hostnamectl is part of systemd, and provides a proper API for dealing with setting a server's hostnames in a standardized way.
$ rpm -qf $(type -P hostnamectl)
Previously each distro that did not use systemd, had their own methods for doing this which made for a lot of unnecessary complexity.
hostnamectl may be used to query and change the system hostname and
This tool distinguishes three different hostnames: the high-level
"pretty" hostname which might include all kinds of special characters
(e.g. "Lennart's Laptop"), the static hostname which is used to
initialize the kernel hostname at boot (e.g. "lennarts-laptop"), and the
transient hostname which is a default received from network
configuration. If a static hostname is set, and is valid (something
other than localhost), then the transient hostname is not used.
Note that the pretty hostname has little restrictions on the characters
used, while the static and transient hostnames are limited to the
usually accepted characters of Internet domain names.
The static hostname is stored in /etc/hostname, see hostname(5) for
more information. The pretty hostname, chassis type, and icon name are
stored in /etc/machine-info, see machine-info(5).
Use systemd-firstboot(1) to initialize the system host name for mounted
(but not booted) system images.
hostnamectl also pulls a lot of disparate data together into a single location to boot:
Static hostname: centos7
Icon name: computer-vm
Machine ID: 1ec1e304541e429e8876ba9b8942a14a
Boot ID: 37c39a452464482da8d261f0ee46dfa5
Operating System: CentOS Linux 7 (Core)
CPE OS Name: cpe:/o:centos:centos:7
Kernel: Linux 3.10.0-693.21.1.el7.x86_64
The info here is coming from
uname -a, etc. including the hostname of the server.
What about the files?
Incidentally, everything is still in files,
hostnamectl is merely simplifying how we have to interact with these files or know their every location.
As proof of this you can use
strace -s 2000 hostnamectl and see what files it's pulling from:
$ strace -s 2000 hostnamectl |& grep ^open | tail -5
open("/lib64/libattr.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
open("/proc/self/stat", O_RDONLY|O_CLOEXEC) = 3
open("/etc/machine-id", O_RDONLY|O_NOCTTY|O_CLOEXEC) = 4
open("/proc/sys/kernel/random/boot_id", O_RDONLY|O_NOCTTY|O_CLOEXEC) = 4
To the astute observer, you should notice in the above
strace that not all files are present.
hostnamectl is actually interacting with a service,
systemd-hostnamectl.service which in fact does the "interacting" with most of the files that most admins would be familiar with, such as
Therefore when you run
hostnamectl you're getting details from the service. This is a ondemand service, so you won't see if running all the time. Only when
hostnamectl runs. You can see it if you run a
watch command, and then start running
hostnamectl multiple times:
$ watch "ps -eaf|grep [h]ostname"
root 3162 1 0 10:35 ? 00:00:00 /usr/lib/systemd/systemd-hostnamed
The source for it is here: https://github.com/systemd/systemd/blob/master/src/hostname/hostnamed.c and if you look through it, you'll see the references to