Difference between "a=b" and "export a=b" in bash

Question

What's the difference between:

a=b

and

export a=b

In bash?

I understand that they both define environment variables, but I don't fully understand the difference.

Answer 1

export propagates the variable to subprocesses.

For example, if you did

FOO=bar

then a subprocess that checked for FOO wouldn't find the variable whereas

export FOO=bar

would allow the subprocess to find it.

But if FOO has already been defined as an environment variable, then FOO=bar will modify the value of that environment variable.

For example:

FOO=one     # Not an environment variable
export FOO  # Now FOO is an environment variable
FOO=two     # Update the environment variable, so sub processes will see $FOO = "two"

Older shells didn't support the export FOO=bar syntax; you had to write FOO=bar; export FOO.

Answer 2

If you don't use export, you're not defining an environment variable; just a shell variable.

Shell variables are only available to the shell process; environment variables are available to any subsequent process, not just shells.

Answer 3

Also, if you want to have the variable available to the calling shell without using export you can do this:

File a.ksh is -

#!/bin/ksh
FOO=bar

On the prompt, run this is

> . a.ksh

This will run the commands within the same shell and $FOO will be available.

Whereas,

> a.ksh

Will make $FOO available only within a.ksh, after the call to a.ksh it would not exist.

Answer 4

In addition to what has already been answered, both of these statement do not necessarily define (i.e. create vs set) an environment variable as "a" might already exist as a shell or environment variable.

In the latter case, both statements are strictly equivalent.