Scheduling Jobs in CentOS Linux

As a system administrator, you may need to run some programs automatically at regular intervals or execute one or more commands at a specified time in the future. Your Linux system includes the facilities to schedule jobs to run at any future date or time you want. You can set up the system to perform a task periodically or just once. Here are some typical tasks you can perform by scheduling jobs on your Linux system:

Back up the files in the middle of the night
Download large files in the early morning when the system is not busy
Send yourself messages as reminders of meetings
Analyze the system logs periodically and look for any abnormal activities

You can perform these tasks by using the at command or the crontab facility of CentOS Linux. The next few sections introduce these job-scheduling features of CentOS Linux.

Scheduling One-Time Jobs

You can use the at command to schedule the execution of one or more commands at a later time. The atd daemon-a program designed to process jobs submitted using at-executes the commands at the specified time, and mails the output to you.

Insider Insight

Before you try out the at command, note that the following configuration files control, which users can schedule tasks using the at command:

/etc/at.allow contains the names of the users who may submit jobs using the at command.
/etc/at.deny contains the names of users not allowed to submit jobs using the at command.
If these files are not present, or if there is an empty /etc/at.deny file, any user can submit jobs using the at command. The default in CentOS Linux is an empty /etc/at.deny file, so anyone can use the at command. If you do not want some users to use at, simply list those usernames in the /etc/at.deny file.

To use at to schedule a one-time job for execution at a later time, follow these steps:

Run the at command with the date or time when you want your commands executed. When you press Enter, the at> prompt appears, as follows:
```
at 21:30
at>  
```
This is the simplest way to indicate the time when you want to execute one or more commands-simply specify the time in a 24-hour format. In this case, you want to execute the commands at 9:30 p.m. tonight (or tomorrow, if it's already past 9:30 p.m.). You can, however, specify the execution time in many different ways (see Table 20-3 for examples).

At the at> prompt, type the commands you want to execute as if typing input at the shell prompt. After each command, press Enter and continue with the next command. When you are finished entering the commands you want to execute, press Ctrl-D to indicate the end. Here is an example showing a single command:

at> ps
at> <EOT>
job 1 at 2003-01-21 21:30

After you press Ctrl-D, the at command responds with a job number and the date and time when the job will execute.

Table 20-3: Specifying the Time of Execution with the at Command
Command	When the Job Is Run
`at now`	Immediately
`at now + 15 minutes`	15 minutes from the current time
`at now + 4 hours`	4 hours from the current time
`at now + 7 days`	7 days from the current time
`at noon`	At noontime today (or tomorrow, if already past noon)
`at now next hour`	Exactly 60 minutes from now
`at now next day`	At the same time tomorrow
`at 17:00 tomorrow`	At 5:00 p.m. tomorrow
`at 4:45pm`	At 4:45 p.m. today (or tomorrow, if already past 4:45 p.m.)
`at 3:00 Aug 16, 2003`	At 3:00 a.m. on August 16, 2003

After you enter one or more jobs, you can view the current list of scheduled jobs with the atq command:

atq
4       2003-04-19 03:00 a
5       2003-08-16 21:57 a
6       2003-10-26 16:45 a

The first field on each line shows the job number-the same number that the at command displays when you submit the job. The next field shows the year, month, day, and time of execution. The last field shows the jobs pending in the queue named a.

If you want to cancel a job, use the atrm command to remove that job from the queue. When removing a job with the atrm command, refer to the job by its number, as follows:

atrm 4

This deletes job 4 scheduled for 3:00 a.m. April 19, 2003.

When a job executes, the output is mailed to you. Type mail to read your mail and to view the output from your jobs.

Note that at is useful for scheduling jobs that need to run irregularly-for instance, if you want to clean up the /var/logs directory because the /var filesystem is over 85 percent full, you might use at to archive the old log files and then delete them to reclaim the disk space.

Scheduling Recurring Jobs

Although at is good for running commands at a specific time, it's not useful for running a program automatically at repeated intervals. You have to use crontab to schedule such recurring jobs, also called cron jobs because they are processed by the cron daemon (crond). You need to do this, for example, if you want to back up your files to tape at midnight every day.

Two files control who can schedule cron jobs using crontab:

/etc/cron.allow contains the names of the users who may submit jobs using the crontab command.
/etc/cron.deny contains the names of users not allowed to submit jobs using the crontab command.

If the /etc/cron.allow file exists, only users listed in this file can schedule cron jobs. If only the /etc/cron.deny file exists, users listed in this file cannot schedule cron jobs. Neither file exists in CentOS Linux, so any user can submit cron jobs.

You schedule recurring jobs by placing job information in a file with a specific format and submitting this file with the crontab command. The cron daemon-crond-checks the job information every minute and executes the recurring jobs at the specified times.

Any output from the job is mailed to the user who submits the job. (In the submitted job-information file, you may specify a different recipient for the mailed output.)

To submit a cron job, perform the following steps:

Prepare a shell script (or an executable program in any programming language) that can perform the recurring task you want to perform. You can skip this step if you want to execute an existing program periodically.

Prepare a text file with information about the times when you want the shell script or program (from Step 1) to execute. Submit this file using crontab. You can submit several recurring jobs with a single file. Each line with timing information about a job has a standard format with six fields-the first five specify when the job runs, and the sixth and subsequent fields constitute the actual command that runs. For example, here is a line that executes the myjob shell script in a user's home directory at 5 minutes past midnight each day:

  5 0 * * * $HOME/myjob

Table 20-4 shows the meaning of the first five fields. Note that an asterisk (*) means all possible values for that field. Also, an entry in any of the first five fields can be a single number, a comma-separated list of numbers, a pair of numbers separated by a dash (indicating a range of numbers), or an asterisk.

Table 20-4: Specifying the Time of Execution in crontab Files
Field Number	Meaning of Field	Acceptable Range of Values*
1	Minute	0-59
2	Hour of the day	0-23
3	Day of the month	0-31
4	Month	1-12 (1 means January, 2 means February, and so on) or the names of months using the first letters (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec)
5	Day of the week	0-6 (0 means Sunday, 1 means Monday, and so on) or the three-letter abbreviations of the week (Sun, Mon, Tue, Wed, Thu, Fri, Sat)

* An asterisk in a field means all possible values for that field. For example, if an asterisk appears in the third field, the job is executed every day.

Suppose the text file jobinfo (in the current directory) contains the job information. Submit this information to crontab with the following command:
```
  crontab jobinfo
```
That's it! You should be set with the cron job. From now on, the cron job should run at regular intervals (as specified in the job information file), and you should receive mail messages with the output from the job.
To verify that the job is indeed scheduled, type the following command:
```
crontab -l
```
The output of the crontab -l command shows the cron jobs currently installed in your name. To remove your cron jobs, type crontab -r.

If you log in as root, you can also set up, examine, and remove cron jobs for any user. To set up cron jobs for a user, use this command:

crontab -u username filename

Here, username is the user for whom you install the cron jobs, and filename is the file that contains information about the jobs.

Use the following form of crontab command to view the cron jobs for a user:

crontab -u username -l

The cron daemon (crond) also executes the cron jobs listed in the systemwide cron-job file /etc/crontab. Here's the default /etc/crontab file in CentOS Linux (type cat /etc/crontab to view the file):

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 set up several environment variables for the jobs listed in this file. Note that the MAILTO environment variable specifies the user who receives the mail message with the output from the cron jobs in this file.

The line that begins with a # is a comment line. The four lines following the run-parts comment execute the run-parts shell script (located in the /usr/bin directory) at various times with the name of a specific directory as argument. Each of the arguments to run-parts-/etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly-are directories. Essentially, run-parts executes all scripts located in the directory that you provide as an argument. This means that you can place scripts in these directories and get crond to execute them at the appointed time. For example, a script placed in the /etc/cron.daily gets executed at a certain time every day. Table 20-5 lists the directories and when they are executed. You have to look at the scripts in these directories to learn what gets executed at these periodic intervals.

To remove a user's cron jobs, use the following command:

crontab -u username -r

Table 20-5: Script Directories for run-parts
Directory Name	Contents
`/etc/cron.hourly`	Scripts executed every hour
`/etc/cron.daily`	Scripts executed each day at 4:02 a.m.
`/etc/cron.weekly`	Scripts executed weekly on Sunday at 4:22 a.m.
`/etc/cron.monthly`	Scripts to be executed at 4:42 a.m. on the first day of each month