How to automate tasks with CRON on Ubuntu

Photo by Alex Knight on Unsplash

A cron job is a Linux command used for scheduling tasks to be executed sometime in the future with a pre-decided time and date pattern. This is normally used to schedule a job that is executed periodically — for example, to send out an email notification every morning.

Pre-requisites

• Access to Ubuntu 18.04 or higher
• Script to be executed with cron
• CRON daemon setup (We’ll cover it anyways)

As a quick guide, here is our sample Python script in place

mock1.py

from datetime import datetime
print("CRON executed at - \n")
print(datetime.now().strftime("%D %H:%M:%S"))

Virtual environments & CRON

Your .py script would be mostly built around the virtual environment bubble containing necessary python packages.

And to ensure that your cron job accesses the environment-specific modules associated with its own virtual environment, just quickly run below command in your activated virtual environment:

>> which python3
/home/user1/mock_project/my_venv/bin/python3

..which returns above python executable path. Keep it handy for our next step

Next we’ll install and setup cron daemon in our Ubuntu environment:

• Update your computer’s local package dependencies

sudo apt update  # remove sudo if you do not require root privileges

• Install cron

sudo apt install cron

• Make sure its set to run in the background

sudo systemctl enable cron

• Check cron active/inactive status

sudo systemctl status cron

• Add your cron job(s) by enabling crontab

crontab -e

If its your first time setting up cron, you’ll be prompted to select a default text editor when editing jobs within crontab.

You can choice either of the options comfortable with. For now let’s go with nano editor —

Now, you’ll notice there is no Time zone indicated — by default UTC. We can change that with our local time zone — example IST (Indian Standard Time)

TZ = "IST" # first LOC below documentation comments

Now, there are essentially 3 parts of the cron job command we need to focus on —

cron pattern | python path | python script

Writing the correct syntax patterns for cron jobs seems a tedious task but I personally find crontab.guru very easy to experiment with.

Snapshot crontab.guru

Our python script would be executed every Friday at 11.03 am.

3 11 * * FRI /home/user1/mock_project/my_venv/bin/python3 /home/user1/mock_project/mock1.py

Optionally, you can track error logs if the cron job fails at any given point by appending a simple parameter to our cron command:

3 11 * * FRI /home/user1/mock_project/my_venv/bin/python3 /home/user1/mock_project/mock1.py > /custom_path/abcd.log 2>&1

Save and close the crontab; view all listed cron jobs(read mode only):

crontab -l

To view currently running cron jobs execute:

sudo grep CRON /var/log/syslog

If you are interested in a quick test-run of cron daemon on your Ubuntu system just enable below cron job that will print current date every minute:

* * * * * /bin/bash -l -c 'date > ~/cron-test.txt'

Useful tips

As part of your Web applications that are IaaS based and require periodic database backups, you can setup daily/weekly CRON jobs.

For example — Django provides a single LOC to dump database at specific location. You may go a step ahead and dump your DB files encrypted in private Cloud storage!

And that’s all needed, hope it serves as a quick guide. Always appreciate feedback and thoughts.

Other articles that helped me along the way

https://www.digitalocean.com/community/tutorials/how-to-use-cron-to-automate-tasks-ubuntu-1804