From 47d4646eebac34c0b94951cfcf1b81ed2ca513e1 Mon Sep 17 00:00:00 2001 From: Russ Cox Date: Mon, 4 May 2020 18:34:19 -0400 Subject: rc: add recursive descent parser MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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. --- src/cmd/rc/syn.y | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) (limited to 'src/cmd/rc/syn.y') diff --git a/src/cmd/rc/syn.y b/src/cmd/rc/syn.y index c7de3531..5c98ef80 100644 --- a/src/cmd/rc/syn.y +++ b/src/cmd/rc/syn.y @@ -1,5 +1,5 @@ %term FOR IN WHILE IF NOT TWIDDLE BANG SUBSHELL SWITCH FN -%term WORD REDIR DUP PIPE SUB +%term WORD REDIR REDIRW DUP PIPE SUB %term SIMPLE ARGLIST WORDS BRACE PAREN PCMD PIPEFD /* not used in syntax */ /* operator priorities -- lowest first */ %left IF WHILE FOR SWITCH ')' NOT @@ -19,7 +19,7 @@ %type line paren brace body cmdsa cmdsan assign epilog redir %type cmd simple first word comword keyword words %type NOT FOR IN WHILE IF TWIDDLE BANG SUBSHELL SWITCH FN -%type WORD REDIR DUP PIPE +%type WORD REDIR REDIRW DUP PIPE %% rc: { return 1;} | line '\n' {return !compile($1);} @@ -45,7 +45,7 @@ cmd: {$$=0;} | IF NOT {skipnl();} cmd {$$=mung1($2, $4);} | FOR '(' word IN words ')' {skipnl();} cmd /* - * if ``words'' is nil, we need a tree element to distinguish between + * if ``words'' is nil, we need a tree element to distinguish between * for(i in ) and for(i), the former being a loop over the empty set * and the latter being the implicit argument loop. so if $5 is nil * (the empty set), we represent it as "()". don't parenthesize non-nil @@ -73,7 +73,7 @@ cmd: {$$=0;} simple: first | simple word {$$=tree2(ARGLIST, $1, $2);} | simple redir {$$=tree2(ARGLIST, $1, $2);} -first: comword +first: comword | first '^' word {$$=tree2('^', $1, $3);} word: keyword {lastword=1; $1->type=WORD;} | comword @@ -85,7 +85,7 @@ comword: '$' word {$$=tree1('$', $2);} | WORD | '`' brace {$$=tree1('`', $2);} | '(' words ')' {$$=tree1(PAREN, $2);} -| REDIR brace {$$=mung1($1, $2); $$->type=PIPEFD;} +| REDIRW brace {$$=mung1($1, $2); $$->type=PIPEFD;} keyword: FOR|IN|WHILE|IF|NOT|TWIDDLE|BANG|SUBSHELL|SWITCH|FN words: {$$=(struct tree*)0;} | words word {$$=tree2(WORDS, $1, $2);} -- cgit v1.2.3