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