Cron is a daemon that can be used to schedule the execution of recurring tasks according to a combination of the time, day of the month, month, day of the week, and week.

Cron assumes that the system is on continuously. If the system is not on when a task is scheduled, it is not executed.

To use the cron service, the vixie-cron RPM package must be installed and the crond service must be running. To determine if the package is installed, use the rpm -q vixie-cron command. To determine if the service is running, use the command /sbin/service crond status.

The main configuration file for cron, /etc/crontab, contains the following lines:

SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root HOME=/ 
# run-parts
01 * * * * root run-parts /etc/cron.hourly
02 4 * * * root run-parts /etc/cron.daily
22 4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts /etc/cron.monthly

The first four lines are variables used to configure the environment in which the cron tasks are run. The SHELL variable tells the system which shell environment to use (in this example the bash shell), while the PATH variable defines the path used to execute commands. The output of the cron tasks are emailed to the username defined with the MAILTO variable. If the MAILTO variable is defined as an empty string (MAILTO=""), email is not sent. The HOME variable can be used to set the home directory to use when executing commands or scripts.

Each line in the /etc/crontab file represents a task and has the following format:

minute  hour  day  month  dayofweek  command

minute — any integer from 0 to 59
hour — any integer from 0 to 23
day — any integer from 1 to 31 (must be a valid day if a month is specified)
month — any integer from 1 to 12 (or the short name of the month such as jan or feb)
dayofweek — any integer from 0 to 7, where 0 or 7 represents Sunday (or the short name of the week such as sun or mon)
command — the command to execute (the command can either be a command such as ls /proc >> /tmp/proc or the command to execute a custom script)

For any of the above values, an asterisk (*) can be used to specify all valid values. For example, an asterisk for the month value means execute the command every month within the constraints of the other values.

A hyphen (-) between integers specifies a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4.

A list of values separated by commas (,) specifies a list. For example, 3, 4, 6, 8 indicates those four specific integers.

The forward slash (/) can be used to specify step values. The value of an integer can be skipped within a range by following the range with /<integer>. For example, 0-59/2 can be used to define every other minute in the minute field. Step values can also be used with an asterisk. For instance, the value */3 can be used in the month field to run the task every third month.

Any lines that begin with a hash mark (#) are comments and are not processed.

As shown in the /etc/crontab file, the run-parts script executes the scripts in the /etc/cron.hourly/, /etc/cron.daily/, /etc/cron.weekly/, and /etc/cron.monthly/ directories on an hourly, daily, weekly, or monthly basis respectively. The files in these directories should be shell scripts.

If a cron task is required to be executed on a schedule other than hourly, daily, weekly, or monthly, it can be added to the /etc/cron.d/ directory. All files in this directory use the same syntax as /etc/crontab.

Here are some examples:

# record the memory usage of the system every monday  
# at 3:30AM in the file /tmp/meminfo 
30 3 * * mon cat /proc/meminfo >> /tmp/meminfo 
# run custom script the first day of every month at 4:10AM 
10 4 1 * * /root/scripts/backup.sh