SYSTEMD and Services

What is SYSTEMD?

Systemd is a Linux initialization system and service manager that is responsible for bootstrapping the system, managing system processes, and providing a standardized approach to manage services, tasks, and logs.
Systemd replaced older init systems like SysVinit and Upstart in most modern Linux distributions due to its efficiency and enhanced features.
All the major distributions, such as RHEL, CentOS, Fedora, Ubuntu, Debian, adopted systemd as their init system.

What is a service unit?

A service unit file is a file with the .service extension contains information about a process which is managed by systemd.
This needs to be created at /etc/systemd/system/<service-name>.service location.
With systemd, the service events are automatically logged (without any additional configuration).

It is composed by three main sections:

Unit (optional)

Unit: This section contains the description of the unit itself, and information about its behavior and its dependencies: (to work correctly a service can depend on another one). It consists of following options (few of many):

  $ cat /etc/systemd/system/project-mercury.service
  [Unit]
  Description=Python Django for Project Mercury       # description of the unit/service
  Documentation=http://wiki.caleston-dev.ca/mercury   # details of the service and documentation related to it
  After=postgresql.service                            # use to specify that our unit/service should be started after the units we provide in the form of a space-separated list

Service (required)

Service: In this section, we can specify things as the command to be executed when the service is started, or the type of the service itself.

  [Service]
  ExecStart=/bin/bash /usr/bin/project-mercury.sh    # script to start
  User=project_mercury                               # service accoutn to be used to start the service instead of root
  Restart=on-failure                                 # defines how and when to restart the service
  RestartSec=10                                      # to set the time in seconds to wait before the system attempt to restart the service

Install (optional)
- Install: This section contains information about the installation of the unit. This allow the service to be enabled during boot.
```
  [Install]
  WantedBy=graphical.target    #  This allow the service to be enabled during boot
```

Commands

Start a service:
```
  $ systemctl start <service_name>
```
Stop a service:
```
  $ systemctl stop <service_name>
```
Restart a service:
```
  $ systemctl restart <service_name>
```
Enable a service to start at boot:
```
  $ systemctl enable <service_name>
```
Disable a service from starting at boot:
```
  $ systemctl disable <service_name>
```
Check the status of a service:
```
  $ systemctl status <service_name>
```
Reload service configuration without stopping it:
```
  $ systemctl reload <service_name>
```
Reload systemd to recognize new or modified unit files:
```
  $ systemctl daemon-reload
```

SYSTEMD Tools

SYSTEMD consists of 2 major tools i.e. systemctl and journalctl.

SYSTEMCTL

Systemctl is the main command used to manage services on a SYSTEMD managed server.
It can be used to manage services such as START/STOP/RESTART/RELOAD as well as ENABLE/DISABLE services during the system boot.
It is also used to LIST AND MANAGE UNITS and LIST AND UPDATE TARGETS.

Commands

systemctl start <service-name>: To start a service, for example to start a docker service:
```
  $ systemctl start docker
```
systemctl stop <service-name>: To stop a service, for example to stop a docker service:
```
  $ systemctl stop docker
```
systemctl restart <service-name>: To restart a service, this will stop and start the service again.
```
  $ systemctl restart docker
```
systemctl reload <service-name>: To reload a service, this will reload all the configuration without interrupting the normal functionality of the service.
```
  $ systemctl reload docker
```
systemctl enable <service-name>: To enable a service and make it persistent accross reboots.
```
  [~]$ systemctl enable docker
```
systemctl disable <service-name>: To disable a service at boot.
```
  [~]$ systemctl disable docker
```
systemctl status <service-name>: To know the status of the service, this command provided the state of the service. If running properly is should show active (running) state.
```
  [~]$ systemctl status docker
```
systemctl deamon-reaload: To reload systemd to recognize new or modified unit/service files, it reloads the system manager configuration and makes the systemd aware of the changes.
```
  $ systemctl daemon-reload
```
systemctl edit <service-file> --full: To edit the service file, this will open a text editor, you can make the changes and re-write the settings as needed, making changing this way applied immediately without running the systemctl daemon reload command.
```
  $ systemctl edit project-mercury.service --full
```
systemctl get-default: To see the current runlevel.
```
  $ systemctl get default
```
systemctl set-default <target-name>: To change the runlevel to a different target.
```
  $ systemctl set-default multi-user.target
```
systemctl list-units --all: To list all the units that systemd has loaded, this lists all the unit which are active, inactive or another state.
```
  $ systemctl list-units --all
```
systemctl list-units: To list only active units.
```
  $ systemctl list-units
```
systemctl cat <service-name>: To view, and also locate a unit file, a comment line containing the path to the unit file is printed as the first line of output.
```
  $ systemctl cat project-mercury.service
```

JOURNALCTL

Journalctl is a command for quering/viewing logs collected by systemd.
The systemd-journald service is responsible for systemd’s log collection, and it retrieves messages from the kernel systemd services, and other sources.
Very useful when you are troubleshooting issues with systemd services.

Commands

journalctl: To print all the log entries from oldest to the newest.
```
  $ journalctl
```
journalctl -b: To print all the logs from the current boot.
```
  $ journalctl -b
```
journalctl -u <service-name/unit-name>: To print all the logs specific to the unit specified.
```
  $ journalctl -u docker.service
```
journalctl -u docker.service --since <time>: To print all the logs specific to the unit specified since the given time.
```
  [~]$ journalctl -u docker.service --since "2022-01-01 13:45:00"
```

07 - Service Management with SYSTEMD

Table of contents

SYSTEMD and Services

What is SYSTEMD?

What is a service unit?

Commands

SYSTEMD Tools

SYSTEMCTL

Commands

JOURNALCTL

Commands