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

int run_command(char** arg_list, int rd, int wd) {
  pid_t child_pid;

  /* Duplicate this process. */
  child_pid = fork();

  if (child_pid != 0){
    /* This is the parent process. */
    return child_pid;
  }else {
    if (rd != STDIN_FILENO){
      if(dup2(rd, STDIN_FILENO) != STDIN_FILENO){
    fprintf(stderr, "Error: failed to redirect standard input\n");
        return -1;
      }
    }

    if (wd != STDOUT_FILENO){
      printf("redirect stdout to %d.", wd);
      if(dup2(wd, STDOUT_FILENO) != STDOUT_FILENO){
        fprintf(stderr, "Error: failed to redirect standard output\n");
        return -1;
      }
    }
    /* Now execute PROGRAM, searching for it in the path.  */
    execvp (arg_list[0], arg_list);
    /* The execvp  function returns only if an error occurs.  */
    fprintf (stderr,  "an error occurred in execvp\n");
    abort ();
  }
}

int main () {
  /*  The argument list to pass to the "ls" command.  */
  char* arg_list1[] = {
    "ls",     /* argv[0], the name of the program.  */
    "-l",
    NULL      /* The argument list must end with a NULL.  */
  };

  char* arg_list2[] = {
    "wc",     /* argv[0], the name of the program.  */
    "-l",
    NULL      /* The argument list must end with a NULL.  */
  };

  char* arg_list3[] = {
    "wc",     /* argv[0], the name of the program.  */
    "-l",
    NULL      /* The argument list must end with a NULL.  */
  };

  int rd;
  int wd;
  int fds[2];
  if (pipe(fds) != 0) {
    fprintf(stderr, "Error: unable to pipe command '%s'\n", arg_list1[0]);
    return -1;
  }
  rd = STDIN_FILENO;
  wd = fds[1];

  // run the first command
  run_command(arg_list1, rd, wd);
  close(wd);

  // run the second command
  rd = fds[0];
  if (pipe(fds) != 0) {
    fprintf(stderr, "Error: unable to pipe command '%s'\n", arg_list2[0]);
    return -1;
  }
  wd = fds[1];

  run_command(arg_list2, rd, wd);
  close(wd);

  // run the third command
  rd = fds[0];
  wd = STDOUT_FILENO;

  run_command(arg_list3, rd, wd);
  close(wd);
  fprintf (stderr, "done with main program\n");
  return 0;
}