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Job Control

This chapter disusses what job control is, how it works, and how Bash allows you to access its facilities.

Job Control Basics

Job control refers to the ability to selectively stop (suspend) the execution of processes and continue (resume) their execution at a later point. A user typically employs this facility via an interactive interface supplied jointly by the system's terminal driver and Bash.

The shell associates a job with each pipeline. It keeps a table of currently executing jobs, which may be listed with the jobs command. When Bash starts a job asynchronously (in the background), it prints a line that looks like:

[1] 25647
indicating that this job is job number 1 and that the process ID of the last process in the pipeline associated with this job is 25647. All of the processes in a single pipeline are members of the same job. Bash uses the job abstraction as the basis for job control.

To facilitate the implementation of the user interface to job control, the system maintains the notion of a current terminal process group ID. Members of this process group (processes whose process group ID is equal to the current terminal process group ID) receive keyboard-generated signals such as SIGINT. These processes are said to be in the foreground. Background processes are those whose process group ID differs from the terminal's; such processes are immune to keyboard-generated signals. Only foreground processes are allowed to read from or write to the terminal. Background processes which attempt to read from (write to) the terminal are sent a SIGTTIN (SIGTTOU) signal by the terminal driver, which, unless caught, suspends the process.

If the operating system on which Bash is running supports job control, Bash allows you to use it. Typing the suspend character (typically `^Z', Control-Z) while a process is running causes that process to be stopped and returns you to Bash. Typing the delayed suspend character (typically `^Y', Control-Y) causes the process to be stopped when it attempts to read input from the terminal, and control to be returned to Bash. You may then manipulate the state of this job, using the bg command to continue it in the background, the fg command to continue it in the foreground, or the kill command to kill it. A `^Z' takes effect immediately, and has the additional side effect of causing pending output and typeahead to be discarded.

There are a number of ways to refer to a job in the shell. The character `%' introduces a job name. Job number n may be referred to as `%n'. A job may also be referred to using a prefix of the name used to start it, or using a substring that appears in its command line. For example, `%ce' refers to a stopped ce job. Using `%?ce', on the other hand, refers to any job containing the string `ce' in its command line. If the prefix or substring matches more than one job, Bash reports an error. The symbols `%%' and `%+' refer to the shell's notion of the current job, which is the last job stopped while it was in the foreground. The previous job may be referenced using `%-'. In output pertaining to jobs (e.g., the output of the jobs command), the current job is always flagged with a `+', and the previous job with a `-'.

Simply naming a job can be used to bring it into the foreground: `%1' is a synonym for `fg %1' bringing job 1 from the background into the foreground. Similarly, `%1 &' resumes job 1 in the background, equivalent to `bg %1'

The shell learns immediately whenever a job changes state. Normally, Bash waits until it is about to print a prompt before reporting changes in a job's status so as to not interrupt any other output. If the the -b option to the set builtin is set, Bash reports such changes immediately (see section The Set Builtin). This feature is also controlled by the variable notify.

If you attempt to exit bash while jobs are stopped, the shell prints a message warning you. You may then use the jobs command to inspect their status. If you do this, or try to exit again immediately, you are not warned again, and the stopped jobs are terminated.

Job Control Builtins

bg [jobspec]
Place jobspec into the background, as if it had been started with `&'. If jobspec is not supplied, the current job is used.

fg [jobspec]
Bring jobspec into the foreground and make it the current job. If jobspec is not supplied, the current job is used.

jobs [-lpn] [jobspec]
jobs -x command [jobspec]

The first form lists the active jobs. The -l option lists process IDs in addition to the normal information; the -p option lists only the process ID of the job's process group leader. The -n option displays only jobs that have changed status since last notfied. If jobspec is given, output is restricted to information about that job. If jobspec is not supplied, the status of all jobs is listed.

If the -x option is supplied, jobs replaces any jobspec found in command or arguments with the corresponding process group ID, and executes command, passing it arguments, returning its exit status.

suspend [-f]
Suspend the execution of this shell until it receives a SIGCONT signal. The -f option means to suspend even if the shell is a login shell.

When job control is active, the kill and wait builtins also accept jobspec arguments.

Job Control Variables

This variable controls how the shell interacts with the user and job control. If this variable exists then single word simple commands without redirects are treated as candidates for resumption of an existing job. There is no ambiguity allowed; if you have more than one job beginning with the string that you have typed, then the most recently accessed job will be selected. The name of a stopped job, in this context, is the command line used to start it. If this variable is set to the value exact, the string supplied must match the name of a stopped job exactly; if set to substring, the string supplied needs to match a substring of the name of a stopped job. The substring value provides functionality analogous to the %? job id (see section Job Control Basics). If set to any other value, the supplied string must be a prefix of a stopped job's name; this provides functionality analogous to the % job id.

Setting this variable to a value is equivalent to `set -b'; unsetting it is equivalent to `set +b' (see section The Set Builtin).

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