This guide covers how to use Cron (crontab) to run desired tasks automatically at specific times.
[01] What is Cron?
A time-based job scheduler on Linux. Use it when you need to run a task periodically at a specific time, date, or interval. Configure and run jobs using crontab.
[02] Install
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sudo apt-get install update
sudo apt-get install cron
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[03] Commands
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# View the current user's crontab
crontab -l
# Edit the current user's crontab
crontab -e
# Remove the current user's crontab
crontab -r
# ex)
root@emaster:~# crontab -l
no crontab for root
root@master:~# crontab -e
no crontab for root - using an empty one
crontab: installing new crontab
root@master:~# crontab -l
# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').
#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command
51 14 19 * * /root/code/.venv/bin/python3 /root/code/test.py
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[04] Configuration (Rules)
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* * * * * command
m h dom mon dow
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- minute (0-59), hour (0-23), day of month (1-31), month (1-12), day of week (0-6: 0 = Sunday, 6 = Saturday)
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# On the 19th of every month, at 14:51,
# run the /root/code/test.py program
# inside the /root/code/.venv/bin/python3 Python virtual environment
51 14 19 * * /root/code/.venv/bin/python3 /root/code/test.py
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Reference: Verify schedule timing (crontab.guru)
[05] Service Control
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# Check status
service cron status
# Start
service cron start
# Stop
service cron stop
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When you want a Python script to run inside a virtual environment, include the shell shebang line at the top of the Python file.
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#! /root/code/.venv/bin/python3 /root/code/test.py
print("Hello")
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