Presentation on Administrative Task Scheduling¶

Scheduling Jobs and Tasks¶

General Objectives¶

By the end of this session, you will learn:

How to schedule cron jobs using cron, at, batch, and systemd.
The difference between user crontabs and system crontabs.
How to edit crontabs and configure the /etc/crontab file.
Time specifications used in crontab job scheduling.
Cron tab variables.
How to create system and user crons.
How to configure access to cron.
How to use systemd to manage crons and alternatives to at.

Introduction¶

As a system administrator, scheduling tasks is crucial for automating repetitive activities and ensuring system efficiency.

Cron Jobs¶

Cron is a daemon that runs in the background and checks for scheduled tasks every minute. Anacron is a program used to schedule jobs on systems that may be powered off and can only be edited by root users.

Differences Between User Cron and System Cron¶

User Cron: Stored in /var/spool/cron/.
System Cron: Stored in /etc/crontab and /etc/cron.d/.

Special Characters in Cron Jobs¶

Symbol	Description	Example
`*`	Any possible value	`* * * * * command`
`,`	List of values	`* 1,2,3 * * * command`
`-`	Range of values	`1-30 * * * * command`
`/`	Step values (repetition)	`/30 * * * command`

Adding a Cron Job¶

Edit the crontab file using:

crontab -e

Or edit the system crontab:

vim /etc/crontab

Special Files for Scheduling Cron Jobs¶

File	Function
`/etc/cron.d/`	Contains anacron facilities
`/etc/cron.daily/`	Executes scripts daily at midnight
`/etc/cron.hourly/`	Executes scripts hourly
`/etc/cron.monthly/`	Executes scripts monthly at midnight
`/etc/cron.weekly/`	Executes scripts weekly on Sunday at midnight

Special Time Specifications¶

Specification	Function
`@hourly`	Executes every hour
`@daily`	Executes every day
`@weekly`	Executes every week
`@monthly`	Executes every month
`@yearly`	Executes every year

Cron Variables¶

Variable	Description
`PATH`	Specifies the absolute path to the task
`SHELL`	Indicates the shell used to execute the task
`MAILTO`	Specifies the email address for notifications
`HOME`	Specifies the home directory for the task

Practical Examples¶

Example 1: Run a Script Every Two Minutes¶

crontab -e
* */2 * * * /home/username/directory/hello.sh

Example 2: Remove a Scheduled Cron Job¶

crontab -r

Example 3: List Scheduled Cron Jobs¶

crontab -l

Example 4: Check Mail for Cron Notifications¶

cat /var/spool/mail/username

Scheduling with `at` Command¶

The at command is used for one-time job scheduling.

Examples¶

at now +5 minutes echo "My first at job" > file.txt
at teatime cat file.txt  # teatime = 16:00 (4:00 PM)

Commands¶

atq: Lists pending at jobs.
atrm: Removes a specific at job.

Using `systemd` for Scheduling¶

systemd provides an alternative to cron with .timer services.

Example: Creating a Timer¶

Create a file file.timer in /etc/systemd/system/: ```ini [Unit] Description=Run the timing task

[Timer] OnCalendar=--* 00:0/2:00 Persistent=true

[Install] WantedBy=timers.target ```

Create a corresponding service file file.service: ```ini [Unit] Description=Run my command OnFailure=mail-systemd-failure@%n.service

[Service] Type=oneshot ExecStart=/usr/local/bin/custom-command User=dedicated-user Group=dedicated-user ```

Reload and enable the timer: bash systemctl daemon-reload systemctl enable file.timer systemctl start file.timer
List active timers: bash systemctl list-timers