Howto setup crontab

    The purpose of this guide is to show you how to setup crontab task.

    Creating a crontab file

    You can edit crontab by entering the following terminal command for current user:

    $crontab -e
    

    Entering the above command will open a terminal editor with crontab file.

    Crontab syntax

    A crontab file has six fields for specifying minute, hour, day of month, month, day of week and the command to be run at that interval. Seebelow:

    *     *     *     *     *  command to be executed
    -     -     -     -     -
    |     |     |     |     |
    |     |     |     |     +----- day of week (0 - 6) (Sunday=0)
    |     |     |     +------- month (1 - 12)
    |     |     +--------- day of month (1 - 31)
    |     +----------- hour (0 - 23)
    +------------- min (0 - 59)
    

    Examples:

    * * * * * <command> #Runs every minute
    30 * * * * <command> #Runs at 30 minutes past the hour
    45 6 * * * <command> #Runs at 6:45 am every day
    00 1 * * 0 <command> #Runs at 1:00 am every Sunday
    00 1 * * 7 <command> #Runs at 1:00 am every Sunday
    30 8 1 * * <command> #Runs at 8:30 am on the first day of every month
    

    Optional setup

    Disabling email notifications

    By default a cron job will send an email to the user account executing the cronjob. If this is not needed put the following command at the end of the cron job line:

    >/dev/null 2>&1
    

    Specifying crontab to user

    $sudo crontab -u <username> -e
    

    If you want to regularly run a command requiring administrative permissions, edit the root crontab file:

    $sudo crontab -e
    

    Removing a crontab file

    $crontab -r
    

    Change Crontab Email Settings

    The cron will look at “MAILTO” if it has any reason to send mail as a result of running commands in “this” crontab. If MAILTO is defined (and non-empty), mail is sent to the user so named. First open your crontab file:

    $crontab -e
    

    To send email to targetuser@example.com, enter:

    MAILTO=targetuser@example.com
    

    If MAILTO is defined but empty (MAILTO=””), no mail will be sent.

    MAILTO=""
    

    Otherwise mail is sent to the owner of the crontab.

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