
when it is typed.
</p>
<p>A job being run in the background will suspend if it tries to read
from the terminal.
</p>
<p>Note that if the job running in the foreground is a shell function,
then suspending it will have the effect of causing the shell to fork.
This is necessary to separate the function’s state from that of the
parent shell performing the job control, so that the latter can return
to the command line prompt. As a result, even if <tt>fg</tt> is
used to continue the job the function will no longer be part of the
parent shell, and any variables set by the function will not be visible
in the parent shell. Thus the behaviour is different from the case
where the function was never suspended. Zsh is different from many
other shells in this regard.
</p>
<p>One additional side effect is that use of <tt>disown</tt> with a job
created by suspending shell code in this fashion is delayed: the
job can only be disowned once any process started from the parent
shell has terminated. At that point, the disowned job disappears
silently from the job list.
</p>
<p>The same behaviour is found when the shell is executing code as the
right hand side of a pipeline or any complex shell construct such as
<tt>if</tt>, <tt>for</tt>, etc., in order that the entire block of code
can be managed as a single job.
<a name="index-background-jobs_002c-I_002fO"></a>
<a name="index-jobs_002c-background_002c-I_002fO"></a>
Background jobs are normally allowed to produce output,
but this can be disabled by giving the command ‘<tt>stty tostop</tt>’.
If you set this
tty option, then background jobs will suspend when they try to produce
output like they do when they try to read input.
</p>
<p>When a command is suspended and continued later with the <tt>fg</tt> or
<tt>wait</tt> builtins, zsh restores tty modes that were in effect when it was
suspended. This (intentionally) does not apply if the command is
continued via ‘<tt>kill -CONT</tt>’, nor when it is continued with <tt>bg</tt>.
</p>
<a name="index-jobs_002c-referring-to"></a>
<a name="index-referring-to-jobs"></a>
<p>There are several ways to refer to jobs in the shell.
A job can be referred to by the process ID of any process of the job
or by one of the following:
</p>
<dl compact="compact">
<dt><tt>%</tt><var>number</var></dt>
<dd><p>The job with the given number.
</p></dd>
<dt><tt>%</tt><var>string</var></dt>
<dd><p>The last job whose command line begins with <var>string</var>.
</p></dd>
<dt><tt>%?</tt><var>string</var></dt>
<dd><p>The last job whose command line contains <var>string</var>.
</p></dd>
<dt><tt>%%</tt></dt>
<dd><p>Current job.
</p></dd>
<dt><tt>%+</tt></dt>
<dd><p>Equivalent to ‘<tt>%%</tt>’.
</p></dd>
<dt><tt>%-</tt></dt>
<dd><p>Previous job.
</p></dd>
</dl>
<p>The shell learns immediately whenever a process changes state.
<a name="index-NOTIFY_002c-use-of"></a>
It normally informs you whenever a job becomes blocked so that
no further progress is possible. If the <tt>NOTIFY</tt> option is not set,
it waits until just before it prints a prompt before it informs you.
All such notifications are sent directly to the terminal, not to
the standard output or standard error.
</p>
<p>When the monitor mode is on, each background job that completes
triggers any trap set for <tt>CHLD</tt>.
</p>
<p>When you try to leave the shell while jobs are running or suspended, you will
be warned that ‘You have suspended (running) jobs’.
You may use the <tt>jobs</tt> command to see what they are.
If you do this or immediately try to
exit again, the shell will not warn you a second time; the suspended
jobs will be terminated, and the running jobs will be sent
a <tt>SIGHUP</tt> signal, if the <tt>HUP</tt> option is set.
<a name="index-HUP_002c-use-of"></a>
</p>
<a name="index-jobs_002c-disowning"></a>
<a name="index-disowning-jobs"></a>
<a name="index-disown_002c-use-of"></a>
<p>To avoid having the shell terminate the running jobs, either
use the <cite>nohup</cite> command (see man page nohup(1))
or the <tt>disown</tt> builtin.
</p>
<hr>
<a name="Signals"></a>
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</tr></table>
<h2 class="section">10.2 Signals</h2>
<p>The <tt>INT</tt> and <tt>QUIT</tt> signals for an invoked
command are ignored if the command is followed by
‘<tt>&</tt>’ and the <tt>MONITOR</tt> option is not active.
The shell itself always ignores the <tt>QUIT</tt> signal.
Otherwise, signals have the values
inherited by the shell from its parent
(but see the <tt>TRAP</tt><var>NAL</var> special functions in <a href="Functions.html#Functions">Functions</a>).
</p>
<a name="index-exiting-shell_002c-and-asynchronous-jobs"></a>
<a name="index-asynchronous-jobs_002c-and-exiting-shell"></a>
<a name="index-jobs_002c-asynchronous_002c-and-exiting-shell"></a>
<p>Certain jobs are run asynchronously by the shell other than those
explicitly put into the background; even in cases where the shell
would usually wait for such jobs, an explicit <tt>exit</tt> command
or exit due to the option <tt>ERR_EXIT</tt> will cause the shell to
exit without waiting. Examples of such asynchronous jobs are
process substitution, see
<a href="Expansion.html#Process-Substitution">Process Substitution</a>, and the handler processes for
multios, see
the section Multios in <a href="Redirection.html#Redirection">Redirection</a>.
</p><hr>
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