#open terminal and type:
$ nano .bashrc

#scroll to the bottom of the script and type:
export <place variable name in here> = '<place variable value here>'

#e.g
export ANDROID_SDK_ROOT='/home/<$USER>/Android/Sdk'

#once you,re finished typing in your variable hit Ctrl+X then press enter

#Ubuntu 20.04: It's better to put $PATH at the beginning that way
#the new path environment variable being added will append to the old one.
echo 'export PATH=$PATH:$HOME/Android/Sdk/emulator' >> ~/.bashrc

#update current shell
source ~/.bashrc