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, 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, 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
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 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 specification (jobspec).
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.
A single ‘%’ (with no accompanying job specification) also refers
to the current job.
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
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 job whose command name begins with ‘ce’. 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 (see The Set Builtin).
Any trap on
SIGCHLD is executed for each child process
If an attempt to exit Bash is made while jobs are stopped, (or running, if
checkjobs option is enabled – see The Shopt Builtin), the
shell prints a warning message, and if the
checkjobs option is
enabled, lists the jobs and their statuses.
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 any stopped jobs are terminated.
When the shell is waiting for a job or process using the
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.