I nt rod uction to systemd
System startup and server processes a re managed by the systemd System and Service Manager.
This progra m provides a method for activati ng system resou rces, server daemons, and other
processes, both at boot time and on a running system.
Daemons a re processes that wait or run i n the backg round performing va rious tasks. To l isten for
con nections, a daemon uses a socket. Sockets may be created by daemons o r may be separated
from the daemon and be created by a nother process, such as systemd, which then passes the
socket to the daemon when a connection is esta b l ished by a client.
A service often refers to one or more daemons, but starting or stopping a service may
instead make a one-time change to the state of the system (for exa mple, to confi g u re network
i nterfaces), which does not i nvo lve leaving a daemon process running afterwa rd.
A bit of history
For many years, process ID 1 of Linux and U N I X systems has been the init process. This process
was responsi b l e for activating other services on the system. Frequently used daemons were
started on systems at boot time with System V a n d Linux Sta ndard Base (LSB) init scripts.
Less freq uently used daemons were started on demand by a n other service, such as initd or
xinetd. These systems have severa l l i m itations, which a re addressed with systemd.
I n Red H at E nterprise L i n u x 7, process ID 1 is syst emd, the new i n it system. A few of the new
featu res provided by systemd include:
• Pa ra l l e l ization capa b i l ities, which increase the boot speed of a system.
• On-demand sta rting of daemons without req uiring a sepa rate service.
• Automatic service dependency management prevents long timeouts, such as not sta rting a
network service when the network is not ava i l ab le.
• A method of t racking related processes together using L i n u x control g rou ps.
2
N ote
With systemd, shel l-based service scripts a re used only for a few legacy services.
Therefore, confi g u ration files with shel l va ria b l es, such as those fou n d i n
/etc/sysconfig, a re being replaced. Those sti l l i n use a re i n c l uded as systemd
enviro n m e nt files and read as N A M E=VA L U E pairs. They a re no longer sourced as a
shel l script.
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