If [[$# -Eq 0]] ; Then In Unix

$? is the return code form a previous command or function
in unix, 0 is the standard return code for a successful completion

if [$? -ne 0]
The whole line means

‘if the previous return code was not equal to 0 ‘ ….

It will then be followed by

then
statement # do something
(optional) else
(optional) statement # do something else
fi # closure of the ‘if’ conditional