I have a hard time remembering the crontab file format, so I thought I'd share an example crontab file here today. This is the root crontab file from a CentOS Linux server, specifically a server I use in a test environment.

As you can see in the example crontab file below, I include a crontab format statement at the top of my crontab file to help me remember the format. After that initial documentation, there are a series of Linux commands that I run at various times during the day to help keep things running smoothly.

Given that introduction, here's my example CentOS Linux crontab file:

#--------------------------------------------------
# example unix/linux crontab file format:
#--------------------------------------------------
# min,hour,dayOfMonth,month,dayOfWeek command
#
# field          allowed values
# -----          --------------
# minute         0-59
# hour           0-23
# 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)
#
#--------------------------------------------------

# run the drupal cron process every hour of every day
0 * * * * /usr/bin/wget -O - -q -t 1 http://localhost/cron.php

# run this apache kludge every minute of every day
* * * * * /var/www/devdaily.com/bin/check-apache.sh

# generate links to new blog posts twice a day
5 10,22 * * * /var/www/devdaily.com/bin/mk-new-links.php

# run the backup scripts at 4:30am
30 4 * * * /var/www/devdaily.com/bin/create-all-backups.sh

# re-generate the blog "categories" list (four times a day)
5 0,4,10,16 * * * /var/www/devdaily.com/bin/create-cat-list.sh

# reset the contact form just after midnight
5 0 * * * /var/www/devdaily.com/bin/resetContactForm.sh

# rotate the ad banners every five minutes

0,20,40  * * * * /var/www/bin/ads/freshMint.sh
5,25,45  * * * * /var/www/bin/ads/greenTaffy.sh
10,30,50 * * * * /var/www/bin/ads/raspberry.sh
15,35,55 * * * * /var/www/bin/ads/robinsEgg.sh

Edit the crontab file with "crontab -e"

As mentioned, this is the root crontab file on a CentOS Linux system, and when I'm logged in as the root user, I edit this file using the following crontab command:

crontab -e

If you haven't seen that command before, here's what it does:

1. It loads the proper crontab file into your editor. In my case, logged in as root, it loads the root crontab file into the vi editor for me.
2. If you change the file during the editing process it signals the crontab daemon that changes have been made. (I assume it sends a kill -HUP command to that daemon, but I don't know that for a fact).

(Hmm, I'll dig into what this actually does, and put another post out here when I learn that.)

The crontab man page

For more information on the Linux crontab format, you can view the crontab man page by issuing the following command:

man 5 crontab

(The regular man crontab command gives you help on the crontab command, not the crontab file format.)

This command gives you the following useful information (and much more):

The time and date fields are:

field          allowed values
-----          --------------
minute         0-59
hour           0-23
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".

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 doesn’t  matter).

Ranges or lists of names are not allowed.

Unix and Linux crontab man pages

For more information on the Unix and Linux crontab system, here are two links to local copies of the CentOS Linux crontab man pages (i.e., crontab help/support documentation):

• The Linux crontab man page that describes the crontab command
• The Linux crontab man page that describes the crontab file format