Linux and Unix are multitasking operating systems i.e. a system that can run multiple tasks (process) during the same period of time. In this new blog series, I am going to list the Linux and Unix job control commands that you can use for multitasking with the Bash or Korn or POSIX shell.
### What is a job control? ###
Job control is nothing but the ability to stop/suspend the execution of processes (commands) and continue/resume their execution as per your requirements. This is done using your operating system and shell such as bash/ksh or POSIX shell.
### Who provides a facility to control jobs? ###
The Bash / Korn shell, or POSIX shell provides a facility to control jobs.
### Say hello to job table ###
Your shell keeps a table of current jobs, called job table. When you type command the shell assigns a jobID (also known as JOB_SPEC). A jobID or JOB_SPEC is nothing but small integer numbers.
#### #1: Creating your first Linux/Unix job ####
I am going to run a command called xeyes that displays two googly eyes on screen, enter:
*Fig.01: Running the xeyes command in the background*
I started a job in the background with an ampersand (&). The shell prints a line that looks like the following:
[1] 6891
In this example, two numbers are output as follows
- [1] : The xeyes job, which was started in the background, was job number 1.
- 6891 : A process ID of job number 1.
I am going to start a few more jobs:
## Start a text editor, system load average display for X, and sleep command ##
gedit /tmp/hello.c &
xload &
sleep 100000 &
#### #2: List the current jobs ####
To [see the status of active jobs in the current shell][1], type:
$ jobs
$ jobs -l
Sample outputs:
[1] 9379 Running xeyes &
[2] 9380 Running gedit /tmp/hello.c &
[3]- 9420 Running xload &
[4]+ 9421 Running sleep 100000 &
A brief description of each field is given below:
(注：表格部分，这样发表出来应该会方便看一点)
<tableborder="1"><tbody><tr><th>Field</th><th>Value</th><th>Description</th><th>Example(s)</th></tr><tr><td>1</td><td>[1]</td><td><strong>jobID</strong> or <strong>JOB_SPEC</strong> - Job number to use with the fg, bg, wait, kill and other shell commands. You must prefix the job number with a percent sign (<kbd><strong>%</strong></kbd>).<br>A plus sign (<kbd>+</kbd>) identifies the default or current job.<br>A minus sign (<kbd>-</kbd>) identifies the previous job.</td><td><kbd>%1</kbd><br><kbd>fg %1</kbd><br><kbd>kill %2</kbd></td></tr><tr><td>2</td><td>9379</td><td>P<strong>rocess ID</strong> - An identification unique number that is automatically assigned to each process when it is created on the system.</td><td>kill 9379</td></tr><tr><td>3</td><td>Running</td><td><strong>state</strong> - The state of job:<br><kbd><strong>Running</strong></kbd> - The job is currently running and has not been suspended by a signal.<br><kbd><strong>Stopped</strong></kbd> - The job was suspended.<br></td><td>N/A</td></tr><tr><td>4</td><td>xeyes &</td><td><strong>command</strong> - The command that was given to the shell.</td><td>script &<br>firefox url&</td></tr></tbody></table>
You can also use ps command to list the processes running on the system:
$ ps
#### #3: Stop or suspend running jobs ####
Hit the [Ctrl]-[Z] key or use [kill command][2] as follows:
kill -s stop PID
In this example, start [ping command][3] and use the Ctrl-Z key sequence to stop the ping command job:
As soon as I logout from shell, pdfwriter.py job will be killed by my shell. To overcome this problem use [disown shell builting command to tell the shell not to send a HUP signal][14], type:
$ ~/scripts/www/pdfwriter.py --profile=faq .... &
$ disown
$ exit
#### #8 Prevent job from being killed on logout using an external command called nohup ####
You can also use [nohup command to execute jobs after you exit from a shell prompt][15]:
#### Linux and Unix job control command list summery ####
<tableborder="1"><tbody><tr><th>Command</th><th>Description</th><th>Example(s)</th></tr><tr><td><kbd><strong>&</strong></kbd></td><td>Put the job in the background</td><td><kbd>command &</kbd></td></tr><tr><td><kbd><strong>%n</strong></kbd></td><td>Set the job with the given n (number)</td><td><kbd>command %1</kbd></td></tr><tr><td><kbd><strong>%Word</strong></kbd></td><td>Refer the job whose command line begins with the Word</td><td><kbd>command %yum</kbd></td></tr><tr><td><kbd><strong>%?Word</strong></kbd></td><td>Refer any job whose command line contains the Word</td><td><kbd>command %?ping</kbd></td></tr><tr><td><kbd><strong>%%</strong></kbd><br><kbd><strong>%+</strong></kbd></td><td>Refer to the current job</td><td><kbd>kill %%<br>kill %+</kbd></td></tr><tr><td><kbd><strong>%-</strong></kbd></td><td>Refer to the previous job</td><td><kbd>bg %-</kbd></td></tr><tr><td><kbd><strong>CTRL-Z</strong><br><kbd><strong>kill -s stop jobID</strong></kbd></kbd></td><td>Suspend or stop the job</td><td><kbd>kill -s stop %ping</kbd></td></tr><tr><td><kbd><strong>jobs</strong><br><kbd><strong>jobs -l</strong></kbd></kbd></td><td>List the active jobs</td><td><kbd>jobs -l</kbd></td></tr><tr><td><kbd><strong>bg</strong></kbd></td><td>Put jobs to the background</td><td><kbd>bg %1<br>bg %ping</kbd></td></tr><tr><td><kbd><strong>fg</strong></kbd></td><td>Put job to the foreground</td><td><kbd>fg %2<br>fg %apt-get</kbd></td></tr></tbody></table>
#### A note about shell built-in and external commands ####
Run the following type command to find out whether given command is internal or external:
type -a fg bg jobs disown
Sample outputs:
fg is a shell builtin
fg is /usr/bin/fg
bg is a shell builtin
bg is /usr/bin/bg
jobs is a shell builtin
jobs is /usr/bin/jobs
disown is a shell builtin
In almost all cases, you need to use shell built-in commands. All external commands such as /usr/bin/fg or /usr/bin/jobs works in a different shell environment, and can not use parent shell's environment.
#### Conclusion ####
I hope you enjoyed this blog post series ([rss feed][16]) and I suggest that you read the following for more information:
- See our faq section for [disown command examples][17], [jobs command examples][18], [bg command examples][19], and [fg command examples][20] on Linux/Unix process management.
- Man pages [bash(1)][21], [ksh(1)][22], [ps(1)][23], [kill(1)][24]
- [Korn shell (ksh93) documentation][25].
- [NU bash shell documentation][26].
I am planning to add more in-depth tutorial in this series. Please let me know if you need to see specific topic in the comment section below.
