A cron job is a way of periodically executing a command line program. Cron jobs are executed by the cron daemon. cron reads jobs from a cron table (also known as a crontab), which is a text file that holds one job per line. Cron jobs can be created using cPanel.
On free hosting, cron jobs are only allowed to run once every five minutes for each user. This limit includes all of a user's cron jobs. For example, if a user has two jobs, each can only run on average every ten minutes.
 crontab Format
Blank lines and leading spaces and tabs are ignored. Lines whose first non-space character is a pound-sign (#) are comments, and are ignored. Note that comments are not allowed on the same line as cron commands, since they will be taken to be part of the command.
Each line has five time and date fields followed by a command. The fields are separated by whitespace; the amount doesn't matter. As a result, the first five field cannot have any whitespace within the field. Commands are executed by cron when the minute, hour, and month of year fields match the current time, and when at least one of the two day fields (day of month, or day of week) matches the current time*. The time and date fields are:
|day of month||1-31|
|month||1-12 (or names, see below)|
|day of week||0-7 (0 or 7 is Sun, or use names)|
A field may be an asterisk (*), which always stands for "first-last" (meaning all values for that field).
Ranges of numbers are allowed. Ranges are two numbers separated with a hyphen. The specified range is inclusive. For example, 8-11 for an "hours" entry specifies execution at hours 8, 9, 10 and 11.Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: "1,2,5,9", "0-4,8-12".Step values can be used in conjunction with ranges. Following a range with "/<number>" specifies skips of the number's value through the range. For example, "0-23/2" can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is "0,2,4,6,8,10,12,14,16,18,20,22"). Steps are also permitted after an asterisk, so if you want to say "every two hours", just use "*/2".
Names can also be used for the ``month and ``day of week fields. Use the first three letters of the particular day or month (case does not matter). Ranges or lists of names are not allowed.The "sixth" field (the rest of the line) specifies the command to be run. The entire command portion of the line, up to a newline or % character, will be executed by /bin/sh. Percent-signs (%) in the command, unless escaped with backslash (\), will be changed into newline characters, and all data after the first % will be sent to the command as standard input.
For the command to be a valid, the first item must be a valid executable file. The PATH environment variable for the cron process is limited, so you generally need to include the full path to the executable. If the job is to execute a script, the executable should be the interpreter, not the script. For example, to execute a PHP script named "foo.php" that resides in your home directory, use "
*Note: The day of a command's execution can be specified by two fields -- day of month, and day of week. If both fields are restricted (ie, are not *), the command will be run when either field matches the current time. For example, "30 4 1,15 * 5" would cause a command to be run at 4:30 am on the 1st and 15th of each month, plus every Friday.
Run 'foo.php' every Sunday, capturing output:
* * * * Sun /usr/local/bin/php $HOME/foo.php >> $HOME/log/foo.txt 2>&1
Run 'email.php' every day at midnight:
0 0 * * * /usr/local/bin/php $HOME/email.php
Run job1 every 10 minutes (except on the hour), job2 on the hour (except at midnight), job3 at midnight (except Monday) and job4 at midnight on Monday, capturing output:
10-50/10 * * * * $HOME/bin/job1 >> $HOME/log/jobs.txt 2>&1 0 1-23 * * * $HOME/bin/job2 >> $HOME/log/jobs.txt 2>&1 0 0 * * 0,2-6 $HOME/bin/job3 >> $HOME/log/jobs.txt 2>&1 0 0 * * Mon $HOME/bin/job4 >> $HOME/log/jobs.txt 2>&1
With 4 jobs that are to run every 5, 10, 15 and hourly, there will be many conflicts when multiple jobs run at the same time. Resolve this conflict by skipping a more frequent job when it would run at the same time as a less frequent job, which is easily done using commas to specify a set of run times:
5,25,35,55 * * * * $HOME/periodic/5m 10,20,40,50 * * * * $HOME/periodic/10m 15,30,45 * * * * $HOME/periodic/15m 0 * * * * $HOME/periodic/hourly
Or, just to obfuscate things a bit:
5/30,25/30 * * * * $HOME/periodic/5m 10/30,20/30 * * * * $HOME/periodic/10m 15-45/15 * * * * $HOME/periodic/15m 0 * * * * $HOME/periodic/hourly
- Wikipedia entry on cron