https://linuxize.com/post/how-to-change-hostname-on-ubuntu-18-04/

hostnamectl

hostnamectl
bamboo

sudo hostnamectl set-hostname jira
hostnamectl
jira

hosts

sudo cp /etc/hosts /etc/hosts.bak
sudo nano /etc/hosts
127.0.1.1 bamboo

127.0.0.1 jira

EDIT cloud.cfg ( if cloud-init installed )

ls -l /etc/cloud/cloud.cfg

setup@bamboo:~$ ls -la /etc/cloud/cloud.cfg
-rw-r--r-- 1 root root 3612 May 11  2019 /etc/cloud/cloud.cfg

Search for preserve_hostname and change the value from false to true:

sudo nano /etc/cloud/cloud.cfg

# This will cause the set+update hostname module to not operate (if true)
preserve_hostname: false

# This will cause the set+update hostname module to not operate (if true)
preserve_hostname: true