Have you ever considered automating basic tasks on your desktop, such as sending timely emails to friends or downloading files from a certain source?

Well, it's possible if you know job scheduling in operating systems.

In this article, I will show how to schedule a job in Linux (I will be using Ubuntu 22.04 here).

Cron

In Linux, "cron" command utility is used for job scheduling. Cron is derived from a Greek word "Chronos" meaning time. All the cron along with the task to be done, is stored in a crontab file.

The syntax of cron is given below

 ┌───────────── minute (0 - 59)
 │ ┌───────────── hour (0 - 23)
 │ │ ┌───────────── day of the month (1 - 31)
 │ │ │ ┌───────────── month (1 - 12)
 │ │ │ │ ┌───────────── day of the week (0 - )
 │ │ │ │ │                                 
 │ │ │ │ │
 │ │ │ │ │
 * * * * * <command>

For example, if you want to represent "25 May 18:30", then it's cron representation will be

30 18 25 5 *

Using Crontab in Ubuntu

Let's run a Python script automatically using Crontab. I will write a Python program to make a simple GUI app using tkinter module.

from tkinter import *

window = Tk(className="App")
window.geometry("300x300")

label = Label(window, text="Hello World")
label.pack()

window.mainloop()

Now save this Python script in a directory and copy the file location.

Open terminal and type

crontab -e

This command will open Crontab file in that terminal (vim/nano/emacs editor based on your preference. I am using nano.).

Remove all the comments if you want.
Add an extra line in that file

43 13 25 5 * /usr/bin/python3 -u /path/of/file/script.py

Note: Specify the path of python3 interpreter (usr/bin/python3 in Ubuntu) while entering a job.
Execute whereis python3 in your terminal to get the path.

So, you will get a new window automatically on time.

There are many useful crontab commands, I will list out here.

crontab -e // opens a new crontab file
crontab -l // shows list of job scheduled
crontab -r // deletes user's crontab
crontab -h // to display list of commands available in crontab 

Accessing Crontab using python-crontab module

Opening Crontab files and entering the jobs manually is a tedious task.
There is a module in Python python-crontab which helps to
schedule the job easily.

Install python-crontab.

pip3 install python-crontab

Create a new Python file. Import and initialise Crontab object with current user

from crontab import CronTab
cron = CronTab(user="<username>")

Note: Type whoami in terminal to get the username.

Set a new command using cron.new() function. Let's execute the same file.

job = cron.new("/usr/bin/python3 -u /path/of/file/script.py")

Now schedule the job.

job.minute.on(43)
job.hour.on(13)
job.day.on(25)
job.month.on(5) # That's 25th May 13:43

Save the changes.

cron.write()

Run this program and check crontab -l to ensure whether job is scheduled or not.

That's it! You have scheduled a job using Python.

Wait, hold on

This is my first article in dev.to. I was working on a project, then I came across job scheduling in Linux, which makes really easy to automate tasks. If you like this article, make sure you follow me on dev.to for more interesting articles.

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