If you want to use unifdef for plain text (not C code), use the -t option, which disables this parsing for C comments and quotes.
You specify which symbols you want defined (-Dsym) or undefined (-Usym) and the lines inside those ifdefs will be copied to the output or removed as appropriate. The ifdef, ifndef, else, and endif lines associated with sym will also be removed. Ifdefs involving symbols you don't specify and ``#if'' control lines are untouched and copied out along with their associated ifdef, else, and endif lines. If an ifdef X occurs nested inside another ifdef X, then the inside ifdef is treated as if it were an unrecognized symbol. If the same symbol appears in more than one argument, the last occurrence dominates.
The -l option causes unifdef to replace removed lines with blank lines instead of deleting them.
If your C code uses ifdefs to delimit non-C lines, such as comments or code which is under construction, then you must tell unifdef which symbols are used for that purpose so that it won't try to parse for quotes and comments inside those ifdefs. You specify ignored ifdefs with -iDsym and -iUsym similar to -Dsym and -Usym above.
Unifdef copies its output to stdout and will take its input from stdin if no file argument is given. If the -c argument is specified, then the operation of unifdef is complemented, i.e. the lines that would have been removed or blanked are retained and vice versa.
Unifdef works nicely with the -Dsym option added to diff(1) as of the 4.1 Berkeley Software Distribution.
Exit status is 0 if output is exact copy of input, 1 if not, 2 if trouble.