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This chapter disusses what job control is, how it works, and
how Bash allows you to access its facilities.
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
jobs command. When Bash starts a job
asynchronously (in the background), it prints a line that looks
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
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
signal by the terminal driver, which, unless caught, suspends the
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
command to continue it in the background, the
command to continue it in the foreground, or the
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
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
builtin is set, Bash reports such changes immediately (see
section The Set Builtin). This
feature is also controlled by the variable
If you attempt to exit bash while jobs are stopped, the shell
prints a message warning you. You may then use the
command to inspect their status. If you do this, or try to exit
again immediately, you are not warned again, and the stopped jobs
Place jobspec into the background, as if it
had been started with &. If jobspec
is not supplied, the current job is used.
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
option lists process IDs in addition to the normal
-p option lists only the
process ID of the job's process group leader. The
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.
-x option is supplied,
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 the execution of this shell until it receives a
-f option means to suspend even
if the shell is a login shell.
When job control is active, the
builtins also accept jobspec arguments.
- auto_resume 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
exact, the string supplied must match
the name of a stopped job exactly; if set to
the string supplied needs to match a substring of the
name of a stopped job. The
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
- notify 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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