Installation guide

198 Chapter 22. Automated Tasks
• 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 ex-
ample, 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.
Example 22-1. Examples of crontabs
# 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
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 direc-
tory should be shell scripts.
If a cron tasks 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.
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, login as that user and type the
command 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.