xv6

Hide the details of what we are connected to: could be a file, a I/O device or a pipe.

With the two separated, shell has a chance to redict I/O without disturbing the main shell.

Example:

char *argv[2];

argv[0] = "cat";
argv[1] = 0;
if (fork() == 0) { // child process
  close(0);
  open("input.txt", O_RDONLY);
  exec("cat", argv);
}

dup duplicates an existing file descriptor and shares the offset with the original fd.

Note that if two fd do not share offsets if they are not created with dup , even if we are using open calls to the same file.