In Linux, tasks can be configured to run automatically within a specified period of time, on a specified date, or when the system load average is below a specified number. Red Hat Linux comes preconfigured to run important system tasks to keep the system updated. For example, the slocate database used by the locate command is updated daily. A system administrator can use automated tasks to perform periodic backups, monitor the system, run custom scripts, and more.
Red Hat Linux comes with four automated tasks utilities: cron, anacron, at, and batch.
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 configure tasks based on time periods instead of exact times, refer to Section 28.2 Anacron. To schedule one-time tasks, refer to Section 28.3 At and Batch.
To use the cron service, you must have the vixie-cron RPM package 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 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 MAILTO variable. 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 represents a task and 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.
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. Refer to Example 28-1 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 28-1. 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.
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, all users listed in cron.deny are not allowed to use cron.
To start the cron service, use the command /sbin/service crond start. To stop the service, use the command /sbin/service crond stop. It is recommended that you start the service at boot time. Refer to Chapter 14 Controlling Access to Services for details on starting the cron service automatically at boot time.