0

在尝试创建一个非常基本的命令行解释器时，我遇到了一个我似乎无法弄清楚的部分。当我检查所需分隔符的令牌时，我似乎无法正确启用 && 和 || 功能。下面列出的是插入 args 然后创建进程的循环。

我现在只关注 && 并计划使用该实现来帮助 || 功能。你们可以看看并帮助我指出正确的方向吗？

旁注：这也是我第一次用 C 编写程序，因此代码中可能会出现一些错误。

这是一个家庭作业问题。

谢谢

更新了我到目前为止的完整程序代码。

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#define ARG_SIZE 1<<16
#define MAXLINE 100

char *args[ARG_SIZE];
int place = 0;
int return_status;

void insert( char *token ){
    args[ place ] = token;
    place++;
}

void create() {
    size_t nargs = place;
    pid_t pid;

    if ( nargs == 0 ) return;
    if ( !strcmp( args[0], "exit" ) ) exit(0);
    pid = fork();
    if ( pid ) {
        pid = wait( return_status );
    } else {
        if ( access( args[ 0 ], X_OK ) ){
            if( execvp( args[ 0 ], args ) ) {
                puts( strerror( errno ) );
                exit( 127 );
            }
        }else{
            puts( strerror( errno ) );
            exit( 127 );
        }
    }

    int i = 0;
    for ( i ; i < place + 1; i++)
        args[ i ] = NULL;
    place = 0;
}

int main( int argc, char *argv[] ){

    char line[MAXLINE+1];                       /* an input line */
    int c;                                      /* a single input char or EOF */
    int n;                                      /* line length */
    char *token;                                /* pointer to an input token */

    for(;;) {                                   /* repeat until end of file */ 
        write( 1, "#>", 2 );

        if (fgets(line,MAXLINE,stdin) == NULL)  /* end of file? */
            exit(0);

        n = strlen(line);                       /* get length of line */

        if (n == 0)                             /* if line is empty */
            continue;

        if (line[n-1] != '\n') {                /* does input end with '\n' ? */
            fprintf(stderr,"Line too long.\n"); /* no, so line is too long. */

            /*--------------------------------------------*/
            /* Read and ignore input through end of line. */
            /*--------------------------------------------*/
            while ((c = fgetc(stdin)) != '\n') {
                if (c == EOF) {
                    fprintf(stderr,"Unexpected end of file\n");
                    exit(1);
                }
            }
            continue;
        }

        write( 1, line, strlen( line ) );
        line[n-1] = '\0';                      /* remove the end of line */
        /*-------------------------------------------------------------*/
        /* Identify and process each token (i.e. sequence of non-blank */
        /* characters delimited by whitespace). For "ordinary" tokens  */
        /* (i.e. "words") we just display them. For the ||, &&, and ;  */
        /* items we display them with a textual explanation.           */
        /*-------------------------------------------------------------*/
        token = strtok(line," \t");
        while (token != NULL ) {
            insert( token );
            if (!strcmp(token,"&&")){
                place--;
                args[ place ] = NULL;
                create();
                if( return_status == -1)
                    write( 1, "Wait failed.\n", 12 );
                if( return_status & 0xff ){
                    int i = 0;
                    for( i; i < place; i++)
                        args[ i ] = NULL;
                    place = 0;
                    while( strcmp( token, "||" ) || strcmp( token, ";" ) ) ;
                }
            }
            else if (!strcmp(token,"||")){
                place--;
                args[ place ] = NULL;
                completed = 0;
                create();                
            }
            else if (!strcmp(token,";")){
                place--;
                args[ place ] = NULL;
                create();
            }
            else{
            }
            token = strtok(NULL," \t");
        }
        create();
    }
    return 0;
}
4

2 回答 2

1

wait() 接受一个整数指针，而不是整数。应该有一个编译器警告。如果没有，我建议您使用警告标志。

#include <sys.wait.h>
wait(&return_code);

此检查不正确，因为 access() 在成功时返回 0。似乎您不能使用访问权限来检查不在当前目录中的命令，除非它们提供了可执行文件的完整路径或者您可以扩展它。例如，“ls”需要扩展为“/bin/ls”。

if ( access( args[ 0 ], X_OK ) ){
            if( execvp( args[ 0 ], args ) ) {
                puts( strerror( errno ) );
                exit( 127 );
            }

对 return_status 的检查正在查看第一个字节，但被 forked() 或 execvp() 的程序在第二个字节中返回其状态。检查它返回的内容：

if( return_status & 0xff ){

应该是：

if ( WEXITSTATUS(return_status) != 0 ){

或者

if ( (return_status & 0xff00) >> 8 ){

if 正文：

      for( int i = 0; i < place; i++)
                        args[ i ] = NULL;
       place = 0;
       while( strcmp( token, "||" ) || strcmp( token, ";" ) ) ;

可以替换为：

 break;

因为您正在执行 for 循环并在 create() 中将 place 设置为 0，而 while 循环对我来说是无限的。只是在结束命令序列时打破封闭，让您回到阅读另一个键入的命令行。

  //if ( return_status == -1)

可以使用 wait() 状态宏：

  if ( ! WIFEXITED(return_status) )
于 2012-09-24T17:16:17.637 回答
0

此代码显示了我在另一个答案中提到的内容，适用于“false && ls”，并在 && 失败后更新以解决更多命令。

    #include <unistd.h>
    #include <string.h>
    #include <errno.h>
    #include <sys/types.h>
    #include <sys/wait.h>
    #define ARG_SIZE 1 << 16
    #define MAXLINE 100

    char *args[ARG_SIZE];
    int place = 0;
    int return_status;

    char message[200];

    void insert( char *token ){
        args[ place ] = token;
        place++;
    }

    void reset_args(void)
    {
        for (int i = 0 ; i < place + 1; i++)
            args[ i ] = NULL;
        place = 0;
    }

    #define EXECVP_FAILED 127

    char last_command[200];

    void create() {
        size_t nargs = place;
        pid_t pid;

        if ( nargs == 0 ) return;
        if ( args[0] )
            strcpy(last_command,args[0]);
        else
           last_command[0] = '\0';
        if ( !strcmp( args[0], "exit" ) ) exit(0);
        pid = fork();
        if ( pid ) {
            pid = wait( &return_status );
        } else {
            if( execvp( args[ 0 ], args ) ) 
            {
               puts( strerror( errno ) );
               exit( EXECVP_FAILED );
            }
        }
        reset_args();
    }

    int main( void ){

       char line[MAXLINE+1];                       /* an input line */
       int c;                                      /* a single input char or EOF */
       int n;                                      /* line length */
       char *token;                                /* pointer to an input token */
       enum states { NORMAL, AND_CMD_FAILED };
       enum states state  = NORMAL;
       for(;;) {                                   /* repeat until end of file */ 
          state = NORMAL;
          write( 1, "#>", 2 );

          if (fgets(line,MAXLINE,stdin) == NULL)  /* end of file? */
             exit(0);

          n = strlen(line);                       /* get length of line */

          if (n == 0)                             /* if line is empty */
             continue;

          if (line[n-1] != '\n') {                /* does input end with '\n' ? */
             fprintf(stderr,"Line too long.\n"); /* no, so line is too long. */

             /*--------------------------------------------*/
             /* Read and ignore input through end of line. */
             /*--------------------------------------------*/
             while ((c = fgetc(stdin)) != '\n') {
                if (c == EOF) {
                   fprintf(stderr,"Unexpected end of file\n");
                   exit(1);
                }
             }
             continue;
          }

          write( 1, line, strlen( line ) );
          line[n-1] = '\0';                      /* remove the end of line */
          /*-------------------------------------------------------------*/
          /* Identify and process each token (i.e. sequence of non-blank */
          /* characters delimited by whitespace). For "ordinary" tokens  */
          /* (i.e. "words") we just display them. For the ||, &&, and ;  */
          /* items we display them with a textual explanation.           */
          /*-------------------------------------------------------------*/
          token = strtok(line," \t");
          while (token != NULL ) {
             insert( token );
             if (!strcmp(token,"&&")){
                    place--;
                args[ place ] = NULL;                      
                            if ( state == NORMAL )
                               create();
                            else
                               reset_args();

                if ( ! (WIFEXITED(return_status) ) || ( WEXITSTATUS(return_status)  ) )
                {
                   // command did not execute successfully
                   if ( ! WIFEXITED(return_status) )
                   {
                      sprintf(message,"Wait failed for %s\n",last_command);
                      write( 1, message, strlen(message) );
                   }
                   else if ( WEXITSTATUS(return_status) == EXECVP_FAILED )
                   {
                      sprintf(message,"execvp failed for %s\n",last_command);
                      write( 1, message, strlen(message) );
                   }
                   else  if ( WEXITSTATUS(return_status) != 0 ) {
                      sprintf(message,"%s command failed - exit status %d\n",
                              last_command,WEXITSTATUS(return_status));
                      write( 1, message, strlen(message) );
                   }
                   // bypass next command(s) up to || or ; delimiters
                   state = AND_CMD_FAILED;
                }
             }
             else if (!strcmp(token,"||")){
                place--;
                args[ place ] = NULL;
                //completed = 0;
                if ( state == NORMAL )
                   create();
                else
                   reset_args();
                state = NORMAL;         
             }
             else if (!strcmp(token,";")){
                place--;
                args[ place ] = NULL;
                if ( state == NORMAL )
                   create();
                else
                   reset_args();
                state = NORMAL;
             }
             else{
             }
             token = strtok(NULL," \t");
          }
          if ( state == NORMAL )
             create();
          else
             reset_args();
       }
       return 0;
  }
于 2012-09-25T09:04:08.997 回答