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  1. #1

    crontab explained

    Introduction:

    cron is a utility that you can use to schedule and automate tasks. By defining items in the cron table, called crontab, you can schedule any script or program to run on almost any sort of schedule

    Basics:

    Each user has their own crontab, the scheduled scripts run as that user take this in account with regards to permissions. To edit the crontab use the following command:

    Code:
    $ crontab -e
    You can list what your currnet crontab is using the following command:

    Code:
    $ crontab -l
    root user can view user's crontab

    Code:
    #crontable -u linuxweb
    Crontab Format

    The following is the format entries in a crontab must be.


    MIN HOUR MDAY MON DOW COMMAND 5 * * * *

    Code:
    MIN =  Minute 0-60
    
    HOUR = Hour [24-hour clock] 0-23
    
    MDAY = Day of Month 1-31
    
    MON = Month 1-12 OR jan,feb,mar,apr ...
    
    DOW =  Day of Week 0-6 OR sun,mon,tue,wed,thu,fri,sat
    
    COMMAND = Command to be run Any valid command-line
    Examples:

    Here are a few examples, for crontab entry

    Code:
    #Run command at 7:00am each weekday [mon-fri]
    00 07 * * 1-5 (/sbin/diskusages.sh);
    Code:
    #Run command on 1st of each month, at 5:30pm
    30 17 1 * * (/home/backup.sh);
    Code:
    #Run command at 8:00am,10:00am and 2:00pm every day
    00 8,10,14 * * * (/sbin/sendstatus.sh);
    Code:
    #Run command every 5 minutes during market hours
    */5 6-13 * * mon-fri get_stock_quote.script

    Special Characters in Crontab


    You can use an asterisk in any category to mean for every item, such as every day or every month.

    You can use commas in any category to specify multiple values. For example: mon,wed,fri

    You can use dashes to specify ranges. For example: mon-fri, or 9-17

    You can use forward slash to specify a repeating range. For example: */5 for every five minutes, hours, days

    Special Entries

    There are several special entries, some which are just shortcuts, that you can use instead of specifying the full cron entry.

    The most useful of these is probably @reboot which allows you to run a command each time the computer gets reboot. This could be useful if you want to start up a server or daemon under a particular user, or if you do not have access to the rc.d/init.d files.

    Example Usage:
    Code:
    # restart freevo servers
    @reboot freevo webserver start
    @reboot freevo recordserver start
    The complete list:

    Entry Description Equivalent To

    Code:
    @reboot Run once, at startup. None
    
    @yearly Run once a year 0 0 1 1 *
    
    @annually (same as @yearly) 0 0 1 1 *
    
    @monthly Run once a month 0 0 1 * *
    
    @weekly Run once a week 0 0 * * 0
    
    @daily Run once a day 0 0 * * *
    
    @midnight (same as @daily) 0 0 * * *
    
    @hourly Run once an hour 0 * * * *

    Miscelleanous Issues


    Script Output

    If there is any output from your script or command it will be sent to that user's e-mail account, on that box. Using the default mailer which must be setup properly.

    You can set the variable MAILTO in the crontab to specify a separate e-mail address to use. For example:


    MAILTO="admin@linuxwebadmin.info"

  2. #2
    Join Date
    Mar 2006
    Posts
    418
    If you dont wish to recieve any output, even though the script you're running returns some (for example a script runing every 5 mins and returning some output would quickly spam your email account), you can add this to the end of the script command: > /dev/null 2>&1

    Example:
    Code:
    #Run command every 5 minutes during market hours
    */5 6-13 * * mon-fri get_stock_quote.script > /dev/null 2>&1

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