Background, running jobs in - Revision history

Jimbo: Reverted edits by DavidYoung (talk) to last revision by Jimbo

2012-08-25T21:25:44Z

Reverted edits by DavidYoung (talk) to last revision by Jimbo

DavidYoung: minor updates

2012-08-16T23:19:16Z

minor updates

Jimbo at 22:41, 29 February 2008

2008-02-29T22:41:53Z

Jimbo at 22:40, 29 February 2008

2008-02-29T22:40:00Z

New page

==Overview==
There are three basic methods to run jobs in the background: by '''ampersand''', by [[cron]], and by [[at]].

==Ampersand==
This is the simplest method: when executing a command from the [[shell]], simply append an ampersand to it, and the shell will execute it in the background and return you a process id number:

ph34r# '''/usr/bin/perl /home/jimbo/dostuff.pl &'''
[1] 84190
ph34r# '''ps waux | grep 84190'''
root 84190 0.0 0.1 1460 788 ?? Is 4:02PM 0:02.19 /usr/bin/perl /home/jimbo/dostuff.pl

However, if you should exit the shell, the background process will also be killed:

ph34r# exit
% ps waux | grep 84190
%

Our job got killed when we exited the shell because it was a [[child process]] of the shell. If we wanted to be able to close the shell and still keep the job running, we should have used [[at]].

Another interesting use of the ampersand character from the [[shell]] is to cause one task to run ''after'' another task finishes:

ph34r# '''echo this && echo is && echo just && echo an && echo example'''
this
is
just
an
example
ph34r#

You can also append a single ampersand at the end of a bunch of double-ampersand linked commands in order to cause the whole kit and kaboodle to run in the background, while still making each individual command wait on the last one to finish before it starts.

==cron==
[[cron]] is the system's automated scheduling utility for repeated tasks. Its usage is complex enough that it's better for you to visit the [[cron]] article itself if you want to learn how to use it; but in a nutshell special files called [[crontab]]s are used to tell '''cron''' what tasks you want it to perform, under which user contexts, and when. '''cron''' jobs are not bound to any particular shell, and will fire off on schedule without any user intervention and regardless of whether any particular user - or anyone at all - is interactively logged into the system.

==at==
[[at]] is the system's scheduling utility for single-execution jobs: when you want something to happen at some particular point in time, but only once, you schedule it with [[at]]. Once every five minutes, [[cron]] fires up [[atrun]], which checks the [[at]] queue and pops jobs that are due to process off the queue and runs them.

[[at]] jobs are ''not'' children of any shell process, and just like cron jobs, will run regardless of whether any particular user or even any user at all is logged into the system interactively. This makes [[at]] a handy way to cause a long-running job to start up immediately, while allowing you to close your shell and go elsewhere (or to make sure a REALLY long-running job will continue running even if your shell closes for some reason out of your control, such as an internet outage).

When using [[at]] to schedule a job to run immediately, however, you may get annoyed by having to wait for the next 5-minute mark for the job to run: if so, you can always fire up [[atrun]] yourself, rather than waiting for [[cron]] to do it for you. This won't hurt anything, and it will go ahead and pop jobs that are due to run out of the queue immediately so you don't have to sit around and wait.

ph34r# '''date'''
Fri Feb 29 15:50:29 EST 2008
ph34r# '''at now'''
'''/bin/sh /home/jimbo/some_long_job.sh'''
'''^D''' ''(user pressed ctrl-D)''
Job 9 will be executed using /bin/sh
ph34r# '''atq'''
Date Owner Queue Job#
Fri Feb 29 15:50:48 EST 2008 root c 9
ph34r# '''/usr/libexec/atrun'''
ph34r# '''atq'''
ph34r# exit
% '''ps waux | grep some_long_job'''
root 20472 0.0 0.1 1460 788 ?? Ss 12:02PM 0:02.19 /bin/sh /home/jimbo/some_long_job.sh
%

As you can see, we scheduled the job for immediate execution, saw that it was pushed into the queue, manually fired off [[atrun]], then checked to see that it had popped from the queue. Seeing that it had popped from the queue, we exited our shell (note the transition from the "ph34r# " prompt to the "% " prompt) and checked to see that the job was still running - and sure enough, it was. Miller time!

[[Category: Common Tasks]] [[Category: FreeBSD for Servers]]

← Older revision		Revision as of 21:25, 25 August 2012
Line 60:		Line 60:

	[[Category: Common Tasks]] [[Category: FreeBSD for Servers]]		[[Category: Common Tasks]] [[Category: FreeBSD for Servers]]
−
−	~~== Research Reveals Largest Ancient Dam Built by Maya Civilization ==~~
−
−	Recent excavations, sediment coring and mapping by a multi-university team led by the University of Cincinnati at the pre-Columbian city of Tikal, a paramount urban center of the ancient Maya, have identified new landscaping and engineering feats, including the largest ancient dam built by the Maya of Central America.
−
−	~~[[http://goodvillenews.com/Research-Reveals-Largest-Ancient-Dam-Built-by-Maya-Civilizat-cff.html Research Reveals Largest Ancient Dam Built by Maya Civilization]]~~
−
−	~~[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]~~
−
−	~~== Cat Saves Owner Hours After Adoption ==~~
−
−	~~A newly-adopted cat repaid his owners loving gesture earlier this month by saving her from a medical emergency just hours after he was brought home, the Green Bay Press Gazette reports.~~
−
−	~~[[http://goodvillenews.com/Cat-Saves-Owner-Hours-After-Adoption-ixHpp7.html Cat Saves Owner Hours After Adoption]]~~
−
−	~~[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]~~
−
−	~~== 11 Things You Should Start Doing for Yourself Today ==~~
−
−	Enjoy everything that happens in your life, but never make your happiness or success dependent on an attachment to any person, place or thing. Wayne DyerYou deserve to live a more balanced, harmonious and happier life, starting today and starting now. Today, not tomorrow, nor the day after tomorrow is where your life is, where your life starts.
−
−	~~[[http://goodvillenews.com/11-Things-You-Should-Start-Doing-for-Yourself-Today-e883xI.html 11 Things You Should Start Doing for Yourself Today]]~~
−
−	~~[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]~~
−
−	~~== Discovering My Own Values ==~~
−
−	At the end of your life a friend once asked, What do you hope to have happened? I thought it a great question and decided to give him a thoughtful answer, so I pocketed it for later and bought myself a month for the assignment. For a while my mind flooded with questions of plot.
−
−	~~[[http://goodvillenews.com/Discovering-My-Own-Values-4UdImK.html Discovering My Own Values]]~~
−
−	~~[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]~~
−
−	~~== What Ive Learned About Learning ==~~
−
−	We learn more by looking for the answer to a question and not finding it than we do from learning the answer itself. ~Lloyd AlexanderI am a teacher and an avid learner, and Im passionate about both.Im a teacher because I help Eva homeschool our kids OK, she does most of the work, but I do help, mostly with math but with everything else too.
−
−	~~[[http://goodvillenews.com/What-Ive-Learned-About-Learning-I45BZI.html What Ive Learned About Learning]]~~
−
−	~~[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]~~

← Older revision		Revision as of 23:19, 16 August 2012
Line 60:		Line 60:

	[[Category: Common Tasks]] [[Category: FreeBSD for Servers]]		[[Category: Common Tasks]] [[Category: FreeBSD for Servers]]
		+
		+	== Research Reveals Largest Ancient Dam Built by Maya Civilization ==
		+
		+	Recent excavations, sediment coring and mapping by a multi-university team led by the University of Cincinnati at the pre-Columbian city of Tikal, a paramount urban center of the ancient Maya, have identified new landscaping and engineering feats, including the largest ancient dam built by the Maya of Central America.
		+
		+	[[http://goodvillenews.com/Research-Reveals-Largest-Ancient-Dam-Built-by-Maya-Civilizat-cff.html Research Reveals Largest Ancient Dam Built by Maya Civilization]]
		+
		+	[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]
		+
		+	== Cat Saves Owner Hours After Adoption ==
		+
		+	A newly-adopted cat repaid his owners loving gesture earlier this month by saving her from a medical emergency just hours after he was brought home, the Green Bay Press Gazette reports.
		+
		+	[[http://goodvillenews.com/Cat-Saves-Owner-Hours-After-Adoption-ixHpp7.html Cat Saves Owner Hours After Adoption]]
		+
		+	[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]
		+
		+	== 11 Things You Should Start Doing for Yourself Today ==
		+
		+	Enjoy everything that happens in your life, but never make your happiness or success dependent on an attachment to any person, place or thing. Wayne DyerYou deserve to live a more balanced, harmonious and happier life, starting today and starting now. Today, not tomorrow, nor the day after tomorrow is where your life is, where your life starts.
		+
		+	[[http://goodvillenews.com/11-Things-You-Should-Start-Doing-for-Yourself-Today-e883xI.html 11 Things You Should Start Doing for Yourself Today]]
		+
		+	[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]
		+
		+	== Discovering My Own Values ==
		+
		+	At the end of your life a friend once asked, What do you hope to have happened? I thought it a great question and decided to give him a thoughtful answer, so I pocketed it for later and bought myself a month for the assignment. For a while my mind flooded with questions of plot.
		+
		+	[[http://goodvillenews.com/Discovering-My-Own-Values-4UdImK.html Discovering My Own Values]]
		+
		+	[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]
		+
		+	== What Ive Learned About Learning ==
		+
		+	We learn more by looking for the answer to a question and not finding it than we do from learning the answer itself. ~Lloyd AlexanderI am a teacher and an avid learner, and Im passionate about both.Im a teacher because I help Eva homeschool our kids OK, she does most of the work, but I do help, mostly with math but with everything else too.
		+
		+	[[http://goodvillenews.com/What-Ive-Learned-About-Learning-I45BZI.html What Ive Learned About Learning]]
		+
		+	[[http://goodvillenews.com/wk.html GoodvilleNews.com - good, positive news, inspirational stories, articles]]

@@ Line 1: / Line 1: @@
-==Overview==
+There are three basic methods to run jobs in the background: by '''ampersand''' from the shell, by the [[cron]] scheduler , and by the [[at]] scheduler.  We will cover the basics of all three methods here.
 There are three basic methods to run jobs in the background: by '''ampersand''', by [[cron]], and by [[at]].
 ==Ampersand==