====== Schedule Tasks Using At And Cron ======

**General Information**

Using system utilities to schedule one-off or reoccurring jobs. 

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===== at =====

at is good for one-off jobs that don't need to reoccur regularly.

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Install, Enable, Start
<code bash>
yum install at
systemctl enable atd
systemctl start atd
</code>

\\
Example Times (teatime is 4pm)
<code bash>
at now +5 minutes
at now +5 hours
at 12:00am
at teatime
<enter command> <ctrl+d>
</code>

\\
Example job
<code bash>
at now +5 minutes
at>logger "System uptime is:$(uptime)"
at> <ctrl+d>
</code>

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View jobs
<code bash>
atq
</code>

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Remove job (cancel job)
<code bash>
atrm <job#>
</code>

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===== cron =====

cron is for scheduling reoccurring jobs/scripts.

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Edit current user's cron
<code bash>
crontab -e
</code>

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System wide
<code bash>
/etc/crontab

# .---------------- minute (0 - 59)
# |  .------------- hour (0 - 23)
# |  |  .---------- day of month (1 - 31)
# |  |  |  .------- month (1 - 12) OR jan,feb,mar,apr ...
# |  |  |  |  .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# |  |  |  |  |
# *  *  *  *  * user-name  command to be executed
</code>

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More preferred method: Place scripts inside one of the cron.* directories.
  * /etc/cron.d/ = system executed jobs (format => /etc/crontab)
  * /etc/cron.daily/ = daily executed scripts (format => bash script)
  * /etc/cron.hourly/ = hourly executed scripts (format => bash script)
  * /etc/cron.monthly/ = monthly executed scripts (format => bash script)
  * /etc/cron.weekly/ = weekly executed scripts (format => bash script)

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Example custom script in /etc/cron.d
<code bash>
*/5 * * * * root /home/user/uptimelog
</code>
  * Executes /home/user/uptimelog as root, every 5 mins

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===== Anacron =====

Anacron runs all scripts in cron.* directories. This allows jobs to be run if the scheduled time was while a system was powered off.

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Main config and contents: /etc/anacrontab
<code bash>
#period in days   delay in minutes   job-identifier   command
1	5	cron.daily		nice run-parts /etc/cron.daily
7	25	cron.weekly		nice run-parts /etc/cron.weekly
@monthly 45	cron.monthly		nice run-parts /etc/cron.monthly
</code>

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Run all anacron jobs(in /etc/anacrontab), ignore timestamps (note that the "delay in minutes" setting is still in effect)
<code bash>
anacron -f
</code>

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View timestamps for anacron jobs (cron.daily in this example)
<code bash>
cat /var/spool/anacron/cron.daily
</code>

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Run all jobs now, ignore delay specifications in /etc/anacrontab
<code bash>
anacron -n
</code>

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