Prefixing variable assignment doesn't work with echo

Question

As far as I know, if you prefix a bash command with variable assignment the variable will take effect immediately. And it will only have effect within that command.

I tried this command in bash:

V=1 echo $V

However 1 is not printed out in the terminal which is not what I expected. So why it doesn't work?

Answer 1

The variable will be passed in the environment of the command following it, not when the command is being evaluated (expanded). Any variable expansion will be done earlier by shell.

$ V=1 env | grep V=
V=1

To get it working:

$ V=1; echo $V
1