hostname — Local host name configuration file
/etc/hostname
The /etc/hostname file
                configures the name of the local system that is set
                during boot, with the
                sethostname(2)
                system call. It should contain a single
                newline-terminated host name string. The
                host name may be a free-form string up to 64 characters
                in length, however it is recommended that it consists
                only of 7bit 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 host name as well, however only as fallback.
You may use hostnamectl(1) to change the value of this file from the command line.