Question

I asked about pipes in a previous question, got that working perfectly. However I had some questions about output redirection like >> in a shell normally does. There doesn’t seem to be a whole lot of info on the Internet about it. Here is what I have so far. Is there a better/easier way to do this, it’s messy and im not even super sure that I understand it. Part of it was psuedocode given in notes, i kinda filled them in, but even im not super sure.

void do_redirect(char** cmd, char** file) {
  int fds[2]; 
  int count; 
  int fd;     
  char i;    
  pid_t pid;  
  pipe(fds);
  //File Descriptors/pipe and redirecting char variables (i)
  //fd is used with the open command, basically stores the 

  //Child 1
  if (fork() == 0) {
    //Open the file with read/write commands, 0_CREAT creates the file if it does not exist
    fd = open(file[0], O_RDWR | O_CREAT, 0777);
    dup2(fds[0], 0);

    //Close STDOUT
    close(fds[1]);

    //Read from STDOUT
    while ((count = read(0, &i, 1)) > 0)
    write(fd, &i, 1); // Write to file.

    exit(0);

  //Child 2
  } else if ((pid = fork()) == 0) {
    dup2(fds[1], 1);

    //Close STDIN
    close(fds[0]);

    //Output contents to the given file.
    execvp(cmd[0], cmd);
    perror("execvp failed");

  Parent
  } else {
    waitpid(pid, NULL, 0);
    close(fds[0]);
    close(fds[1]);
  }
}

Answer 1

You’ll need O_APPEND if it’s >> you are looking for.