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--- howtos:general_admin:task_scheduling [2013/10/17 00:18 (UTC)]
mfillpot uncommented links to non-existent articles and added a note asking or contributions
+++ howtos:general_admin:task_scheduling [2015/06/24 19:36 (UTC)] (current)
tdrssb
@@ Line 3: / Line 3: @@
 ====== Task Scheduling in Linux ======
 =====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:
@@ Line 10: / Line 10: @@
    * ''[[howtos:software:anacron|anacron]]'' -- anachronistic cron; a periodic scheduler that doesn't rely on the system being left on 24x7
-<note>
+=====Using at=====
-The links in red are to non-existent articles.
+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.
-If you are experienced with those tools, please feel free to write the articles.
+<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
+      2015-06-22 12:10 p tux
+      2015-06-22 12:15 p root
+</code>
+The command **//atq//** (**__at__ __q__ueue**) is the same as **//at -l//**:
+<code>
+darkstar:~%  atq
+      2015-06-22 12:10 p tux
+      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=====
+**//cron//** is a daemon that runs tasks in the background at specific times. For example, if you want to automate downloads of patches on a specific day (Monday), date (2 July), or time (1300), **//cron//** will allow you to set this up in a variety of ways. The flexibility inherent in **//cron//** can allow administrators and power users to automate repetitive tasks, such as creating backups and system maintenance.
+**//cron//** is usually configured using a //crontab// file. The following command will open your user account //crontab// file:
+<code>
+darkstar:~% crontab -e
+</code>
+To edit the system-level //crontab//, first log into the root account:
+<code>
+darkstar:~# crontab -e
+</code>
+If your system has **//sudo//** installed, type in:
+<code>
+darkstar:~% sudo crontab -e
+</code>
+The //crontab// file syntax is:
+<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 (0 - 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:
+<code>
+#For more information see the manual pages of crontab(5) and cron(8)
+#
+# min hr day month weekday command
+#
+#
+11 * 1-3 * /home/user/example_script.sh
+</code>
+=====Using anacron=====
+<note>
+**//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>
+**//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.
 ====== Sources ======
@@ Line 21: / Line 119: @@
 <!-- Authors are allowed to give credit to themselves! -->
   * Originally written by [[wiki:user:vharishankar | vharishankar]]
-  * Contrbutions by [[wiki:user:mfillpot |mfillpot]]
+  * 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.-->
 <!-- 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}}

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