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285 lines
13 KiB
Markdown
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10 Linux/Unix Bash and KSH Shell Job Control Examples
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================================================================================
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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.
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### What is a job control? ###
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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.
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### Who provides a facility to control jobs? ###
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The Bash / Korn shell, or POSIX shell provides a facility to control jobs.
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### Say hello to job table ###
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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.
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#### #1: Creating your first Linux/Unix job ####
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I am going to run a command called xeyes that displays two googly eyes on screen, enter:
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$ xeyes &
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Sample outputs:
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[][4]
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*Fig.01: Running the xeyes command in the background*
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I started a job in the background with an ampersand (&). The shell prints a line that looks like the following:
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[1] 6891
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In this example, two numbers are output as follows
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- [1] : The xeyes job, which was started in the background, was job number 1.
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- 6891 : A process ID of job number 1.
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I am going to start a few more jobs:
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## Start a text editor, system load average display for X, and sleep command ##
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gedit /tmp/hello.c &
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xload &
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sleep 100000 &
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#### #2: List the current jobs ####
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To [see the status of active jobs in the current shell][1], type:
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$ jobs
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$ jobs -l
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Sample outputs:
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[1] 9379 Running xeyes &
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[2] 9380 Running gedit /tmp/hello.c &
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[3]- 9420 Running xload &
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[4]+ 9421 Running sleep 100000 &
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A brief description of each field is given below:
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(注：表格部分，这样发表出来应该会方便看一点)
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<table border="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>
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You can also use ps command to list the processes running on the system:
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$ ps
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#### #3: Stop or suspend running jobs ####
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Hit the [Ctrl]-[Z] key or use [kill command][2] as follows:
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kill -s stop PID
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In this example, start [ping command][3] and use the Ctrl-Z key sequence to stop the ping command job:
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[][5]
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*Animated gif 01: Suspending ping command job*
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#### #4: Resume suspended/stopped job in the foreground ####
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Let us [resume or bring stopped ping job to the foreground][6] and make it the current job with the help of [fg command][7]. The syntax is as follows:
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## Job id number 5 for ping command ##
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fg %5
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I can also state any job whose command line begins with the string "ping":
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## %String ##
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fg %ping
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Sample outputs:
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64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=3 ttl=53 time=265 ms
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64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=4 ttl=53 time=249 ms
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64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=5 ttl=53 time=267 ms
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^C
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#### #5: Resume suspended/stopped job in the background ####
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In this example, I am going to update all installed packages on Red Hat or CentOS Linux production server using [yum command][8] background job:
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# yum -y update &>/root/patch.log &
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However, due to some reason (say load issue) I decided to stop this job for 20 minutes:
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# kill -s stop %yum
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Sample outputs:
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[7]+ Stopped yum -y update &>/root/patch.log &
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#### Restart a stopped background yum process with bg ####
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Now, I am going to [resume stopped the yum -y update &>/root/patch.log & job][9], type:
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# bg %7
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OR
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# bg %yum
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Sample outputs:
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[7]+ yum -y update &>/root/patch.log &
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#### #6: Kill a job / process ####
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To stop/kill a [yum command][10] process, enter the following [kill command][11] whose jobID was 7:
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# kill %7
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OR
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# kill pid
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Sample outputs:
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[7]+ Terminated yum -y update &>/root/patch.log &
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On Linux/FreeBSD/OS X Unix you can [use killall command to kill process by name instead of PID][12] or jobID.
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#### #7 Why does shell kill off all my background jobs when I logout? ####
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In this example, I am going to start pdfwriter.py job to generate pdf files for [this site][13] in bulk:
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~/scripts/www/pdfwriter.py --profile=faq --type=clean --header=logo\
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--footer-left "nixCraft is GIT UL++++ W+++ C++++ M+ e+++ d-" \
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--footer-right "Page [of] of [total]" &
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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:
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$ ~/scripts/www/pdfwriter.py --profile=faq .... &
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$ disown
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$ exit
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#### #8 Prevent job from being killed on logout using an external command called nohup ####
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You can also use [nohup command to execute jobs after you exit from a shell prompt][15]:
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$ nohup ~/scripts/www/pdfwriter.py --profile=faq .... &
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$ exit
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#### #9: Finding the PID of last job ####
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To find the the process ID of the most recently executed background (asynchronous) command, use bash shell special parameter $!
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$ gedit foo.txt &
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$ echo "PID of most recently executed background job - $!"
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Sample outputs:
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PID of most recently executed background job - 9421
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#### #10: Wait for job completion ####
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The wait command waits for given process ID or jobID (job specification) , and reports its termination status. The syntax is as follows:
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/path/to/large-job/command/foo &
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wait $!
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/path/to/next/job/that-is-dependents/on-foo-command/bar
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Here is one of my working script:
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#!/bin/bash
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# A shell script wrapper to create pdf files for our blog/faq section
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########################################################################
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# init() - Must be run first
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# Purpose - Create index file in $_tmp for all our wordpress databases
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########################################################################
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init(){
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_php="/usr/bin/php"
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_phpargs="-d apc.enabled=0"
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_base="~/scripts"
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_tmp="$_base/tmp"
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_what="$1"
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for i in $_what
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do
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[[ ! -d "$_tmp/$i" ]] && /bin/mkdir "$_tmp/$i"
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$_php $_phpargs -f "$_base/php/rawsqlmaster${i}.php" > "$_tmp/$i/output.txt"
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done
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}
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#####################################################
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# Without index file, we can out generate pdf files
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#####################################################
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init blog
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###########################################################
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# Do not run the rest of the script until init() finished
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###########################################################
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wait $!
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## Alright, create pdf files
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~/scripts/www/pdfwriter.py --profile=blog --type=clean --header=logo\
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--footer-left "nixCraft is GIT UL++++ W+++ C++++ M+ e+++ d-" \
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--footer-right "Page [of] of [total]"
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#### Linux and Unix job control command list summery ####
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<table border="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>
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#### A note about shell built-in and external commands ####
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Run the following type command to find out whether given command is internal or external:
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type -a fg bg jobs disown
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Sample outputs:
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fg is a shell builtin
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fg is /usr/bin/fg
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bg is a shell builtin
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bg is /usr/bin/bg
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jobs is a shell builtin
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jobs is /usr/bin/jobs
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disown is a shell builtin
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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.
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#### Conclusion ####
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I hope you enjoyed this blog post series ([rss feed][16]) and I suggest that you read the following for more information:
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- 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.
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- Man pages [bash(1)][21], [ksh(1)][22], [ps(1)][23], [kill(1)][24]
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- [Korn shell (ksh93) documentation][25].
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- [NU bash shell documentation][26].
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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.
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--------------------------------------------------------------------------------
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via: http://www.cyberciti.biz/howto/unix-linux-job-control-command-examples-for-bash-ksh-shell/
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译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[1]:http://www.cyberciti.biz/faq/unix-linux-jobs-command-examples-usage-syntax/
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[2]:http://www.cyberciti.biz/faq/unix-kill-command-examples/
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[3]:http://www.cyberciti.biz/faq/unix-ping-command-examples/
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[4]:http://www.cyberciti.biz/howto/unix-linux-job-control-command-list-for-bash-ksh-shell/attachment/run-xeyes-command-in-background/
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[5]:http://www.cyberciti.biz/howto/unix-linux-job-control-command-list-for-bash-ksh-shell/attachment/suspend-unix-job-command/
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[6]:http://www.cyberciti.biz/faq/unix-linux-fg-command-examples-usage-syntax/
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[7]:http://www.cyberciti.biz/faq/unix-linux-fg-command-examples-usage-syntax/
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[8]:http://www.cyberciti.biz/faq/rhel-centos-fedora-linux-yum-command-howto/
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[9]:http://www.cyberciti.biz/faq/unix-linux-bg-command-examples-usage-syntax/
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[10]:http://www.cyberciti.biz/faq/rhel-centos-fedora-linux-yum-command-howto/
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[11]:http://www.cyberciti.biz/faq/unix-kill-command-examples/
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[12]:http://www.cyberciti.biz/faq/unix-linux-killall-command-examples-usage-syntax/
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[13]:http://www.cyberciti.biz/faq/
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[14]:http://www.cyberciti.biz/faq/unix-linux-disown-command-examples-usage-syntax/
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[15]:http://www.cyberciti.biz/tips/nohup-execute-commands-after-you-exit-from-a-shell-prompt.html
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[16]:http://www.cyberciti.biz/tutorials/practical-unixlinux-series/feed/
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[17]:http://www.cyberciti.biz/faq/unix-linux-disown-command-examples-usage-syntax/
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[18]:http://www.cyberciti.biz/faq/unix-linux-jobs-command-examples-usage-syntax/
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[19]:http://www.cyberciti.biz/faq/unix-linux-bg-command-examples-usage-syntax/
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[20]:http://www.cyberciti.biz/faq/unix-linux-fg-command-examples-usage-syntax/
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[21]:http://www.manpager.com/linux/man1/bash.1.html
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[22]:http://www.manpager.com/linux/man1/ksh.1.html
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[23]:http://www.manpager.com/linux/man1/ps.1.html
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[24]:http://www.manpager.com/linux/man1/kill.1.html
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[25]:http://www2.research.att.com/sw/download/man/man1/ksh.html
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[26]:https://www.gnu.org/software/bash/manual/bashref.html
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