hostname — Local hostname configuration file
/etc/hostname
The /etc/hostname file configures the
name of the local system that is set during boot using the
sethostname(2)
system call. It should contain a single newline-terminated
hostname string. The hostname may be a free-form string up to 64
characters in length; however, it is recommended that it consists
only of 7-bit ASCII lower-case characters and no spaces or dots,
and limits itself to the format allowed for DNS domain name
labels, even though this is not a strict requirement.
Depending on the operating system, other configuration files might be checked for configuration of the hostname as well, however only as fallback.
You may use hostnamectl(1) to change the value of this file during runtime from the command line. Use systemd-firstboot(1) to initialize it on mounted (but not booted) system images.