You have to edit three files to set a permanent environment variable as follow:

• ~/.bashrc

  When you open any terminal window this file will be run. Therefore, if you wish to have a permanent environment variable in all of your terminal windows you have to add the following line at the end of this file:
  export DISPLAY=0

• ~/.profile

Same as bashrc you have to put the mentioned command line at the end of this file to have your environment variable in the every login of your OS.

• /etc/environment

If you want your environment variable in every windows or application ( not just terminal window ) you have to edit this file. Add the following command at the end of this file:
DISPLAY=0
Note that in this file you do not have to write export command

Normally you have to restart your computer to apply this changes. But you can apply changes in bashrc and profile by these commands:

$ source ~/.bashrc
$ source ~/.profile

But for /etc/environment you have no choice but restarting ( as far as I know )

* A Simple Solution

I’ve written a simple script for this procedures to do all those work. You just have to set name and value of your environment variable.

#!/bin/bash

echo "Enter variable name: "

read variable_name

echo "Enter variable value: "

read variable_value

echo "adding " $variable_name " to environment variables: " $variable_value

echo "export "$variable_name"="$variable_value>>~/.bashrc

echo $variable_name"="$variable_value>>~/.profile

echo $variable_name"="$variable_value>>/etc/environment

source ~/.bashrc

source ~/.profile

echo "do you want to restart your computer to apply changes in /etc/environment file? yes(y)no(n)"

read restart

case $restart in

    y) sudo shutdown -r 0;;

    n) echo "don't forget to restart your computer manually";;

esac

exit

Save this lines in a shfile then make it executable and just run it!