implement pipe in unix shell

[twolf]

I am trying to learn Unix C programming and doing some exercises for practice. I am currently working on writing my own shell that works similar to the linux bash shell.

The code I have below provides for a fairly basic shell. It supports I/O redirection. I am trying to add support for piping. Initially, I just want to add support for a single pipe.

I have tried to go through some tutorials online but can't quite figure out where I am going wrong. I am missing something but not sure what.

Here is what I have so far.

#include <stdlib.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/types.h>
#include <string.h>
#include <sys/wait.h>
#include <fcntl.h>

#define TRUE 1

int main(void)
{
  char *arg_list[10];

  int status;

  //counter for tokenizing/parsing
  int x = 0;
  
  //counter for while loop to find input file, output file, etc
  int y = 0;

  pid_t pid;
  char buf[100];

  char inFile[10];
  char outFile[10];

  int fdIn, fdOut;

  int fd[2];

  pipe(fd);

  while(TRUE)
  {
     printf("> ");
     if (!fgets(buf, 100, stdin))
       return 0;

    pid = fork();
    switch(pid)
    {
      case -1:
      return 1;
   
      case 0:
      {
        arg_list[x] = strtok(buf, " \n");
            
        x = 0;
         
         while(arg_list[x] != NULL)
         {
           counter++;
           arg_list[x] = strtok(NULL, " \n");
         }
            
         y = 0;
         
         while(arg_list[y] != NULL)
         {
           if(!strcmp(arg_list[y], "<"))
           {
             if(arg_list[y + 1] != NULL)
             {
               fdIn = open(arg_list[y + 1], O_RDONLY);
               dup2(fdIn, STDIN_FILENO);
             }
             else
             {
               printf("No input file specified");
             }
             arg_list[y] = 0;
           }
           else if(!strcmp(arg_list[y], ">"))
           {
             if(arg_list[y + 1] != NULL)
             {
 fdOut = open(arg_list[y+1], O_CREAT | O_WRONLY | O_TRUNC, 0666);
               dup2(fdOut, STDOUT_FILENO);
             }
             else
             {
               printf("No output file specified");
             }
             arg_list[y] = 0;
           }
           else if(!strcmp(arg_list[y], "|"))
           {
             //is the code in this "if else" correct?
				 //or should something else be done when the
				 //pipe character is seen?
				 if(arg_list[y + 1] != NULL)
             {
               close(fd[1]);

               if(fd[0] != STDIN_FILENO)
               {
                 if(dup2(fd[0], STDIN_FILENO) == -1)
                 {
                   perror("dup2");
                   exit(1);
                 }
                 close(fd[0]);
               }

             }
             arg_list[y] = 0;
           }

           y++;
         }

        execvp(arg_list[0], arg_list);
        break;
      }

      default:
      waitpid(-1, &status, 0);
      close(fd[0]);

      if(fd[1] != STDOUT_FILENO)
      {
        if(dup2(fd[1], STDOUT_FILENO) == -1)
        {
          perror("dup2");
          exit(2);
        }
        close(fd[1]);
      }

      break;
    }
  }
  
  return 0;
}

If somebody could help with this, it would be greatly appreciated.

[Lakshmipathi]

Just a thought , W. Richard Stevens Unix book (don't remember the exact book name) has complete code for shell implementation . That might be helpful.