Reference/!ifdef

!ifdef

gflag [bcheck gflag [...]]]

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 gflag is globally defined (using !define or the /D switch), then the contained lines will be compiled. Otherwise, they will be skipped. 'bcheck' can be specified as & (boolean and) or | (boolean or) along with more gflags -- precedence is simple, left to right.

!define SOMETHING
!ifdef SOMETHING
  !echo "SOMETHING is defined"
!endif
!undef SOMETHING
!ifdef SOMETHING
  !echo "SOMETHING is defined" # will never be printed
!endif

Command introduced with NSIS v1.2