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man pages section 1: User Commands

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Updated: Thursday, June 13, 2019
 
 

fg(1)

Name

jobs, fg, bg, stop, notify - control process execution

Synopsis

sh

jobs [-p | -l] [% job_id...]
jobs -x command [arguments]
fg [% job_id...]
bg [% job_id...]
stop % job_id...
stop pid...

csh

jobs [-l]
fg [% job_id]
bg [% job_id]...
notify [% job_id]...
stop % job_id...
stop pid...

ksh88

jobs [-lnp] [% job_id...]
fg [% job_id...]
bg [% job_id...]
stop % job_id...
stop pid...

ksh

jobs [-lnp] [job_id...]
fg [job_id...]
bg [job_id...]

Description

sh

When Job Control is enabled, the Bourne shell built-in jobs reports all jobs that are stopped or executing in the background. If %job_id is omitted, all jobs that are stopped or running in the background is reported. The following options modify or enhance the output of jobs:

–l

Reports the process group ID and working directory of the jobs.

–p

Reports only the process group ID of the jobs.

–x

Replaces any job_id found in command or arguments with the corresponding process group ID, and then executes command passing it arguments.

When the shell is invoked as jsh, Job Control is enabled in addition to all of the functionality described previously for sh. Typically Job Control is enabled for the interactive shell only. Non-interactive shells typically do not benefit from the added functionality of Job Control.

With Job Control enabled every command or pipeline the user enters at the terminal is called a job_id. All jobs exist in one of the following states: foreground, background or stopped. These terms are defined as follows:

  1. A job in the foreground has read and write access to the controlling terminal.

  2. A job in the background is denied read access and has conditional write access to the controlling terminal (see stty(1))

  3. A stopped job is a job that has been placed in a suspended state, usually as a result of a SIGTSTP signal (see signal.h(3HEAD)).

Every job that the shell starts is assigned a positive integer, called a job_id number which is tracked by the shell and are used as an identifier to indicate a specific job. Additionally, the shell keeps track of the current and previous jobs. The current job is the most recent job to be started or restarted. The previous job is the first non-current job.

The acceptable syntax for a Job Identifier is of the form:

%job_id

where job_id can be specified in any of the following formats:

% or +

for the current job

for the previous job

?<string>

specify the job for which the command line uniquely contains string.

n

for job number n, where n is a job number

pref

where pref is a unique prefix of the command name (for example, if the command ls −l name were running in the background, it could be referred to as %ls); pref cannot contain blanks unless it is quoted.

When Job Control is enabled, fg resumes the execution of a stopped job in the foreground, also moves an executing background job into the foreground. If %job_id is omitted the current job is assumed.

When Job Control is enabled, bg resumes the execution of a stopped job in the background. If %job_id is omitted the current job is assumed.

stop stops the execution of a background job(s) by using its job_id, or of any process by using its pid; see ps(1).

csh

The C shell built-in, jobs, without an argument, lists the active jobs under job control.

–l

List process IDs, in addition to the normal information.

The shell associates a numbered job_id with each command sequence to keep track of those commands that are running in the background or have been stopped with TSTP signals (typically Control-Z). When a command or command sequence (semicolon-separated list) is started in the background using the & metacharacter, the shell displays a line with the job number in brackets and a list of associated process numbers:

[1] 1234

To see the current list of jobs, use the jobs built-in command. The job most recently stopped (or put into the background if none are stopped) is referred to as the current job and is indicated with a `+'. The previous job is indicated with a `'; when the current job is terminated or moved to the foreground, this job takes its place (becomes the new current job).

To manipulate jobs, refer to the bg, fg, kill, stop, and % built-in commands.

A reference to a job begins with a `%'. By itself, the percent sign refers to the current job.

% %+ %%

The current job.

%−

The previous job.

%j

Refer to job j as in: `kill –9 %j'. j can be a job number, or a string that uniquely specifies the command line by which it was started; `fg %vi' might bring a stopped vi job to the foreground, for instance.

%?string

Specify the job for which the command line uniquely contains string.

A job running in the background stops when it attempts to read from the terminal. Background jobs can normally produce output, but this can be suppressed using the `stty tostop' command.

fg brings the current or specified job_id into the foreground.

bg runs the current or specified jobs in the background.

stop stops the execution of a background job(s) by using its job_id, or of any process by using its pid; see ps(1).

notify notifies the user asynchronously when the status of the current job or specified jobs changes.

ksh88

jobs displays the status of the jobs that were started in the current shell environment. When jobs reports the termination status of a job, the shell removes its process ID from the list of those known in the current shell execution environment.

job_id specifies the jobs for which the status is to be displayed. If no job_id is specified, the status information for all jobs are displayed.

The following options modify or enhance the output of jobs:

–l

(The letter ell.) Provides more information about each job listed. This information includes the job number, current job, process group ID, state and the command that formed the job.

–n

Displays only jobs that have stopped or exited since last notified.

–p

Displays only the process IDs for the process group leaders of the selected jobs.

By default, jobs displays the status of all the stopped jobs, running background jobs, and all jobs whose status has changed and have not been reported by the shell.

If the monitor option of the set command is turned on, an interactive shell associates a job with each pipeline. It keeps a table of current jobs, printed by the jobs command, and assigns them small integer numbers. When a job is started asynchronously with &, the shell prints a line which looks like:

[1] 1234

indicating that the job, which was started asynchronously, was job number 1 and had one (top-level) process, whose process id was 1234.

If you are running a job and wish to do something else you can hit the key ^Z (Control-Z) which sends a STOP signal to the current job. The shell then normally indicates that the job has been “Stopped” (see OUTPUT below), and print another prompt. You can then manipulate the state of this job, putting it in the background with the bg command, or run some other commands and then eventually bring the job back into the foreground with the foreground command fg. A ^Z takes effect immediately and is like an interrupt, in that pending output and unread input are discarded when it is typed.

There are several ways to refer to jobs in the shell. A job can be referred to by the process id of any process of the job or by one of the following:

%number

The job with the specified number.

%string

Any job whose command line begins with string; works only in the interactive mode when the history file is active.

%?string

Any job whose command line contains string; works only in the interactive mode when the history file is active.

%%

Current job.

%+

Equivalent to %%.

%−

Previous job.

The shell learns immediately whenever a process changes state. It normally informs you whenever a job becomes blocked so that no further progress is possible, but only just before it prints a prompt. This is done so that it does not otherwise disturb your work. When the monitor mode is on, each background job that completes triggers any trap set for CHLD. When you try to leave the shell while jobs are running or stopped, you are warned that `You have stopped (running) jobs.' You can use the jobs command to see what they are. If you do this or immediately try to exit again, the shell does not warn you a second time, and the stopped jobs are terminated.

fg moves a background job from the current environment into the foreground. Using fg to place a job in the foreground removes its process ID from the list of those known in the current shell execution environment. The fg command is available only on systems that support job control. If job_id is not specified, the current job is brought into the foreground.

bg resumes suspended jobs from the current environment by running them as background jobs. If the job specified by job_id is already a running background job, bg has no effect and exits successfully. Using bg to place a job into the background causes its process ID to become `known in the current shell execution environment, as if it had been started as an asynchronous list. The bg command is available only on systems that support job control. If job_id is not specified, the current job is placed in the background.

stop stops the execution of a background job(s) by using its job_id, or of any process by using its pid. See ps(1).

ksh

jobs displays information about specified jobs that were started by the current shell environment on standard output. The information contains the job number enclosed in [...], the status, and the command line that started the job.

If job_id is omitted, jobs displays the status of all stopped jobs, background jobs, and all jobs whose status has changed since last reported by the shell.

When jobs reports the termination status of a job, the shell removes the job from the list of known jobs in the current shell environment.

The following options modify or enhances the output of jobs:

–l

Displays process IDs after the job number in addition to the usual information.

–n

Displays only the jobs whose status has changed since the last prompt was displayed.

–p

Displays the process group leader IDs for the specified jobs.

job_id can be specified to jobs, fg, and bg as one of the following:

number

The process id of job.

-number

The process group id of job.

%number

The job number.

%string

The job whose name begins with string.

%?string

The job whose name contains string.

%+
%%

The current job.

%-

The previous job.

fg places the specified jobs into the foreground in sequence and sends a CONT signal to start each running. If job_id is omitted, the most recently started or stopped background job is moved to the foreground.

bg places the specified jobs into the background and sends a CONT signal to start them running. If job_id is omitted, the most recently started or stopped background job is resumed or continued in the background.

OUTPUT

If the –p option is specified, the output consists of one line for each process ID:

"%d\n", "process ID"

Otherwise, if the –l option is not specified, the output is a series of lines of the form:

"[%d] %c %s %s\n", job-number, current, state, command

where the fields are as follows:

current

The character + identifies the job that would be used as a default for the fg or bg commands. This job can also be specified using the job_id %+ or %% . The character identifies the job that would become the default if the current default job were to exit; this job can also be specified using the job_id %− . For other jobs, this field is a space character. At most, one job can be identified with + and at most one job can be identified with . If there is any suspended job, then the current job is a suspended job. If there are at least two suspended jobs, then the previous job is also a suspended job.

job-number

A number that can be used to identify the process group to the wait, fg, bg, and kill utilities. Using these utilities, the job can be identified by prefixing the job number with %.

state

One of the following strings in the POSIX Locale:

Running

Indicates that the job has not been suspended by a signal and has not exited.

Done

Indicates that the job completed and returned exit status zero.

Done(code)

Indicates that the job completed normally and that it exited with the specified non-zero exit status, code, expressed as a decimal number.

Stopped

Indicates that the job was stopped.

Stopped(SIGTSTP)

Indicates that the job was suspended by the SIGTSTP signal.

Stopped(SIGSTOP)

Indicates that the job was suspended by the SIGSTOP signal.

Stopped(SIGTTIN)

Indicates that the job was suspended by the SIGTTIN signal.

Stopped(SIGTTOU)

Indicates that the job was suspended by the SIGTTOU signal.

The implementation can substitute the string Suspended in place of Stopped. If the job was terminated by a signal, the format of state is unspecified, but it is visibly distinct from all of the other state formats shown here and indicates the name or description of the signal causing the termination.

command

The associated command that was specified to the shell.

If the –l option is specified, a field containing the process group ID is inserted before the state field. Also, more processes in a process group can be output on separate lines, using only the process ID and command fields.

Environment Variables

See environ(7) for descriptions of the following environment variables that affect the execution of jobs, fg, and bg: LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, and NLSPATH.

Exit Status

sh, csh, ksh88

The following exit values are returned for jobs, fg, and bg:

0

Successful completion.

>0

An error occurred.

ksh

The following exit values are returned for jobs:

0

The information for each job is written to standard output.

>0

One or more jobs does not exist.

The following exit values are returned for fg:

exit status of last job

One or more jobs has been brought into the foreground.

non-zero

One or more jobs does not exist or has completed.

The following exit values are returned for bg:

0

All background jobs are started.

>0

One more jobs does not exist or there are no background jobs.

Attributes

See attributes(7) for descriptions of the following attributes:

csh, sh, ksh88

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Availability
system/core-os
Interface Stability
Committed
Standard

ksh

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Availability
system/core-os
Interface Stability
Uncommitted

See Also

csh(1), kill(1), ksh(1), ksh88(1), ps(1), sh(1), shell_builtins(1), stop(1), stty(1), wait(1), signal.h(3HEAD), attributes(7), environ(7), standards(7)