gNU Bourne-Again SHell (GNU Bourne-Again SHell)
JOB CONTROL
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 operating system
kernel'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 operating 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, if the user so specifies with stty
tostop, write to the terminal. Background processes which
attempt to read from (write to when stty tostop is in effect) the
terminal are sent a SIGTTIN (SIGTTOU) signal by the kernel's
terminal driver, which, unless caught, suspends the process.
If the operating system on which bash is running supports job
control, bash contains facilities to use it. Typing the suspend
character (typically ^Z, Control-Z) while a process is running
causes that process to be stopped and returns control 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. The user
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 specification (jobspec). 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 job whose command name begins with ce. If a prefix
matches more than one job, bash reports an error. Using %?ce, on
the other hand, refers to any job containing the string ce in its
command line. If the 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 or started in the background. The previous
job may be referenced using %-. If there is only a single job,
%+ and %- can both be used to refer to that job. 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 -. A single % (with no accompanying job specification) also
refers to the current job.
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 -b option to the set builtin command is
enabled, bash reports such changes immediately. Any trap on
SIGCHLD is executed for each child that exits.
If an attempt to exit bash is made while jobs are stopped (or, if
the checkjobs shell option has been enabled using the shopt
builtin, running), the shell prints a warning message, and, if
the checkjobs option is enabled, lists the jobs and their
statuses. The jobs command may then be used to inspect their
status. If a second attempt to exit is made without an
intervening command, the shell does not print another warning,
and any stopped jobs are terminated.
When the shell is waiting for a job or process using the wait
builtin, and job control is enabled, wait will return when the
job changes state. The -f option causes wait to wait until the
job or process terminates before returning.
