rajarshig.github.io

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

References:

This site is open source. Improve this page.