Systemd

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Description

This article gives summarized information on systemd which is aimed at replacing several bootstrap services on most Linux flavors. Systemd comes on RHEL 7+.

Systemd is a system and service manager for Linux operating systems. It is designed to be backwards compatible with SysV init scripts, and 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. In Red Hat Enterprise Linux 7, systemd replaces Upstart as the default init system.

Systemd introduces the concept of systemd units. These units are represented by unit configuration files located in one of the directories listed below, and encapsulate information about system services, listening sockets, saved system state snapshots, and other objects that are relevant to the init system.


Unit File Locations

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.

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.


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