Setting environment variable for one program call in bash using env

Question

I am trying to invoke a shell command with a modified environment via the command env.

According to the manual

env HELLO=\'Hello World\'         


        

Answer 1

I think what happens is similar to this situation in which I was also puzzled.

In a nutshell, the variable expansion in the first case is done by the current shell which doesn't have $HELLO in its environment. In the second case, though, single quotes prevent the current shell from doing the variable expansion, so everything works as expected.

Note how changing single quotes to double quotes prevents this command from working the way you want:

HELLO='Hello World' bash -c "echo $HELLO"

Now this will be failing for the same reason as the first command in your question.