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rc: add recursive descent parser The old yacc-based parser is available with the -Y flag, which will probably be removed at some point. The new -D flag dumps a parse tree of the input, without executing it. This allows comparing the output of rc -D and rc -DY on different scripts to see that the two parsers behave the same. The rc paper ends by saying: It is remarkable that in the four most recent editions of the UNIX system programmer’s manual the Bourne shell grammar described in the manual page does not admit the command who|wc. This is surely an oversight, but it suggests something darker: nobody really knows what the Bourne shell’s grammar is. Even examination of the source code is little help. The parser is implemented by recursive descent, but the routines corresponding to the syntactic categories all have a flag argument that subtly changes their operation depending on the context. Rc’s parser is implemented using yacc, so I can say precisely what the grammar is. The new recursive descent parser here has no such flags. It is a straightforward translation of the yacc. The new parser will make it easier to handle free carats in more generality as well as potentially allow the use of unquoted = as a word character. Going through this exercise has highlighted a few dark corners here as well. For example, I was surprised to find that x >f | y >f x | y are different commands (the latter redirects y's output). It is similarly surprising that a=b x | y sets a during the execution of y. It is also a bit counter-intuitive x | y | z x | if(c) y | z are not both 3-phase pipelines. These are certainly not things we should change, but they are not entirely obvious from the man page description, undercutting the quoted claim a bit. On the other hand, who | wc is clearly accepted by the grammar in the manual page, and the new parser still handles that test case.
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 1 0b917997 2004-03-04 devnull /*

 2 0b917997 2004-03-04 devnull  * on Mac OS X, err is something else,

 3 0b917997 2004-03-04 devnull  * and assigning to it causes a bus error.

 4 0b917997 2004-03-04 devnull  * what a crappy linker.

 5 0b917997 2004-03-04 devnull  */

 6 0b917997 2004-03-04 devnull #define err rc_err

 7 f08fdedc 2003-11-23 devnull #define	EOF	(-1)

 8 f08fdedc 2003-11-23 devnull #define	NBUF	512

 9 f08fdedc 2003-11-23 devnull struct io{

10 f08fdedc 2003-11-23 devnull 	int fd;

11 f08fdedc 2003-11-23 devnull 	char *bufp, *ebuf, *strp, buf[NBUF];

12 f08fdedc 2003-11-23 devnull };

13 f08fdedc 2003-11-23 devnull io *err;

14 f08fdedc 2003-11-23 devnull io *openfd(int), *openstr(void), *opencore(char *, int);

15 f08fdedc 2003-11-23 devnull int emptybuf(io*);

16 f08fdedc 2003-11-23 devnull void pchr(io*, int);

17 f08fdedc 2003-11-23 devnull int rchr(io*);

18 f08fdedc 2003-11-23 devnull void closeio(io*);

19 f08fdedc 2003-11-23 devnull void flush(io*);

20 f08fdedc 2003-11-23 devnull int fullbuf(io*, int);

21 c8f53842 2007-03-26 devnull void pdec(io*, int);

22 c8f53842 2007-03-26 devnull void poct(io*, unsigned);

23 c8f53842 2007-03-26 devnull void pptr(io*, void*);

24 f08fdedc 2003-11-23 devnull void pquo(io*, char*);

25 f08fdedc 2003-11-23 devnull void pwrd(io*, char*);

26 f08fdedc 2003-11-23 devnull void pstr(io*, char*);

27 f08fdedc 2003-11-23 devnull void pcmd(io*, tree*);

28 47d4646e 2020-05-05 rsc void pcmdu(io*, tree*);

29 f08fdedc 2003-11-23 devnull void pval(io*, word*);

30 f08fdedc 2003-11-23 devnull void pfnc(io*, thread*);

31 f08fdedc 2003-11-23 devnull void pfmt(io*, char*, ...);