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howtos:general_admin:task_scheduling [2015/06/24 14:06 (UTC)]
tdrssb
howtos:general_admin:task_scheduling [2015/06/24 19:36 (UTC)] (current)
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 ====== Task Scheduling in Linux ====== ====== Task Scheduling in Linux ======
 =====Overview===== =====Overview=====
-This article discusses some tools used in a Linux system to schedule tasks to run automatically at specified time intervals or at any given point of time in the future.+This article discusses some tools used in a Linux system to schedule tasks to run automatically at specified time intervals or at any given point of time in the future. This primer will not cover these commands in-depth; this is just a brief introduction to using these commands. See the individual HOWTOS for each command for an in-depth look at all relevant options and configurations.
  
 Some task-scheduling daemons used in Linux/UNIX are: Some task-scheduling daemons used in Linux/UNIX are:
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    * ''​[[howtos:​software:​cron|cron]]''​ -- the periodic scheduler most commonly used    * ''​[[howtos:​software:​cron|cron]]''​ -- the periodic scheduler most commonly used
    * ''​[[howtos:​software:​anacron|anacron]]''​ -- anachronistic cron; a periodic scheduler that doesn'​t rely on the system being left on 24x7    * ''​[[howtos:​software:​anacron|anacron]]''​ -- anachronistic cron; a periodic scheduler that doesn'​t rely on the system being left on 24x7
 +
 +=====Using at=====
 +The **//at//** command allows a user to execute commands or scripts at a specified time (required) and date (optional). The commands can be entered via standard input, redirection,​ or file. 
 +
 +<​code>​
 +darkstar:~% at
 +</​code>​
 +
 +=====Interactive at=====
 +Using the command **//at//** with standard input (keyboard) is a little more complicated than typing one line in at the prompt. The command uses an internal "​sub-shell"​ to gather the required information. Once the command information entry is complete, <​key>​Ctrl-D</​key>​ (EOT) will signify entry completion. The **//-m//** flag specifies a mail message will be sent to the user when the job is finished, regardless if any output was created.
 +
 +<​code>​
 +darkstar:~% at 12:01 -m
 +warning: commands will be executed using (in order) a) $SHELL b) login shell c) /bin/sh
 +at> ./​my_script.sh
 +at> <EOT>
 +job 4 at 2015-06-22 12:01 
 +darkstar:​~% ​
 +</​code>​
 +
 +=====File-driven at=====
 +Commands can also be contained within a file and run by **//at//**:
 +<​code>​
 +darkstar:~% at 12:32 -m -f /​usr/​local/​bin/​my_script.sh
 +warning: commands will be executed using (in order) a) $SHELL b) login shell c) /bin/sh
 +job 8 at 2015-06-22 12:10
 +</​code>​
 +The **-m** flag will email the user after completion of the command; the **-f** flag specifies the command will read the job from a file, not from standard input. After the command is typed in (and the appropriate warning is displayed), the **//at//** job number((As distinct from a process ID (PID) known to the operating system)) is displayed.
 +
 +=====at Internal Scheduling=====
 +The job numbers provided after a command is typed in, or when a file is read, allow the user to know which internal job will be run in sequential order. If a user wants to delete a specific task, all that needs to be known is this internal job number. To remove the job, the command **//​atrm//​** (**__at__ __r__e__m__ove**) is used:
 +
 +<​code>​
 +darkstar:​~% ​ at -l
 +7      2015-06-22 12:10 p tux
 +8      2015-06-22 12:15 p root
 +</​code>​
 +
 +The command **//atq//** (**__at__ __q__ueue**) is the same as **//at -l//**:
 +
 +<​code>​
 +darkstar:​~% ​ atq
 +7      2015-06-22 12:10 p tux
 +8      2015-06-22 12:15 p root
 +</​code>​
 +
 +To remove the user job, use **//​atrm//​** with the job number:
 +<​code>​
 +darkstar:​~% ​ atrm 7
 +</​code>​
  
 =====Using cron===== =====Using cron=====
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 The //crontab// file syntax is: The //crontab// file syntax is:
  
-minute(0-59hour(0-23) day(1-31) ​month(1-12weekday(0-6command+<​code>​ 
 + # * * * * *  command to execute 
 + # │ │ │ │ │ 
 + # │ │ │ │ │ 
 + # │ │ │ │ └───── day of week (0 - 6(Sun(0) /Mon (1)/Tue (2)/Wed (3)/Thu (4)/Fri (5)/Sat (6)) 
 + # │ │ │ └────────── month (1 12) 
 + # │ │ └─────────────── ​day of month (1 - 31) 
 + # │ └──────────────────── hour (23) 
 + # └───────────────────────── min (0 - 59) 
 +</​code>​
  
 Using an asterisk in any placeholder location, will match any value. For example, the following will run //​example_script.sh//​ at noon (1200) everyday during the first three months of the year: Using an asterisk in any placeholder location, will match any value. For example, the following will run //​example_script.sh//​ at noon (1200) everyday during the first three months of the year:
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 =====Using anacron===== =====Using anacron=====
 <​note>​ <​note>​
-**//​anacron//​** is not installed in Slackware by default. ​+**//​anacron//​** is not installed in Slackware by default.((See [[http://​slackbuilds.org/​repository/​13.37/​system/​anacron/​|Slackbuilds.org]] for more information on **//​anacron//​** on Slackware)) ​
 </​note>​ </​note>​
  
- +**//​anacron//​** is unique from **//​cron//​** ​in the respect that it does not expect the operating system ​to be running continuously like a 24x7 server. If the time of execution passes while the system is turned off**//​anacron//​** executes the command automatically when the machine is turned back on. The reverse is **not** true for **//​cron//​** - if the computer is turned off during the time of scheduled execution, **//​cron//​** will not execute the job. Another key difference between **//​anacron//​** and **//​cron//​** is the minimum chronological "​granularity"​ - **//​anacron//​** can only execute jobs by //day//, versus the ability of **//​cron//​** ​to execute by the //minute//Finally, **//​anacron//​** can __only__ be used by root, while **//​cron//​** can be used by root and normal users.
- +
-<​note>​ +
-The links in red are to non-existent articles. +
- +
-If you are experienced with those toolsplease feel free to write the articles. +
-</note>+
  
 ====== Sources ====== ====== Sources ======
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   * Originally written by [[wiki:​user:​vharishankar | vharishankar]]   * Originally written by [[wiki:​user:​vharishankar | vharishankar]]
   * Contrbutions by [[wiki:​user:​mfillpot |mfillpot]],​ [[wiki:​user:​tdrssb |tdrssb]]   * Contrbutions by [[wiki:​user:​mfillpot |mfillpot]],​ [[wiki:​user:​tdrssb |tdrssb]]
 +  * Example crontab example modified from [[en.wikipedia.org/​wiki/​cron]]
 <!-- Please do not modify anything below, except adding new tags.--> <!-- Please do not modify anything below, except adding new tags.-->
 <!-- You must also remove the tag-word "​template"​ below. Otherwise your page will not show up in the Table of Contents --> <!-- You must also remove the tag-word "​template"​ below. Otherwise your page will not show up in the Table of Contents -->
 {{tag>​howtos task_scheduling needs_attention author_vharishankar author_mfillpot}} {{tag>​howtos task_scheduling needs_attention author_vharishankar author_mfillpot}}
 +

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