How to Schedule Jobs in Linux: At and Crontab Commands

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

How to schedule one-time jobs in Linux

Before you try the at command in Linux, you need to know that the following configuration files control which users can schedule tasks by using the at command:

• /etc/at.allow contains the names of the users who may use the at command to submit jobs.
• /etc/at.deny contains the names of users who are not allowed to use the at command to submit jobs.

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

1. Run the at command with the date or time when you want your commands to be executed.

   When you press Enter, the at> prompt appears, as follows:

   at 21:30
   at>

   This method 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 tonight (or tomorrow, if it’s already past 9:30 p.m.). You can, however, specify the execution time in many ways.
2. At the at> prompt, type the commands you want to execute as though you were typing at the shell prompt. After each command, press Enter and continue with the next command.
3. When you finish entering the commands you want to execute, press Ctrl+D to indicate the end.

   Here’s an example that shows how to execute the ps command at a future time:

   at> ps
   at> <EOT>
   job 1 at 2018-12-28 21:30

   After you press Ctrl+D, the at command responds with the <EOT> message, a job number, and the date and time when the job will execute.

4 2018-12-28 03:00 a root
5 2018-10-26 21:57 a root
6 2018-10-26 16:45 a root

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

atrm 4

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

How to schedule recurring jobs in Linux

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. Because the cron daemon processes recurring jobs, such jobs are also referred to as cron jobs.

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

Two configuration files control who can schedule cron jobs in Linux by using crontab:

• /etc/cron.allow contains the names of the users who are allowed to use the crontab command to submit jobs.
• /etc/cron.deny contains the names of users who are not allowed to use the crontab command to submit jobs.

To submit a cron job in Linux, follow these steps:

1. 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.

2. Prepare a text file with information about the times when you want the shell script or program (from Step 1) to execute; then submit this file by 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 fields specify when the job runs, and the sixth and subsequent fields constitute the command that runs. Here’s 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

   The table below shows the meaning of the first five fields. Note: 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 hyphen (indicating a range of numbers), or an asterisk.

If the text file jobinfo (in the current directory) contains the job information, submit this information to crontab with the following command:

crontab jobinfo

To verify that the job is indeed scheduled in Linux, type the following command:

crontab -l

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 <em>username filename</em>

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

crontab _u <em>username</em> -l

crontab -u <em>username</em> -r

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 line that begins with # 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 argument to run-parts — /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, and /etc/cron.monthly — is a directory. Essentially, run-parts executes all scripts located in the directory that you provide as an argument.

The table below lists the directories where you can find these scripts and when they execute. You have to look at the scripts in these directories to know what executes at these intervals.

Script Directories for cron Jobs