Question

I know there are many threads that talk about this problem but I don’t really understand the way it can be done.

I’m trying to make a shell that can execute a linux command sucha as ps | grep | less
I’ve donne the parsing by puting every command and its args in a simply linked list.

Here’s my implementation that doesn’t work. Hope that’s clear enough.

if ((son = fork()) < 0)
  return printerr_sys("Unable to fork", 0);
if (son == 0)
  {
    if (first > 1 && data->format[first - 1] &&
    is_directing_elt(data->format[first - 1]) == DIRECT_TPIPE)
       dup2(tube_p[0], STDIN_FILENO);
       first = make_argv(data, first, &argv);
    if (next)
     {
      dup2(tube_v[1], STDOUT_FILENO);
      close(tube_v[0]);
     }
    if (execvp(argv[0], argv) < 0)
      return printerr_cmd(argv[0], 1);
  }
else
  {
    if (next)
        {
           close(tube_v[1]);
           cmdline_executer(data, next, tube_v);
        }
    waitpid(son, &(data->lastcmd), WUNTRACED);
    data->lastcmd = WEXITSTATUS(data->lastcmd);
  }
return TRUE;

My questions are:

• What would be the correct implementation?
• Is it possible to do it with recursion?
• Do I need to fork from right to left or left to right (logically it give the same result)?

Answer 1

Processes run independently, so you need to set up the pipe for at least the first pair of commands before you fork, but you’re doing that in the child (son == 0). You could code a recursive solution that, as long as there are at least two commands left, creates a pipe, then forks, then runs the first command.