Environment Variable

What is an Environment Variable

An Environment Variable is a name associated with a value (typically numbers and strings or words), that can affect the way a running process or program behaves on a computer

They are part of the environment in which a process or program runs

Environment Variables can be

• system-wide(global) like HOME which stores the directory allotted to a specific user by an operating system
• shell variable or a local variable - which are specific to a shell(zsh or bash) or program or session

echo command can be used to get the value of an environment variable in a Linux Terminal, with $ as the prefix of the variable

echo $HOME
/home/izaku

In windows(Command Prompt, cmd.exe) the variable is enclosed with % characters

echo %HOME%
/home/izaku

Display environment variables

printenv command can be used to display list of environment variables and their values

To retrieve a specific environment variable, its name is passed with printenv command

Set Environment variables

Setting environment variables temporarily

A shell variable DW can be set to /home/izaku/Downloads using a simple = in a Terminal

This shell variable is available to the shell instance, which can be retrieved using echo command

But printenv command doesn't return its value, which shows that it is not an environment variable

To set it as an environment variable, export command can be used with the variable like export DW

Setting Persistent environment variables

To make Environment variables persistent those variables have to be defined in a bash configuration file

In linux, environment variables are typically read from the following files:

• /etc/environment - This file is used to set up system-wide environment variables

  Variables in this file are set in the following format :

  FOO=bar
  VAR_TEST="Test Var"

• /etc/profile - Variables set in this file are loaded whenever a bash login shell is entered

  export is used here to set variables

  export JAVA_HOME="/path/to/java/home"
  export PATH=$PATH:$JAVA_HOME/bin

• Per-user shell specific configuration file

  It can be ~/.bashrc or ~/.zshrc or ~/.bash_profile pr ~/.zsh_profile based on shell being used

  export is used here to define variables

  export PATH="$HOME/bin:$PATH"

  Use source command to load the new environment variables into the current shell session:

  source ~/.bashrc