What is the difference between a shell variable and an environment variable?

You are reading this because you can’t understand why os.getenv('SOMEVAR') doesn’t work when echo $SOMEVAR does; or because you’ve never figured out why this mysterious shell command, export, is needed.

The shell is the software that gives you a prompt and parses and executes the commands you type. But the shell is also a programming language, and, like Python, it has variables. So, here is a version of hello world in Python:

import sys
 
greeting = "Hello, world!"
sys.stdout.write(greeting)

Here is how we would write that program in the shell (there are many different shells, but here we assume bash, which is the most commonly used in GNU/Linux systems):

greeting="Hello, world!"
echo $greeting

Variables such as greeting are called shell variables and they have nothing to do with the environment. In the following example, Python isn’t going to pass the greeting variable to myprogram:

import subprocess
 
greeting = "Hello, world!"
subprocess.call('myprogram')

Likewise, the shell isn’t going to pass the greeting variable to myprogram in this:

greeting="Hello, world!"
myprogram

If you want greeting to become an environment variable, you need to “export it to the environment”:

export greeting

The above works when greeting is already defined. Usually it is convenient to define it and export it at the same time:

export greeting="Hello, world!"

Environment variables usually also work as shell variables. In addition, if greeting is already exported, then executing greeting="Good morning!" will change the environment variable! Finally,

unset greeting

will delete both the shell variable and the environment variable.

Another way to set an environment variable is like this:

greeting="Hello, world!" myprogram

With this syntax, the greeting environment variable is set and is passed (along with any other environment variables) to myprogram, and then it is deleted immediately.

We usually use all caps for environment variables. You can use the shell env command to view all the environment variables.

This is a republication of an old article first published in my Django Deployment site.