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Basic usage of crontab

 ·   ·  ☕ 7 min read

    What is crontab?


    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


    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)

    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

    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/

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

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

    @reboot /home/pi/ > /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/ > /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”.


    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 :

    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


    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, will do this for you

    Ohidur Rahman Bappy
    Ohidur Rahman Bappy
    📚Learner 🐍 Developer