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systemd is the first daemon to start during booting and the last daemon to terminate during shutdown. The systemd daemon serves as the root of the user space's process tree; the first process (PID 1) has a special role on Unix systems, as it replaces the parent of a process when the original parent terminates. Therefore, the first process is ...
The Unix program which spawns all other processes. As of 2016, for major Linux distributions, it has been replaced by systemd. [2] biod [3] Works in cooperation with the remote nfsd to handle client NFS requests. crond [1] Time-based job scheduler, runs jobs in the background. dhcpd: Dynamically configure TCP/IP information for clients. fingerd
Systemd's initialization instructions for each daemon are recorded in a declarative configuration file rather than a shell script. For inter-process communication, systemd makes Unix domain sockets and D-Bus available to the running daemons. Systemd is also capable of aggressive parallelization.
There are three types of services. The type is INTERNAL if the service is provided by xinetd, RPC when it based on Remote procedure call (commonly listed in the /etc/rpc file), or it can be UNLISTED when the service is neither in the /etc/services nor in the /etc/rpc files. The id is the unique identifier of the service.
Although systemd is, as of 2016, used by default in most major Linux distributions, runlevels can still be used through the means provided by the sysvinit project. After the Linux kernel has booted, the /sbin/init program reads the /etc/inittab file to determine the behavior for each runlevel.
As an example of indirect usage, systemd assumes exclusive access to the cgroups facility. A control group (abbreviated as cgroup) is a collection of processes that are bound by the same criteria and associated with a set of parameters or limits. These groups can be hierarchical, meaning that each group inherits limits from its parent group.
The term was coined by the programmers at MIT's Project MAC.According to Fernando J. Corbató, who worked on Project MAC around 1963, his team was the first to use the term daemon, inspired by Maxwell's demon, an imaginary agent in physics and thermodynamics that helped to sort molecules, stating, "We fancifully began to use the word daemon to describe background processes that worked ...
The list of services that will be serviced is given in a configuration file, usually /etc/inetd.conf. A GUI for managing the configuration file is an optional accessory. The daemon may need a signal in order to re-read its configuration. For an example, telnet can be configured as follows (line taken from a machine running AIX version 5.1):