Chapter 27. Automating System Tasks

Tasks, also known as jobs, can be configured to run automatically within a specified period of time, on a specified date, or when the system load average decreases below 0.8.
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 so on.
Red Hat Enterprise Linux comes with the following automated task utilities: cron, anacron, at, and batch.
Every utility is intended for scheduling a different job type: while Cron and Anacron schedule recurring jobs, At and Batch schedule one-time jobs (see Section 27.1, “Cron and Anacron” and Section 27.2, “At and Batch” respectively).

27.1. Cron and Anacron

Both, Cron and Anacron, are daemons that can schedule execution of recurring tasks to a certain point in time defined by the exact time, day of the month, month, day of the week, and week.
Cron jobs can run as often as every minute. However, the utility assumes that the system is running continuously and if the system is not on at the time when a job is scheduled, the job is not executed.
On the other hand, Anacron remembers the scheduled jobs if the system is not running at the time when the job is scheduled. The job is then exectuted as soon as the system is up. However, Anacron can only run a job once a day.

27.1.1. Installing Cron and Anacron

To install Cron and Anacron, you need to install the cronie package with Cron and the cronie-anacron package with Anacron (cronie-anacron is a sub-package of cronie).
To determine if the packages are already installed on your system, issue the rpm -q cronie cronie-anacron command. The command returns full names of the cronie and cronie-anacron packages if already installed or notifies you that the packages are not available.
To install the packages, use the yum command in the following form:
 yum  install  package 
For example, to install both Cron and Anacron, type the following at a shell prompt:
~]# yum install cronie cronie-anacron
Note that you must have superuser privileges (that is, you must be logged in as root) to run this command. For more information on how to install new packages in Red Hat Enterprise Linux, see Section 8.2.4, “Installing Packages”.

27.1.2. Running the Crond Service

The cron and anacron jobs are both picked by the crond service. This section provides information on how to start, stop, and restart the crond service, and shows how to enable it in a particular runlevel. For more information on the concept of runlevels and how to manage system services in Red Hat Enterprise Linux in general, see Chapter 12, Services and Daemons.

27.1.2.1. Starting and Stopping the Cron Service

To determine if the service is running, use the command service crond status.
To run the crond service in the current session, type the following at a shell prompt as root:
 service crond start 
To configure the service to be automatically started at boot time, use the following command:
 chkconfig crond on 
This command enables the service in runlevel 2, 3, 4, and 5. Alternatively, you can use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.

27.1.2.2. Stopping the Cron Service

To stop the crond service, type the following at a shell prompt as root
 service crond stop 
To disable starting the service at boot time, use the following command:
 chkconfig crond off 
This command disables the service in all runlevels. Alternatively, you can use the Service Configuration utility as described in Section 12.2.1.1, “Enabling and Disabling a Service”.

27.1.2.3. Restarting the Cron Service

To restart the crond service, type the following at a shell prompt:
 service crond restart 
This command stops the service and starts it again in quick succession.

27.1.3. Configuring Anacron Jobs

The main configuration file to schedule jobs is the /etc/anacrontab file, which can be only accessed by the root user. The file contains the following:
SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=45
# the jobs will be started during the following hours only
START_HOURS_RANGE=3-22

#period in days   delay in minutes   job-identifier   command
1         5     cron.daily    nice run-parts /etc/cron.daily
7         25    cron.weekly   nice run-parts /etc/cron.weekly
@monthly  45    cron.monthly  nice run-parts /etc/cron.monthly
The first three lines define the variables that configure the environment in which the anacron tasks run:
  • SHELL — shell environment used for running jobs (in the example, the Bash shell)
  • PATH — paths to executable programs
  • MAILTO — user name of the user who receives the output of the anacron jobs by email
    If the MAILTO variable is not defined (MAILTO=), the email is not sent.
The next two variables modify the scheduled time for the defined jobs:
  • RANDOM_DELAY — maximum number of minutes that will be added to the delay in minutes variable which is specified for each job
    The minimum delay value is set, by default, to 6 minutes.
    If RANDOM_DELAY is, for example, set to 12, then between 6 and 12 minutes are added to the delay in minutes for each job in that particular anacrontab. RANDOM_DELAY can also be set to a value below 6, including 0. When set to 0, no random delay is added. This proves to be useful when, for example, more computers that share one network connection need to download the same data every day.
  • START_HOURS_RANGE — interval, when scheduled jobs can be run, in hours
    In case the time interval is missed, for example due to a power failure, the scheduled jobs are not executed that day.
The remaining lines in the /etc/anacrontab file represent scheduled jobs and follow this format:
period in days   delay in minutes   job-identifier   command
  • period in days — frequency of job execution in days
    The property value can be defined as an integer or a macro (@daily, @weekly, @monthly), where @daily denotes the same value as integer 1, @weekly the same as 7, and @monthly specifies that the job is run once a month regarless of the length of the month.
  • delay in minutes — number of minutes anacron waits before executing the job
    The property value is defined as an integer. If the value is set to 0, no delay applies.
  • job-identifier — unique name referring to a particular job used in the log files
  • command — command to be executed
    The command can be either a command such as ls /proc >> /tmp/proc or a command which executes a custom script.
Any lines that begin with a hash sign (#) are comments and are not processed.

27.1.3.1. Examples of Anacron Jobs

The following example shows a simple /etc/anacrontab file:
SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root

# the maximal random delay added to the base delay of the jobs
RANDOM_DELAY=30
# the jobs will be started during the following hours only
START_HOURS_RANGE=16-20

#period in days   delay in minutes   job-identifier   command
1         20    dailyjob      nice run-parts /etc/cron.daily
7         25    weeklyjob     /etc/weeklyjob.bash
@monthly  45    monthlyjob    ls /proc >> /tmp/proc
All jobs defined in this anacrontab file are randomly delayed by 6-30 minutes and can be executed between 16:00 and 20:00.
The first defined job is triggered daily between 16:26 and 16:50 (RANDOM_DELAY is between 6 and 30 minutes; the delay in minutes property adds 20 minutes). The command specified for this job executes all present programs in the /etc/cron.daily directory using the run-parts script (the run-parts scripts accepts a directory as a command-line argument and sequentially executes every program in the directory).
The second job executes the weeklyjob.bash script in the /etc directory once a week.
The third job runs a command, which writes the contents of /proc to the /tmp/proc file (ls /proc >> /tmp/proc) once a month.

27.1.4. Configuring Cron Jobs

The configuration file for cron jobs is the /etc/crontab, which can be only modified by the root user. The file contains the following:
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root
HOME=/
# For details see man 4 crontabs
# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * username  command to be executed
The first three lines contain the same variable definitions as an anacrontab file: SHELL, PATH, and MAILTO. For more information about these variables, see Section 27.1.3, “Configuring Anacron Jobs”.
In addition, the file can define the HOME variable. The HOME variable defines the directory, which will be used as the home directory when executing commands or scripts run by the job.
The remaining lines in the /etc/crontab file represent scheduled jobs and have the following format:
minute   hour   day   month   day of week   username   command
The following define the time when the job is to be run:
  • 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)
  • day of week — any integer from 0 to 7, where 0 or 7 represents Sunday (or the short name of the week such as sun or mon)
The following define other job properties:
  • username — specifies the user under which the jobs are run
  • command — the command to be executed
    The command can be either a command such as ls /proc /tmp/proc or a command which executes a custom script.
For any of the above values, an asterisk (*) can be used to specify all valid values. If you, for example, define the month value as an asterisk, the job will be executed 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 exactly these four integers.
The forward slash (/) can be used to specify step values. The value of an integer will be skipped within a range following the range with /integer. For example, minute value defined as 0-59/2 denotes every other minute in the minute field. Step values can also be used with an asterisk. For instance, if the month value is defined as */3, the task will run every third month.
Any lines that begin with a hash sign (#) are comments and are not processed.
Users other than root can configure cron tasks with the crontab utility. The user-defined crontabs are stored in the /var/spool/cron/ directory and executed as if run by 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 with the editor specified in 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 user name and written to the file /var/spool/cron/username. To list the contents of your crontab file, use the crontab -l command.

Do not specify a user

Do not specify the user when defining a job with the crontab utility.
The /etc/cron.d/ directory contains files that have the same syntax as the /etc/crontab file. Only root is allowed to create and modify files in this directory.

Do not restart the daemon to apply the changes

The cron daemon checks the /etc/anacrontab file, the /etc/crontab file, the /etc/cron.d/ directory, and the /var/spool/cron/ directory every minute for changes and the detected changes are loaded into memory. It is therefore not necessary to restart the daemon after an anacrontab or a crontab file have been changed.

27.1.5. Controlling Access to Cron

To restrict the access to Cron, you can use the /etc/cron.allow and /etc/cron.deny files. These access control files use the same format with one user name on each line. Mind that no whitespace characters are permitted in either file.
If the cron.allow file exists, only users listed in the file are allowed to use cron, and the cron.deny file is ignored.
If the cron.allow file does not exist, users listed in the cron.deny file are not allowed to use Cron.
The Cron daemon (crond) does not have to be restarted if the access control files are modified. The access control files are checked each time a user tries to add or delete a cron job.
The root user can always use cron, regardless of the user names listed in the access control files.
You can control the access also through Pluggable Authentication Modules (PAM). The settings are stored in the /etc/security/access.conf file. For example, after adding the following line to the file, no other user but the root user can create crontabs:
-:ALL EXCEPT root :cron
The forbidden jobs are logged in an appropriate log file or, when using “crontab -e”, returned to the standard output. For more information, see access.conf.5 (that is, man 5 access.conf).

27.1.6. Black and White Listing of Cron Jobs

Black and white listing of jobs is used to define parts of a job that do not need to be executed. This is useful when calling the run-parts script on a Cron directory, such as /etc/cron.daily: if the user adds programs located in the directory to the job black list, the run-parts script will not execute these programs.
To define a black list, create a jobs.deny file in the directory that run-parts scripts will be executing from. For example, if you need to omit a particular program from /etc/cron.daily, create the /etc/cron.daily/jobs.deny file. In this file, specify the names of the programs to be omitted from execution (only programs located in the same directory can be enlisted). If a job runs a command which runs the programs from the cron.daily directory, such as run-parts /etc/cron.daily, the programs defined in the jobs.deny file will not be executed.
To define a white list, create a jobs.allow file.
The principles of jobs.deny and jobs.allow are the same as those of cron.deny and cron.allow described in section Section 27.1.5, “Controlling Access to Cron”.