Google Compute Engine: how to set hostname permanently?

Need to remove file /etc/dhcp/dhclient.d/google_hostname.sh

rm -rf /etc/dhcp/dhclient.d/google_hostname.sh
rm -rf /etc/dhcp/dhclient-exit-hooks.d/google_set_hostname

That isn't possible. Please take a look at this answer. The following article explains that the "hostname" is part of the default metadata entries and it is not possible to manually edit any of the default metadata pairs. As such, you would need to use a script or something else to change the hostname every time the system restarts, otherwise it will automatically get re-synced with the metadata server on every reboot.

You can find information on startup scripts for GCE in this article. You can visit this one for info on how to apply the script to an instance.

The most simple way to achieve it is creating a simple script and that's what I have done.

I have stored the hostname in the instance metadata and then I retrieve it every time the system restarts in order to set the hostname using a cron job.

$ gcloud compute instances add-metadata <instance> --metadata hostname=<new_hostname> 
$ sudo crontab -e

And this is the line that must be appended in crontab

@reboot hostname $(curl --silent "http://metadata.google.internal/computeMetadata/v1/instance/attributes/hostname" -H "Metadata-Flavor: Google")

After these steps, every time you restart your instance it will have the hostname <new_hostname>. You can check it in the prompt or with the command: hostname

Edit rc.local

sudo nano /etc/rc.local 

Add your line under the rest:

hostname *your.hostname.com*

Make sure to run the following after for the script to be executed

chmod +x /etc/rc.d/rc.local

Reboot, and profit.