Systemd is a system and service manager for Linux operating systems.
It provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, support for system state snapshots, or dependency-based service control logic.
Systemd introduces the concept of systemd units. These units are represented by unit configuration files. Bellow is the list of available systemd unit types.
It provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, support for system state snapshots, or dependency-based service control logic.
Systemd introduces the concept of systemd units. These units are represented by unit configuration files. Bellow is the list of available systemd unit types.
| Unit Type | File Extension | Description |
|---|---|---|
| Service unit | .service | A system service. |
| Target unit | .target | A group of systemd units. |
| Automount unit | .automount | A file system automount point. |
| Device unit | .device | A device file recognized by the kernel. |
| Mount unit | .mount | A file system mount point. |
| Path unit | .path | A file or directory in a file system. |
| Scope unit | .scope | An externally created process. |
| Slice unit | .slice | A group of hierarchically organized units that manage system processes. |
| Snapshot unit | .snapshot | A saved state of the systemd manager. |
| Socket unit | .socket | An inter-process communication socket. |
| Swap unit | .swap | A swap device or a swap file. |
| Timer unit | .timer | A systemd timer. |
Location of the systemd unit files
| Directory | Description |
|---|---|
/usr/lib/systemd/system/ | Systemd units distributed with installed RPM packages. |
/run/systemd/system/ | Systemd units created at run time. This directory takes precedence over the directory with installed service units. |
/etc/systemd/system/ | Systemd units created and managed by the system administrator. This directory takes precedence over the directory with runtime units. |
Some of the benefits of systemd over the traditional System V init facility include:
- systemd never loses initial log messages
- systemd can respawn daemons as needed
- systemd records runtime data (i.e., captures stdout/stderr of processes)
- systemd doesn't lose daemon context during runtime
- systemd can kill all components of a service cleanly
Here runlevels are replace by the target as follows:
Traditional runlevel New target name Symbolically linked to... Runlevel 0 | runlevel0.target -> poweroff.target Runlevel 1 | runlevel1.target -> rescue.target Runlevel 2 | runlevel2.target -> multi-user.target Runlevel 3 | runlevel3.target -> multi-user.target Runlevel 4 | runlevel4.target -> multi-user.target Runlevel 5 | runlevel5.target -> graphical.target Runlevel 6 | runlevel6.target -> reboot.target
The default runlevel (which previously set in the /etc/inittab file) is now replaced by a default target. The location of the default target is /etc/systemd/system/default.target, which by default is linked to the multi-user target.
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