ifdef - remove #ifdefs from a file

     ifdef [-t] [-dsymbol] [-Dsymbol] [-Usymbol] [-Isymbol] [file]


     -D   Define symbol permanently

     -I   Ignore symbol

     -U   Undefine symbol permanently

     -d   Define symbol. It may be #undef'ed later

     -t   Produce a table of the symbols on stdout


     ifdef -DUNIX file.c >newfile.c
                         # Define UNIX

     ifdef -D_MINIX -UDOS <x.c >y.c
                         # Define


     Ifdef allows conditional code [ #ifdef ... #endif  ]  to  be  selectively
     removed  from  C  files,  but  at  the  same  time  leaving  all  other C
     preprocessor commands intact such as #define,  #include  etc.   Input  to
     ifdef  is either the file named as the last argument, or stdin if no file
     is named.  Output goes to stdout.

     Symbols may be defined with the -d or -D flags just like cpp, except that
     the  latter  option  ignores  subsequent #undefs.  It is not permitted to
     give values to symbols.  Similarly, -U undefines  a  symbol  and  ignores
     subsequent  #definess.   Symbols  defined with -I are ignored; any #ifdef
     using an ignored symbol will be left intact.