10 Jobs & Signals (zsh)

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# 10 Jobs & Signals

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## 10.1 Jobs

If the `MONITOR` option is set, an interactive shell associates a job with each pipeline. It keeps a table of current jobs, printed by the `jobs` 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 does not have a place in the job table, and is not subject 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 with string.

`%?`string

  The last job whose command line contains string.

`%%`

  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.

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## 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.

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This document was generated on February 16, 2020 using texi2any.

Zsh version 5.8, released on February 14, 2020.