It's not advanced, it's not long enough, and it's not correct either.
> This will set the value of TMP to `/tmp/myproject` only if it has not been set.
This should read "if", not "only if". And it should read: "This will set the value of TMP to `/tmp/myproject` if it has not been set or if it is null."
The form that means 'if it has not been set' has no colon; it is
TMP=${TMP-/tmp/myproject}