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

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.

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.

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.

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.

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.