This one:

VAR=value command

will define VAR for the scope of ‘command’ only (and will make VAR known to ‘command’). But, as you noticed, the shell does variables expansion before the assignment. For instance, doing EDITOR=emacs less file.txt will tell ‘less’ to launch emacs upon pressing ‘v’.

On the other hand, the second form

VAR=value; command

defines VAR for the remaining instance of the shell, but the shell will NOT pass it to subprocesses, so Z=hello; env will NOT display any Z variable, whereas Z=hello; echo $Z will display ‘hello’ (because of shell variable expansion).

A third form is using ‘export’:

export VAR=value; command

. It is like the first form, but VAR will be passed to all subsequent commands.

Cheers,

Pascal