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(bashref.info)Job Control Basics

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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, 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 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 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 then manipulates 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'.  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 `%-'.  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 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.

   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 is enabled, Bash
reports such changes immediately (Note: The Set Builtin).  Any trap
on `SIGCHLD' is executed for each child process that exits.

   If an attempt to exit Bash is while jobs are stopped, the shell
prints a message warning that there are stopped jobs.  The `jobs'
command may then be used to inspect their status.  If a second attempt
to exit is made without an intervening command, Bash does not print
another warning, and the stopped jobs are terminated.