optimization: do not repeatedly use dup()

[Ralf Wildenhues]

Why does the shell snippet
  exec 5>file
  echo foo >&5
  echo bar >&5

cause repeated dup()ing and close()ing of the file descriptor instead of
a simple write() to the already open file descriptor, as pointed out in
[1]?

Looks like a simple optimization to avoid 2 of 3 syscalls (disclaimer: I
have not looked at the bash source code at all).

Regards,
Ralf

[1] http://lists.gnu.org/archive/html/bug-libtool/2005-05/msg00150.html