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10 Jobs & Signals
10.1 Jobs
If the MONITOR
option is set, an interactive shell associates a job
with each pipeline. command, and assigns them small integer numbers.
When a job is started asynchronously with ‘&
’, the shell prints a line
to standard error which looks like:
[1] 1234
indicating that the job which was started asynchronously was job number 1 and had one (top-level) process, whose process ID was 1234.
If a job is started with ‘&|
’ or ‘&!
’, then that job is immediately
disowned. After startup, it to the job control features described here.
If you are running a job and wish to do something else you may hit the
key ^Z (control-Z) which sends a TSTP
signal to the current job: this
key may be redefined by the susp
option of the external stty
command.
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,
putting it in the background
with the bg
command, or run some other commands and then eventually
bring the job back into the foreground with
the foreground command fg
. A
^Z takes effect immediately and is like an interrupt in that pending
output and unread input are discarded when it is typed.
A job being run in the background will suspend if it tries to read from the terminal.
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 fg
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.
One additional side effect is that use of disown
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.
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
if
, for
, etc., in order that the entire block of code can be managed
as a single job.
Background
jobs are normally allowed to produce output, but this can be disabled by
giving the command ‘stty tostop
’. 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.
When a command is suspended and continued later with the fg
or wait
builtins, zsh restores tty modes that were in effect when it was
suspended. This (intentionally) does not apply if the command is
continued via ‘kill -CONT
’, nor when it is continued with bg
.
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:
-
%``number
The job with the given number. -
%``string
The last job whose command line begins withstring
. -
%?``string
The last job whose command line containsstring
. -
%%
Current job. -
%+
Equivalent to ‘%%
’. -
%-
Previous job.
The shell learns immediately whenever a process changes state.
It normally informs you
whenever a job becomes blocked so that no further progress is possible.
If the NOTIFY
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.
When the monitor mode is on, each background job that completes triggers
any trap set for CHLD
.
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
jobs
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 SIGHUP
signal, if the HUP
option is set.
To avoid having the shell terminate the running jobs, either use the
nohup command (see man page nohup(1)) or the disown
builtin.
10.2 Signals
The INT
and QUIT
signals for an invoked command are ignored if the
command is followed by ‘&
’ and the MONITOR
option is not active. The
shell itself always ignores the QUIT
signal. Otherwise, signals have
the values inherited by the shell from its parent (but see the
TRAP``NAL
special functions in Functions).
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 exit
command or exit due to
the option ERR_EXIT
will cause the shell to exit without waiting.
Examples of such asynchronous jobs are process substitution, see
Process Substitution, and the
handler processes for multios, see the section Multios in
Redirection.
This document was generated on February 15, 2020 using
texi2html 5.0.
Zsh version 5.8, released on February 14, 2020.