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
schedule one-time tasks, refer to Section 34.2, “At and Batch”.
34.1.1. Configuring Cron Tasks
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. Refer to Example 34.1, “Crontab Examples” for 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
Example 34.1. Crontab Examples
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.
34.1.2. 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
usernames 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, users listed in cron.deny are not allowed to
use cron.
