Setting environment variable for one program call in bash using env

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.

This works and is good for me

$ MY_VAR='Hello' ANOTHER_VAR='World!!!' && echo "$MY_VAR $ANOTHER_VAR"
Hello World!!!

It's because in your first case, your current shell expands the $HELLO variable before running the commands. And there's no HELLO variable set in your current shell.

env HELLO='Hello World' echo $HELLO

will do this:

• expand any variables given, in this case $HELLO
• run env with the 3 arguments 'HELLO=Hello World', 'echo' and '' (an empty string, since there's no HELLO variable set in the current shell)
• The env command will run and set the HELLO='Hello World' in its environment
• env will run echo with the argument '' (an empty string)

As you see, the current shell expanded the $HELLO variable, which isn't set.

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

will do this:

• set the variable HELLO='Hello World for the following command
• run bash with the 2 arguments '-c' and 'echo $HELLO'
• since the last argument is enclosed in single quotes, nothing inside it is expanded
• the new bash in turn will run the command echo $HELLO
• To run echo $HELLO in the new bash sub-shell, bash first expands anything it can, $HELLO in this case, and the parent shell set that to Hello World for us.
• The subshell runs echo 'Hello World'

If you tried to do e.g. this:

env HELLO='Hello World' echo '$HELLO'

• The current shell would expand anything it can, which is nothing since $HELLO is enclosed in single quotes
• run env with the 3 arguments 'HELLO=Hello World', 'echo' and '$HELLO'
• The env command will run and set the HELLO='Hello World' in its environment
• env will run echo with the argument '$HELLO'

In this case, there's no shell that will expand the $HELLO, so echo receives the string $HELLO and prints out that. Variable expansion is done by shells only.