SystemD

Service Manager

What is Systemd

systemd is a system and service manager for Linux operating systems. It’s designed to be a replacement for the traditional SysV init system, providing more powerful features for managing services, tasks, and other aspects of the system’s operation. systemd is responsible for initializing the system, managing services, handling logging, and more. It’s widely adopted across various Linux distributions due to its efficiency and modern approach to system management.

It’s actually a suite of program, not only an init system like most people think. One of the biggest benefits with systemd is that is gives a common and consistent environment for managing services. E.i SysV relies heavily on script during init/start and these script vary among linux distributions. Is actually also more faster and have better performance due multithreading support. It’s also the de facto standard tool in the enterprise environment.

systemd
systemd

How to install systemD

For Debian and Ubuntu it is installed by default

Termomonlogy and Concept

  1. Unit: A unit is a resource managed by systemd, such as a service, socket, device, mount point, or timer. Each unit is represented by a configuration file typically stored in /etc/systemd/system/ or /usr/lib/systemd/system/.
  2. Service: A service unit controls a process or a group of related processes that provide specific functionality. Services are defined in .service files and can be started, stopped, enabled, disabled, or restarted using systemd commands like systemctl.
  3. Socket: A socket unit represents inter-process communication endpoints used by services. It allows services to listen for incoming connections or communicate with other processes through a socket file.
  4. Target: A target unit is a symbolic link to other units. It represents a specific system state or a group of related services that should be started together. Targets are similar to runlevels in the traditional SysV init system.
  5. Dependency: systemd units can have dependencies on other units. Dependencies define the order in which units are started or stopped. Units can depend on other units being started or stopped before they can start or stop.
  6. SysV Compatibility: systemd provides compatibility with the older SysV init system, allowing it to run SysV init scripts as systemd services. This ensures backward compatibility with older software that relies on SysV init scripts.
  7. Journal: systemd includes a logging subsystem called the journal, which collects and stores log messages from the system and services. The journal provides powerful filtering and querying capabilities and replaces traditional text-based log files stored in /var/log/.
  8. Timer: A timer unit allows scheduling services to run at specific times or intervals. It can be used to automate tasks such as periodic backups, system maintenance, or other recurring operations.
  9. Cgroups: systemd utilizes Linux control groups (cgroups) to manage and isolate system resources such as CPU, memory, and I/O for each service or process. This helps in resource management and control.
  10. Boot Process Management: systemd coordinates the boot process, parallelizing service initialization to reduce boot times. It also handles service restarts, recovery, and dependency resolution during boot.

Folder/files structure and configuration

Systemd unit files usually located at

/etc/systemd/system
/lib/systemd/system
/run/systemd/system
systemd

Configuration Files

Defines settings and parameters for components and services

These files control how the various components of systemd operate. These files are usually located at
/etc/systemd/

Usually stored as key/value pair with a section portion
E.i

[Journal]
#Storage=auto
#Compress=yes
#Seal=yes

Because of the presence of a configuration file does NOT mean that that component is in use… When viewing the content in the configuration file you may se some commented out lines, this mean that theese are the default parameter value.
Also remember that each component are optional, each linux distro decides for itself what component it want have installed.

systemd

Unit Files

Define and manage systemd units

Controls how system and service operation functions. It’s like replacing a bash script for sysV

The 2 main folders for unit files are
/lib/systemd/system
Default location for unit files that come with the distro or/also packages install into
/etc/systemd/system
The location where you put custom (or if you want to modify) unit files. Any unit files in the directory that have the same name as unit files in /lib/systemd/systemtakes precedence!

Here is a list of what unit type exists

  1. Service Units: Represent a process or a group of related processes that provide specific functionality, such as a web server (e.g., Apache or Nginx), a database server (e.g., MySQL or PostgreSQL), or any custom application.
    File postfix: .service (e.g., apache2.service, nginx.service).
  2. Socket Units: Manage inter-process communication endpoints used by services. They allow services to listen for incoming connections or communicate with other processes through a socket file.
    File postfix: .socket (e.g., ssh.socket, dbus.socket).
  3. Device Units: Represent device files in the Linux filesystem. They can be used to manage hardware devices or virtual devices.
    File postfix: .device (e.g., dev-hugepages.device, dev-mqueue.device).
  4. Mount Units: Control the mounting and unmounting of filesystems. They define mount points and options for mounting filesystems.
    File postfix: .mount (e.g., home.mount, var-log.mount).
  5. Automount Units: Automatically mount filesystems on access. They are triggered when a specified path is accessed and automatically mount the filesystem associated with that path.
    File postfix: .automount (e.g., var-lib-nfs-rpc_pipefs.automount, proc-sys-fs-binfmt_misc.automount).
  6. Target Units: Are symbolic links to other units, representing a specific system state or a group of related services that should be started together. Targets are similar to runlevels in the traditional SysV init system.
    File postfix: .target (e.g., multi-user.target, graphical.target).
  7. Snapshot Units: Record the state of the systemd system manager at a specific point in time. They can be used to restore the system to a previous state.(UNSURE of this unit)
    File postfix: .snapshot (e.g., snapshots-working.target, snapshots-default.target).
  8. Timer Units: Schedule services to run at specific times or intervals. They can be used to automate tasks such as periodic backups, system maintenance, or other recurring operations.
    File postfix: .timer (e.g., apt-daily.timer, systemd-tmpfiles-clean.timer).
  9. Path Units: Monitor filesystem paths for changes and trigger actions when changes occur. They can be used to monitor log files, directories, or other filesystem objects.
    File postfix: .path (e.g., var-log-journal.path, tmp-files-clean.path).
  10. Swap Units: Manage swap devices or files used for virtual memory. They control the activation and deactivation of swap space.
    File postfix: .swap (e.g., dev-disk-by\x2duuid-6e5412d7\x2da9f3\x2d42bc\x2d9db9\x2d72de7c9ec7ab.swap, dev-sda5.swap).
systemd

executables

The files that systemd actually uses can be spread out a little bit. To be sure use the command which <executable> to determine the path to file. But the common path is

/lib/systemd/

Sometimes they utilize the standard folder paths
/bin, /sbin, /usr/bin, /usr/sbin

These files are usually not subject for customization or modification…

Manage the services

start = starts the sevice immediately
enable = service start up on boot
stop = stop the running service
restart = stops and then starts the service
reload = reload the service configuration without interrupting its operation
disable = prevent a service from starting automatically at boot time
status = check the status of a service
show = shows detailed information about a service

Start a service
sudo systemctl start <service_name>

Enable a service
sudo systemctl enable <service_name>

Stop a service
sudo systemctl stop <service_name>

Restart a service
sudo systemctl restart <service_name>

Reload a service
sudo systemctl reload <service_name>

Disable a service
sudo systemctl disable <service_name>

Status of a service
sudo systemctl disable <service_name>

Show detailed info about a service
sudo systemctl show <service_name>

Commands

Some commonly used systemd commands
DESCRIPTION COMMAND
List All units files sudo systemctl list-units
List all running services
OR 		sudo systemctl list-units --type=service
sudo systemctl --type=service
List all installed unit files sudo systemctl list-unit-files
To show all states of unit files sudo systemctl list-units --all --state=inactive
sudo systemctl list-units --all --state=active
sudo systemctl list-units --all --state=running

Example of using systemd commands

DESCRIPTION COMMAND
Enable printer service(preferred way)
OR 		sudo systemctl enable cups
sudo systemctl enable cups.service
Enable network service sudo systemctl enable NetworkManager
Enable bluetooth service sudo systemctl enable bluetooth
Enable iscsi service sudo systemctl start iscsid
Enable apache service sudo systemctl enable apache2
Enable nginx service sudo systemctl enable nginx
Enable mysql service sudo systemctl enable mysql
Status of SSH sudo systemctl status ssh
Status of vsftp sudo systemctl status vsftpd

Commands TEST

Some commonly used systemd commands
General

Texttt 111111

Managing services

Texxtt 2