• 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 . As you can see from the /etc/crontab file, it uses the run-parts script to execute 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 needs 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 is an example:

# 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

Users other than root can configure cron tasks by using the crontab utility. All user -defined crontabs are stored in the /var/spool/cron directory and are executed using the usernames of the users that created them. To create a crontab as a user, log in as that user and type crontab -e to edit the user’s crontab using the editor specified by the VISUAL or EDITOR environment variable. The file uses the same format as /etc/crontab . When the changes to the crontab are saved, the crontab is stored according to username and written to the file /var/spool/cron/username .

The cron daemon checks the /etc/crontab file, the /etc/cron.d/ directory, and the /var/spool/cron directory every minute for any changes. If any changes are found, they are loaded into memory. Thus, the daemon does not need to be restarted if a crontab file is changed.

Controlling Access to cron

The /etc/cron.allow and /etc/cron.deny files are used to restrict access to cron. The format of both access control files is one username on each line. Whitespace is not permitted in either file.

The cron daemon ( crond ) does not have to be restarted if the access control files are modified. The access control files are read each time a user tries to add or delete a cron task. The root user can always use cron, regardless of the user names listed in the access control files.

If the file cron.allow exists, only users listed in it are allowed to use cron, and the cron.deny file is ignored. If cron.allow does not exist, all users listed in cron.deny are not allowed to use cron.