Need to implement a pipe, dup2 function to perform "ls | tr a-z A-Z > file.txt"

Question

I have the following C code which does "ls | tr a-z A-Z" but I don't know how to make the output go to file.txt. Do I need to create another pipe or fork thrice? Do I need to use open() function at all since I am saving the output to a file?

int main() {
pid_t child_pid_one, child_pid_two;
int pipe_fd[2]; /* pipe */

/* Before the fork, we need to create the pipe */ 
/* (otherwise no sharing of pipe) */
if (pipe(pipe_fd) < 0) {
  err(EX_OSERR, "pipe error");
}

if ((child_pid_one = fork()) < 0) {
  err(EX_OSERR, "fork error");
}

if (child_pid_one == 0)  { /* CHILD #1's code (ls code) */
  close(pipe_fd[0]); /* we don't need pipe's read end */

  /* Redirecting standard output to pipe write end */
  if (dup2(pipe_fd[1], STDOUT_FILENO) < 0) {
     err(EX_OSERR, "dup2 error");
  }

  execlp("/bin/ls", "/bin/ls", NULL);
  err(EX_OSERR, "exec error");
}  else { /* parent's code */

  /* Creating second child */
  if ((child_pid_two = fork()) < 0) {
     err(EX_OSERR, "fork error");
  } 

  if (child_pid_two == 0)  { /* CHILD #2's code (tr a-z A-Z) */
     close(pipe_fd[1]); /* we don't need pipe's write end */

     /* Redirecting standard input to pipe read end */
     if (dup2(pipe_fd[0], STDIN_FILENO) < 0) {
        err(EX_OSERR, "dup2 error");
     }


     execlp("/usr/bin/tr", "/usr/bin/tr","a-z","A-Z", NULL);
     err(EX_OSERR, "exec error");
  } else {
     /* Parent has no need for the pipe */
     close(pipe_fd[0]);
     close(pipe_fd[1]);

     /* Reaping each children */
     wait(NULL); 
     wait(NULL);
  }
 }

 return 0;
}

Answer 1

Just redirect STDOUT_FILENO of the tr process to a newly opened file:

// write-only (as is stdout), truncate to zero length
int file_fd = open("file.txt", O_WRONLY | O_TRUNC);

if (file_fd < 0) {
    // error handling
}

if (dup2(file_fd, STDOUT_FILENO) < 0) {
    err(EX_OSERR, "dup2 error");
}