Ugly plans to control the time

Once upon time there were cronjobs, now there are systemd timers with a lot of new features.
I will not go deep on systemd, only few notes

To create a simple timer we need to create:

• a script to run something
• a service to start the script execution
• a timer to schedule the service

Create a very very useless script

we will create a simple script that for out test purposes will only create a file.

#!/bin/bash

DATE=$(date +%s)
touch /tmp/$DATE

Create a Systemd Unit Service

What is a Systemd unit?

The systemd unit basically is the smallest entity of Systemd, there are a lot of different unit types.
We are interested in only 2 types

Service

A service ( suffix .service) unit contains information about process which are to be managed by systemd.

Timer

A Timer ( suffix .timer) unit and can control service execution (scheduling).

Each unit resites in a file in one of these locations:

/lib/systemd/system

system default unit files

/etc/systemd/system

user-defined unit files and modifications of installed software units

/usr/lib/systemd/system

unit files for installed software

To view all unit files.

# all units
systemctl list-unit-files

# Only Service units
systemctl list-unit-files --type service

# Only Timer units
systemctl list-unit-files --type timer

A service unit has different sections, for our purpose we need only 2
[Unit] contains metadata, a [Service] contains actions to be executed and a [Install] section.

The [Install] section is used to define the commands to be executed unit is enable disable.

We will use only one field, which is WantedBy=multi-user.target.

If systemctl enable testTimer.timer or testTimer.service is executed then the timer/service will be attached to multi-user.target.

The target refers to a group of related processes, it is like the old init runlevel.
The a targes is reached all the units inside will be started.

When we run the systemctl enable/disabled command, a symbolic link is created/removed inside the multi-user.target.wants directory. For this service we do not need [Install] section.

Create a file named testTimer.service in /etc/systemd/system

[Unit]
Description= A very simple Timer
Wants=testTimer.timer

[Service]
ExecStart=/bin/bash /aboslute/path/to/script.sh
# ExecStartPre: commands to be executed before ExecStart
# ExecStart: commands to be executed where systemctl start is called
# ExecStartPost: commands to be executed after ExecStart
# ExecStop: commands to be run when systemctl stop is called
# ExecReload: commands to be run when systemctl reload is called
# ExecStopPost: commands to be executed after ExecStop automatically
# ....
# ....

Create a Systemd timer

Create a file named testTimer.timer in /etc/systemd/system

[Unit]
Description=Timer to be executed ever 2 minutes
Requires=testTimer.service

[Timer]
# every 2 minutes
OnUnitActiveSec=10
Unit=testTimer.service

[Install]
WantedBy=multi-user.target

You can notice the presence of the same section name [Unit] with metadata, then now we have the [Timer] section

A monolitic timer is stopped when the system is suspended and it is not persistent between reboots.

Run

sudo systemctl enable testTimer.timer
sudo systemctl start testTimer.timer
sudo systemctl status testTimer.timer