Redirect the standard output

Question

Hello and thank you for attention. I am writing my own shell in c and i have problem with redirect the standard output. For example, I get command: ls -l >> file and I need to display output to file. Can you give me some ideas to resolve that problem? Thanks.

have you heard of [dup2](http://man7.org/linux/man-pages/man2/dup.2.html) — Seek Addo, Apr 14 '17 at 20:12
Possible duplicate of [Rerouting stdin and stdout from C](http://stackoverflow.com/questions/584868/rerouting-stdin-and-stdout-from-c) — Support Ukraine, Apr 14 '17 at 20:13
Yes, I have heard about it but I don t know how it exactly work. I read about it but i don t know how to use it — ByQ, Apr 14 '17 at 20:17
@ByQ The man page explain it all and provides some example work. What do you mean you don't know how to use it. — Seek Addo, Apr 14 '17 at 20:52

score 0 · Accepted Answer · answered Apr 14 '17 at 20:24

You may want to use dup() & dup2(), here are two functions I have ready:

void stdout_redirect(int fd, int *stdout_bak)
{
  if (fd <= 0)
    return;
  *stdout_bak = dup(fileno(stdout));
  dup2(fd, fileno(stdout));
  close(fd);
}

void stdout_restore(int stdout_bak)
{
  if (stdout_bak <= 0)
    return;
  dup2(stdout_bak, fileno(stdout));
  close(stdout_bak);
}

Here is how to use it:

int main(void)
{
  int stdout_bak;
  FILE *fd;

  fd = fopen("tmp", "w");
  stdout_redirect(fileno(fd), &stdout_bak);
  /* Your Stuff Here */
  stdout_restore(&stdout_bak);
  return 0;
}

Redirect the standard output

1 Answers1