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 from the command line.