Basic usage of crontab

What is crontab?

Cron

Cron is a service, automatically started at each boot of the Raspberry Pi, which allows the user to execute scheduled commands

Every minute, cron will watch if he has to do something and do it
What we’re going to see today is how to tell cron to execute our command or script when needed

Crontab

A crontab is a tool that will allow us to list what we want to start, in a format understandable by the cron service

A crontab will contain two things:  
– the list of commands to run  
– when to run them

Crontab is also a command. Here’s the syntax :

crontab [-u user] file
crontab [-u user] [-l | -r | -e] [-i] [-s]

What you need to know :

u: crontab is opening the current user crontab by default, but you can specify which one to open (if you have enough privileges)
l: list current crontab content
e: edit current crontab content
r: remove current crontab content

Most of the time you will only use the commands “crontab -l” or “crontab -e”

How to schedule a task?

My first scheduled task

It’s time to take action.
Let’s plan a simple task, so open our crontab for the first time with the command:

crontab -e

First thing, you have to choose an editor. I advise you to stay on nano, so keep the default choice and hit enter :

pi@raspberrypi:~ $ crontab -e
no crontab for pi - using an empty one

Select an editor.  
To change later, run 'select-editor'.
  1. /bin/ed
  2. /bin/nano        <-- – easiest
  3. /usr/bin/vim.tiny

Choose 1-3 [2]:

That’s it. You are now in the editor of crontab, which is empty and can be a little scary if it’s the first time you access it ![🙂]
Do not panic. I’ll explain what to do

First of all, all the lines starting with a # are comments, they do nothing and we can ignore them.
Our changes will be done at the end of the file, move your cursor down with the bottom arrow, and copy this line:

* * * * * echo `date` >> /home/pi/log

Save the file (CTRL+O and Enter) and quit (Ctrl+X)

This simple line in the crontab will allow us to execute a command every minute, which will write the date in a file.
After a few minutes, the file will contain the dates of execution of the command

Save the file (CTRL+O and Enter) and quit (Ctrl+X)

You are probably wondering what the five stars mean
The syntax of an entry in the crontab is as follows:

1 2 3 4 5 command
1: Minute (between 0 and 59)
2: Hour (between 0 and 23)
3: Day (between 1 and 31)
4: Month (between 1 and 12)
5: Day of week (between 0 and 7, starting on Sunday)

Basic example

Now that you understand the theory, let’s look at a simple example to be sure it’s clear
Imagine that you want to run a backup script every Wednesday at midnight
You must add a line like this :

0 0 * * 3 /home/pi/backup.sh

Midnight for the two first 0, and 3 for the day of the week (Wednesday)

Other examples
There are then a lot of possibilities to match the crontab with what you need

Launch a script at fixed hours

0 6,12 * * * /home/pi/backup.sh   => will run at 6 and 12 only
Start a script every 2h
0 */2 * * * /home/pi/backup.sh => will run every 2hours (so 0, 2, 4, 6, ...)
Schedule a script only during the weekdays
0 3 * * 1-5 /home/pi/backup.sh => will not run on Saturday/Sunday
You can also start something on boot
@reboot /home/pi/backup.sh   => will run at every boot

Adding debug
We will see at the end of the article how to debug a cron that does not start, or not at the time you have planned.
But it may be easier to save the displayed messages or script errors in a file

To do this, you must add one of these options in the crontab:

To log in a file what the script would have displayed on the screen if you had launched it manually, you must specify the name of the file with the character “>” :

@reboot /home/pi/backup.sh > /home/pi/log_backup.txt

But this option will erase the file at each run

So if you want to add a new line at the end of the file, you have to add the character “»”, like this :

@reboot /home/pi/backup.sh >> /home/pi/log_backup.txt

Now if you want to log errors in another file you have to add this :

@reboot /home/pi/backup.sh > /home/pi/log_backup.txt 2>/home/pi/errors_backup.txt

And finally, if you want to save errors and the displayed in the same file, you can do this :

@reboot /home/pi/backup.sh > /home/pi/log_backup.txt 2>&1

You should be starting to understand the little tricks by now, but unfortunately in IT things rarely happen as expected.
I will give you some tips to fix the errors with the crons on your Raspberry Pi

Crontab tips

User privileges

A common mistake in creating crons is to forget to consider the privileges of the user who will start the cron

For example, this cron in the default user of the Raspberry (“pi”) will not work :

@reboot /usr/sbin/service ssh start

You will get an error like this :
Failed to start ssh.service: Interactive authentication required.

If you use the current user’s crontab, the cron will run with your current privileges
Pi is not allowed to start a service, so it can’t work

For this to work, you must add this line in the root crontab (sudo crontab -e) or the global crontab found in /etc/crontab

Absolute path

Another widespread mistake using crons is to ignore the file path
You must use the full path to make it work properly

This cron will not work, even in the root crontab :

@reboot service ssh start

If you do not specify the absolute path, cron will not know where the service file is

So you have to write /usr/sbin/service to make this cron work

Also pay attention to the content of your scripts
For example, if you have a PHP script that includes another file (ex: include “file.php”), and you add this script to the crontab, it will not work
You will need to add the absolute path in the function include or do something like this:

@reboot cd /var/www/html; /usr/bin/php test.php

This way, the include will be done in /var/www/html and the PHP script will find the file “file.php”.

Debugging

In addition to what I wrote above, there are two other methods that I will introduce to debug your crons

Emails :

Cron will send an email to the user if there is a problem with one of his scheduled tasks in the crontab
If you have a mail server installed on your Raspberry Pi, you can check the errors in the email file of your user

You can easily install one by doing :

$ sudo apt-get install mailutils

After that, you can type “mail” to read your emails

If you have a well-configured email server, you can redirect emails to your email address by adding something like this to your crontab :

MAIL=yourname@provider.com

Syslog :

Syslog is another valuable help to check what happened with your crons
It’s a log file located in /var/log/syslog

You can read the last messages about crons with this command :

tail -f /var/log/syslog | grep CRON

It will show you the last errors, with real-time refresh if a new cron starts

Conclusion

Now you know what a cron and a crontab is, how to schedule a task or a script on Raspberry Pi with many options and how to find out what didn’t work as you want

Crons are something fundamental in Raspberry Pi and Linux in general.
I hope that you understand better how they work, it will serve you very often

If you have doubts about planning a cron, know that there are websites that allow you either to create your planning or to check if what you did is what you wanted.
For example, crontab.guru will do this for you