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 value of the SHELL variable tells the system which shell environment to use (in this example the bash shell), and the PATH variable defines the path used to execute commands. The output of the cron tasks are emailed to the username defined with the MAILTOvariable. If the MAILTO variable is defined as an empty string (MAILTO=""), email will not be 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 has the 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, feb, and so on)

• dayofweek — any integer from 0 to 7 where 0 or 7 represents Sunday (or the short name of the week such as sun, mon, and so on)

• 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 that you wrote.

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.