CRON
What is a cronjob, and how do I use it?
The cron daemon is a long running process that executes commands at specific
dates and times. To schedule one-time only tasks with cron, use at or batch. For
commands that need to be executed repeatedly (e.g. hourly, daily or weekly),
use crontab, which has the following options:
crontab filename Install filename as your crontab file.
crontab -e Edit your crontab file.
crontab -l Show your crontab file.
crontab -r Remove your crontab file.
MAILTO=user@domain.com Emails the output to the specified address.
The crontab command creates a crontab file containing commands and how
often cron should execute them. Each entry in a crontab file consists of six fields,
specified in the following order:
minute(s) hour(s) day(s) month(s) weekday(s) command(s)
The fields are separated by spaces or tabs. The first five are integer patterns and
the sixth is the command to be executed. The following table briefly describes
each of the fields:
Field Value Description
minute 0-59 The exact minute that the command sequence executes.
hour 0-23 The hour of the day that the command sequence executes.
day 1-31 The day of the month that the command sequence executes.
month 1-12 The month of the year that the command sequence executes.
weekday 0-6 The day of the week that the command sequence executes.
Sunday=0, Monday = 1, Tuesday = 2, and so forth.
command Special The complete command sequence variable that is to be
executed.
Each of the patterns from the first five fields may either be an asterisk (*)
(meaning all legal values) or a list of elements separated by commas. An
element is either a number or two numbers separated by a minus sign (meaning
an inclusive range). Note that the specification of days may be made by two
fields (day of the month and day of the week). If both are specified as a list of
elements, both are followed. For example:
MAILTO=user@domain.com
0 0 1,15 * 1 /big/dom/xdomain/cgi-bin/scriptname.cgi
The cron daemon would run the program scriptname.cgi in the cgi-bin directory
on the first and fifteenth of each month, as well as on every Monday. To specify
days by only one field, the other field should be set to *. For example:
MAILTO=user@domain.com
0 0 * * 1 /big/dom/xdomain/cgi-bin/scriptname.cgi
The program would then only run on Mondays.
If a cron job specified in your crontab entry produces any error messages when it
runs, they will be reported to you via email.
You may create crontab files in notepad (being sure to upload them in ASCII) or
you may create them from the command line (via SSH) by simply typing:
mcedit cronfile.txt
For more information, consult the man pages. man pages are the directions and
tutorials available to you right at the command line. Type any of the following
lines to open the relevant tutorials ([Enter] means to hit the Enter (return) key):
man 5 crontab [Enter]
man 1 crontab [Enter]
man cron [Enter]
man at [Enter]
man batch [Enter]
man 1 cron [Enter]
Note:Your crontab file must end with a line feed - in other words, make sure to
press [Enter] after the last line in the file.
Try It!
Now that you have read an overview of cron, test your skills by following the
steps below. Once completed, you should have a cron file of your own!
Step 1: Create a simple text file using Notepad or any simple text editor that
contains the following text:
MAILTO=yourusername@yourdomain.com [Enter]
58 23 * * * /big/dom/xdomain/cgi-bin/yourscript.pl
[Enter]
Notes for Step 1
You may create this file using your CNC File Manager by navigating to the
/big/dom/xdomain/ directory and clicking 'Create New File' or any other simple
text editor such as Notepad.
[Enter] should not actually be typed. [Enter] means hit the "Enter" (return) key to
begin the next line and to add a blank line feed at the end of the last line of your
cron file. It is important to always remember to do this.
MAILTO: Replace the email address with a valid email address of your own. This
will ensure that when your cron runs, any output from the script, such as an error
message, will be emailed to you.
The second line tells your server when to run this script. In this example, the
script will be run at 11:58 PM Eastern Time every day of the year.
It is very important that you double-check the script path to ensure it is correct
and remember the file names are CaSe-SeNsiTive.
Step 2: Name the text file (example: cronfile.txt).
The cronfile name may be replaced with any name you choose. For instance, if
you are running a cron to trigger an email reminder script, it could be called
reminder.txt. Many choose to simply call it cronfile.txt.
Step 3: Upload the file in ASCII.
Any standard FTP client or your account's CNC upload feature will work for this.
The file must be uploaded in ASCII mode and it is recommended that it be placed
in your /big/dom/xdomain/ directory. It may be placed anywhere in your account
but to prevent browser access (security risks) it is strongly recommended to
place it above your /www directory.
Step 4: Connect to your account via SSH and issue the following command:
crontab /big/dom/xdomain/cronfile.txt
The above tells the server's crontab where the file is located and that you wish to
make it active. Make sure the path to the file is the actual path to where the file
was placed. If successful, you will be returned to the command bash line. If not,
an error will be displayed.
IMPORTANT NOTES
Removing/Stopping the Cron: Deleting the cronfile.txt file from your account will
not stop the cron. You may remove this file at any time, however since the
server's crontab already has the contents, the cron will still run once it has been
made active.
To turn the cron off, you must connect to your account via SSH and issue the
following command:
crontab -r
The crontab -r will deactivate the cronjob and remove the file contents from the
server.
Security Note: If the script the cron is set up to run is in the /cgi-bin/ or /www/
directory, it may be run at any time by anyone with browser access. If the crontab
is all that should run the script and you do not want the public to be able to run
the script, then you will need to place the script in a directory above the /cgi-bin/
and /www/ directories such as: /big/dom/xdom/user/cronscripts/scriptname
