Is the bug in the grammar or in the code?

问题:

I'm not sure if this grammar is correct for a shell command language that should also be able to execute single-quotes and double-quotes. It seems that non-trivial commands work e.g. ls -al | sort | wc -l but the simple one does not work with single-quotes: echo 'foo bar' does not work.

%{
    #include "shellparser.h"
%}

%option reentrant
%option noyywrap

%x SINGLE_QUOTED
%x DOUBLE_QUOTED

%%

"|"                     { return PIPE; }

[ \t\r]                 { }
[\n]                    { return EOL; }

[a-zA-Z0-9_\.\-]+       { return FILENAME; }

[']                     { BEGIN(SINGLE_QUOTED); }
<SINGLE_QUOTED>[^']+    { }
<SINGLE_QUOTED>[']      { BEGIN(INITIAL); return ARGUMENT; }
<SINGLE_QUOTED><<EOF>>  { return -1; }

["]                     { BEGIN(DOUBLE_QUOTED); }
<DOUBLE_QUOTED>[^"]+    { }
<DOUBLE_QUOTED>["]      { BEGIN(INITIAL); return ARGUMENT; }
<DOUBLE_QUOTED><<EOF>>  { return -1; }

[^ \t\r\n|'"]+          { return ARGUMENT; }

%%

My code that scans and parses the shell is

 params[0] = NULL;
    printf("> ");
    i=1;
    do {
        lexCode = yylex(scanner);
        text = strdup(yyget_text(scanner));//yyget_text(scanner);
        /*printf("lexCode %d command %s inc:%d", lexCode, text, i);*/
        ca = text;
        if (lexCode != EOL) {
            params[i++] = text;
        }
        Parse(shellParser, lexCode, text);
        if (lexCode == EOL) {
            dump_argv("Before exec_arguments", i, params);
            exec_arguments(i, params);
            corpse_collector();
            Parse(shellParser, 0, NULL);
            i=1;
        }
    } while (lexCode > 0);

    if (-1 == lexCode) {
        fprintf(stderr, "The scanner encountered an error.\n");
    }

The CMake build file is

cmake_minimum_required(VERSION 3.0)
project(openshell)
find_package(FLEX)
FLEX_TARGET(ShellScanner shellscanner.l shellscanner.c)
set(CMAKE_VERBOSE_MAKEFILE on)
include_directories(/usr/include/readline)
ADD_EXECUTABLE(lemon lemon.c)
add_custom_command(OUTPUT shellparser.c COMMAND lemon -s shellparser.y DEPENDS shellparser.y)
add_executable(openshell shellparser.c ${FLEX_ShellScanner_OUTPUTS} main.c openshell.h errors.c errors.h util.c util.h stack.c stack.h shellscanner.l shellscanner.h)
file(GLOB SOURCES "./*.c")
target_link_libraries(openshell ${READLINE_LIBRARY} ${FLEX_LIBRARIES})
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wall -O3 -std=c99")

My project is available on my github. A typical shell session, where only some commands work due to some bug, is as follows.

> ls -al | sort | wc
argument ::= FILENAME .
argumentList ::= argument .
command ::= FILENAME argumentList .
command ::= FILENAME .
command ::= FILENAME .
commandList ::= command .
commandList ::= command PIPE commandList .
commandList ::= command PIPE commandList .
 {(null)} {ls} {-al} {|} {sort} {|} {wc}
     45     398    2270
3874: child 3881 status 0x0000
in ::= in commandList EOL .
> who
command ::= FILENAME .
commandList ::= command .
 {(null)} {who}
dac      :0           2016-04-18 05:17 (:0)
dac      pts/2        2016-04-18 05:20 (:0)
3874: child 3887 status 0x0000
in ::= in commandList EOL .
> ls -al | awk '{print $1}'
argument ::= FILENAME .
argumentList ::= argument .
command ::= FILENAME argumentList .
argument ::= ARGUMENT .
argumentList ::= argument .
command ::= FILENAME argumentList .
commandList ::= command .
commandList ::= command PIPE commandList .
 {(null)} {ls} {-al} {|} {awk} {'}
awk: cmd. line:1: '
awk: cmd. line:1: ^ invalid char ''' in expression
3874: child 3896 status 0x0100
in ::= in commandList EOL .
> 

I can observe that both commands get the same bug: echo 'foo bar' gets garbled to {echo} {'} when we want it to result in {echo} {foo bar} so that the shell strips the quotes and executes the command like this

char *cmd[] = { "/usr/bin/echo", "foo bar", 0 };

回答1:

The problem is in rule

<SINGLE_QUOTED>[^']+ { }

since it removes all characters inside quotes. All you get as "yytext" is the closing quote (due to rule <SINGLE_QUOTED>['] ...). You have to store somewhere the text and use it when the closing quote is detected. E.g. (very poor coding style, error checking etc. omitted, sorry)

<SINGLE_QUOTED>[^']+    { mystring = strdup(yytext); }

<SINGLE_QUOTED>[']      { BEGIN(INITIAL);
      /*  mystring contains the whole string now,
           yytext contains only "'" */
                          return ARGUMENT; }

回答2:

yytext holds a pointer to the substring which matched the most recently recognized pattern.

So when your scanner returns ARGUMENT at the end of a single quoted string, yytext points to the terminating single quote. As it happens, that is visible in your debugging trace.

If you want to "build up" a token, you should take a look at the flex function yymore(). (And don't forget that the closing single quote is not part of the quoted string.)

---

Returning ARGUMENT for both single- and double- quoted strings is both misleading and imprecise.

It is imprecise because a double-quoted string is handled very differently than a single-quoted string, since enclosed substitution syntaxes are expanded, requiring a recursive call to the parser (and this needs to be done even to recognize the end of the string: consider "$(echo "Hello, world!")", as one simple example).

It is misleading because the end of the quoted segment does not mark the end of a word. Indeed, a simple-minded scanner will not correctly find wird endings. Consider:

x="a b"
printf "[%s]\n" '$x'$x"$x"

---

Finally, it is not clear to me why you chose to use lemon rather than bison/yacc since you are not using the one feature which would make it useful in this case: the fact that it implements a "push" interface, allowing you to call the parser from a lexer rule. Of course, modern bison versions -- and even not-so-modern ones -- also implement this feature. Not that I have any bias against lemon -- I think it could be an excellent match for this problem precisely because of the need to do recursive parsing.