Setup Cron job in Ubuntu 18.04

Published at: 17/5/2020

Cron is a tool for time based job scheduling - found in most of Unix based operating systems. Although it was developed long time ago, its simplicity & effectiveness is remarkable.

Use

Any script or command which is to be executed in some time interval, can be set with Cron.

Points to remember

  • Absolute paths for script or files need to be provided. For eg. I have a python file in a virtual environment which I want to set with Cron. In that case, I have to provide the command as below
/home/username/venv/bin/python  /home/username/venv/project/script.py
  • It need to be planned if the output of the command (if any) need to be saved. It is also possible to setup email integration with a Cron job.

Steps

  • Below command opens the crontab file for current user. At the bottom, we can add multiple cron entries 
    crontab -e
  • A single cron entry follows m h dom mon dow format, which means minute hour dayofmonth month dayofweek command. Detail instruction & example are also included in the file itself
  • To run a python script on 12PM daily, we would add below entry 
    0 12 * * * /home/username/venv/bin/python  /home/username/venv/project/script.py
  • To ignore a field, use *. Then only the non * fields will be considered, as here I have ignored the dom, mon, dow fields, so the Cron will run daily
  • As the minimum time field is minute, we can not use seconds for Cron
  • After entering the entry, use Ctrl+O & Ctrl+X to save

  • Ensure there are no \n character in the entry line, else parsing error will be thrown while saving

  • The Cron dameon will check the entries in its own, so not need to restart it after entry
  • The generated crontab file can be seen at /var/spool/cron/crontabs/username
  • The log of the Cron job can be found in syslog. We can search Cron logs by username 
    cat /var/log/syslog | grep "CRON" | grep "username"

Conclusion

  • Although Cron has its limitation, it provides a very stable way to automate administrative tasks
  • It provides many more features & options for various use cases

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