Cron is a schedule based service
that runs on just about every UNIX
and Linux server. It allows you to
run commands and scripts at a
certain time and date that you can
set in a "crontab". Cron can be set
to run any Perl or PHP script and
basically any command that can be
run from a command line.
In this tutorial I will explain
exactly what cron is and how you can
use it to schedule the execution of
the system's scheduled sending
script (so you can schedule your
email campaigns to be sent at a
later date) on your web server.
I will also show you briefly how to
create a crontab in Plesk Reloaded
and in Cpanel (Xskin) using the
online control panels. I will be
using the system as the application
that I am setting up the cron job
for. The actual scheduled sending
script will be used to send out
emails and autoresponders.
Getting Started
In order to get started with cron,
you must have a server that is
currently running cron. Your host
will also be able to tell you where
you are able to setup crontabs --
whether it's from a web based
control panel, or through a telnet
terminal window.
Cron - Paint by Colors
Lets explore and begin to understand
the crontab command and then we'll
be able to apply it using the method
that we have available (via control
panel or telnet). Look at the
following basic crontab:
A typical crontab command,
explained below.
I'll color code each "section" to
allow me to visually explain the
different parts of this crontab.
A color coded crontab command.
Here's an explanation of the above
cron command:
Minute of the hour
Set to a number between 0 – 59
for the corresponding minute of
the hour or * for every minute
Hour of the day
Set to a number between 0-23 for
the corresponding hour of the
day or * for every hour
Day of the month
Set to number between 1-31 for
the corresponding day of the
month or * for every day
Month of the year
Set to a number between 1-12 for
the corresponding month of the
year or * for every month You
can use the name of the month of
the year as well eg. march (not
case sensitive)
Day of the week
Set to a number between 0-7 for
the corresponding day of the
week (0 or 7 is Sunday) You can
use the name of the day of the
week as well. Eg. Tuesday (not
case sensitive)
Beginning of the command line
Here you specify the path to PHP
or Perl and any flags you might
need.
The path to your script
Specify the path to your script
that you want to run.
What is the asterisk and what
does it do?
The Asterisk specifies that the
script should be run on every
occurrence of a time change. For
example, if you want to set a cron
to run every hour, then you would
place an asterisk in the 'hour'
column. Eg. 0 * 1 1 1 Setting a
range within a column.
You can set a range of values in the
cron command so that it will run
within that range. Eg. 2-5 will run
2, 3, 4 and 5 of the column you set
it in. In the day of the month
column, for example, this 2-5
setting will run on the 2nd, 3rd,
4th and 5th of the month and will
look like this. 0 * 2-5 * * Setting
the frequency within each column.
If you wanted to run a cronjob every
5 minutes, you would be able to use
*/5 in the minutes column. If it was
every 5 hours, then */5 could be
placed in the hours column. Many
would confuse this */5 as a
fraction, but it is actually setting
the frequency that the cron takes
place in that column. It means
'every five' rather than a fifth of
it.
The command line
This section can be any command that
can be run from a command line. In
this case we are setting it up to
run a script.
Path to PHP – UNIX and Linux
have a special variable called
env php which tells you where
PHP is installed on your server.
We'll use this instead of trying
to figure out the path to PHP
manually.
Flags for output – In this
example I've used the –q flag
which runs cron in quiet mode;
this flag stops some unnecessary
messages. You can also add
/dev/null after the server path to
stop error messages being output
as well.
Server Path to your script –
You will need to specify the full
server path directly to your
script. You can find this on the
settings page, as shown below:
The path to the scheduled
sending script from the settings
page.
Setting up cron to run the
scheduling script
Now that we know what the different
parts of the command line actually
do, we can begin to setup a cron job
for a real use. In the system, we
need to run the send_emails.php file
(for sending newsletters) and the
send_autos.php file (for
autoresponders) in the "users"
folder of the application. (Please
see the documentation for exact file
names as this may change with later
versions of the system.)
Decide what frequency you wish
to run the script. For every hour
on the hour, you would use 0 * * *
*
Create the cronjob. Login to
telnet or your control panel and
type crontab -e at
the command line. This will let
you edit your crontab file. Type
this text in:
Setting Crontabs Through Your
Hosting Control Panel
Some 'front ends' to cron allow you
to set the time and the command line
for crontabs through your online
hosting control panel. Look below
for some examples of Cpanel and
Plesk.
Cpanel 9 (Xskin)
Login to your control panel and look
for the cronjobs icon There are two
'modes' to set cron jobs in Cpanel –
you can choose either the standard
or the advanced mode. The standard
mode looks like this:
Setting up a cronjob in Cpanel
using the simple options.
This is a very simple way to add the
frequency of the cron without
needing to know what the asterisk or
the numbers mean. In the command
line you then add your path to PHP
and the full path to your script. In
the top field, you place the email
address where the output will be
sent – this is usually to alert you
of errors etc.
For the advanced mode you will see
the following:
Setting up a cronjob in Cpanel
using the advanced options.
In this mode, you simply place the
full command and then enter your
frequency in the corresponding
columns – they are clearly labeled
so you can see which column are
using as well.
Plesk (Reloaded)
Login to your Plesk control panel
and look for the crontab icon.
Crontabs are called 'tasks' in Plesk.
In order to create a task click on
the add new task icon. All current
tasks will be shown in the tasks
section for the specific user(in the
dropdown).You can enter the email
for cron output for this user in the
email address field.
Setting up a cronjob in Plesk
using the tasks section.
Here's the "Add New Task" screen:
The 'Add New Task' screen in
Plesk's tasks section.
All fields are clearly marked so you
know what time factor you are
dealing with. Then just add your
command to the command field and the
task is complete.
