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:

  1. 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
       

  2. 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
       

  3. 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
  

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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"