Reference/!if

!if

[!] value [op value2]

This command, when paired with an !endif command, will tell the compiler whether or not to compile the lines in between the two lines. If value is non-zero, or the comparison of value and value2 depending on the operator results in true, the contained lines will be compiled. Otherwise, they will be skipped. op can be either == or != (string comparison), <=, < > or >= (float comparison), & (bitwise AND comparison), && or || (boolean comparison). If [!] is set, return value will be switched from true to false and vice versa.

!if 1 < 2
  !echo "1 is smaller than 2!!"
!else if ! 3.1 > 1.99
  !error "this line should never appear"
!else
  !error "neither should this"
!endif

Command introduced with NSIS v2.15