Scheduling Recurring Tasks with cron

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Let’s first see what does cron represent from wiki:

The software utility cron is a time-based job scheduler in Unix-like computer operating systems. People who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals.

cron is most suitable for scheduling repetitive tasks. For example, it runs log file rotation utilities to ensure that your hard drive doesn’t fill up with old log files. You should know how to use cron because it’s just plain useful.

Also see cronjob in K8s.

Install crontab

In CentOS or RedHat, you can run:

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yum install cronie

If you are not sure, try yum provides crontab to see which package will provide this service.

To check if cron service is running or not:

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systemctl status crond

If inactive, enable and restart it.

crontab File

Cron is driven by a crontab(cron table) file, a configuration file that specifies shell commands to run periodically on a given schedule.

The program running through cron is called a cron job. To install a cron job, you’ll create an entry line in your crontab file, usually by running the crontab command.

Each user can have his or her own crontab file, which means that every system may have multiple crontabs, usually found in /var/spool/cron/ folder. the crontab command installs, lists, edits, and removes a user’s crontab.

crontab Commands

For example, run as root, I want to set a recurring task for user dsadm:

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crontab -u dsadm -e

Then edit like this:

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00 21 * * * /home/dsadm/test.sh > /tmp/cron-log 2>&1

This means on everyday at 9:00PM, user dsadm will run test.sh and redirect output to /tmp/cron-log file. You can also put the entries into a file and run:

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crontab -u dsadm <entry file>

The meaning of the entry is:

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# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │                                   7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * command to execute

A * in any field means to match every value.

Now, if you check /var/spool/cron directory, the dsadm crontab file is created there.

To list the dsadm cron job:

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crontab -u dsadm -l

To remove dsadm cron job:

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crontab -u dsadm -r

Run as Non-Root

If you want to run crontab as dsadm, you must set the cron permission:

  • /etc/cron.allow - If this file exists, it must contain your username for you to use cron jobs.
  • /etc/cron.deny - If the cron.allow file does not exist but the /etc/cron.deny file does exist then, to use cron jobs, you must not be listed in the /etc/cron.deny file.

So, if you put dsadm in /etc/cron.allow file, then you can use crontab directly.

System crontab File

Linux distributions normally have an /etc/crontab file. You can also edit here, but the format is a little bit difference:

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# Example of job definition:
# .---------------- 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