How to permanently export a variable in Linux?

You can add it to your shell configuration file, e.g. $HOME/.bashrc or more globally in /etc/environment. After adding these lines the changes won't reflect instantly in GUI based system's you have to exit the terminal or create a new one and in server logout the session and login to reflect these changes.

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 every login of your OS.

• /etc/environment

  If you want your environment variable in every window 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 these 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 these procedures to do all those work. You just have to set the 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 these lines in a shfile then make it executable and just run it!

add the line to your .bashrc or .profile. The variables set in $HOME/.profile are active for the current user, the ones in /etc/profile are global. The .bashrc is pulled on each bash session start.

On Ubuntu systems, use the following locations:

1. System-wide persistent variables in the format of JAVA_PATH=/usr/local/java store in

   /etc/environment
   

2. System-wide persistent variables that reference variables such as
   export PATH="$JAVA_PATH:$PATH" store in

   /etc/.bashrc
   

3. User specific persistent variables in the format of PATH DEFAULT=/usr/bin:usr/local/bin store in

   ~/.pam_environment
   

For more details on #2, check this Ask Ubuntu answer. NOTE: #3 is the Ubuntu recommendation but may have security concerns in the real world.

If it suits anyone, here are some brief guidelines for adding environment variables permanently.

vi ~/.bash_profile

Add the variables to the file:

export DISPLAY=:0
export JAVA_HOME=~/opt/openjdk11

Immediately apply all changes:

source ~/.bash_profile

Source: https://www.serverlab.ca/tutorials/linux/administration-linux/how-to-set-environment-variables-in-linux/