7.1 Job Control Basics
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
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
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
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
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.
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
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.
-b option to the
set builtin is enabled,
Bash reports such changes immediately (see section 4.3 The Set Builtin).
Any trap on
SIGCHLD is executed for each child process
If an attempt to exit Bash is while jobs are stopped, the
shell prints a message warning that there are stopped jobs.
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.