🚀 go-pugleaf

In article <1992Nov3.083527.20756@ugle.unit.no> helgeaa@colargol (Helge O. Aarstein) writes:

   We are trying to make a filter for the lp-command to reject 
   non-postscript files. But, there is a problem: 
	   How can I be sure that the file contains postscript-
	   code?

The simplest way is probably to check to see that the first two
characters of the file are '%!'.  Most programs that generate
PostScript code will at least those two characters at the begining.
The only time I have seen them left out is in hand-written code that
was just to test.  This method is used by the lpr I have used to
distinquish a PostScript file from another kind it might know how to
print.  Of course, '%!' is only a comment, so it can be left out
without affecting the execution of the program, so this method is not
fool-proof.  Since most programs include that code, though, it is
reasonably good (and simple!).  If you wanted to be really sure, I
suppose you could write a grammar checker that verified
PostScript-ness, but such a thing would be complicated and would need
to be updated with future revisions.  So, I recommend the '%!'.

--Peter
-- 
"The brilliant Cerebron, attacking the problem analytically, discovered three
 distinct kinds of dragon: the mythical, the chimerical, and the purely
 hypothetical. They were all, one might say, nonexistent, but each nonexistsed
 in an entirely different way." -- Standislaw Lem _Cyberiad_