Ubuntu 18.04 LTS change hostname permanently

How to display the current Ubuntu hostname

Simply type the following command:
$ hostnamectl

Ubuntu 18.04 LTS change hostname permanently

The procedure to change the computer name on Ubuntu 18.04 LTS:

1. Type the hostnamectl command :
   sudo hostnamectl set-hostname newNameHere
   Delete the old name and setup new name.
2. Next Edit the /etc/hosts file:
   sudo nano /etc/hosts
   Replace any occurrence of the existing computer name with your new one.
3. Reboot the system to changes take effect:
   sudo reboot

How to change hostname on Ubuntu 18.04 without a system restart

Let us see current setting just type the following command:
$ hostnamectl
Next change hostname from localhost to u1804-nixcraft, enter:
$ hostnamectl set-hostname u1804-nixcraft
Verify new changes:
$ hostnamectl

Find your public or private IP address by running the ip command:
$ ip a
$ ip a s eth0

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
    link/ether 40:9f:38:28:f6:b5 brd ff:ff:ff:ff:ff:ff
    inet 192.168.2.24/24 brd 192.168.2.255 scope global noprefixroute enp6s0
       valid_lft forever preferred_lft forever
    inet6 fe80::429f:38ff:fe28:f6b5/64 scope link 
       valid_lft forever preferred_lft forever

Update /etc/hosts file with that IP address such as 192.168.2.24 using a text editor such as vim command/nano command:
$ sudo vi /etc/hosts
OR
$ sudo nano /etc/hosts
Update/append:
192.168.2.24 u1804-nixcraft
Make sure you replace any occurrence of the old computer name with your new one too. Save and close the file in vim.

Conclusion

You learned how to change hostname permanently when using Ubuntu 18.04 LTS as per your needs.