RedHat 的 crontab

Chapter 39. Automated Tasks

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 Enterprise Linux is pre-configured 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 Enterprise Linux comes with several automated tasks utilities: cron, at, and batch.

39.1. Cron

Cron is a daemon that can be used to schedule the execution of recurring jobs 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 job is scheduled, it is not executed. To schedule one-time jobs, refer to Section 39.2, “At and Batch”.

To use the cron service, the vixie-cron RPM package must be 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.

39.1.1. Configuring Cron Jobs

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 jobs 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 jobs 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 job 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 job 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 job 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 39.1, “Sample of /etc/crontab” for examples.

Example 39.1. Sample of /etc/crontab

# 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 jobs 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.

Cron jobs can be run at random intervals, which is useful for highly loaded shared networks in order to avoid overloading the network. Job randomization is disabled by default but it can be configured in the /etc/sysconfig/run-parts file by specifying the following parameters:

• RANDOMIZE — When set to 1, it enables randomize functionality. When set to 0, cron job randomization is disabled.
• RANDOM — Specifies the initial random seed. It has to be set to an integer value greater than or equal to 1.
• RANDOMTIME — When set to an integer value greater than or equal to 1, it provides an additional level of randomization.

Example 39.2. Sample of /etc/sysconfig/run-parts - Job Randomization Setting

RANDOMIZE=1
RANDOM=4
RANDOMTIME=8

39.1.2. 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 job.

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.

39.1.3. Starting and Stopping the Service

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 18, Controlling Access to Services for details on starting the cron service automatically at boot time.