Can I duplicate an input file descriptor and use it for writing data?

Question

The below command makes a copy of the input file descriptor and use the duplicate file descriptor for writing data from the echo command on to the terminal.

sh-4.2$ exec 6<&0
sh-4.2$ echo "hello" >&6
hello

Does that mean we can write to the terminal using the input file descriptor?

What operating system, and which terminal emulator, are you using? It's not documented, but the software on most Unix-like systems open their terminal devices with stdin, stdout, and stderr all duped from the same file descriptor, which was opened for both reading and writing. — Mark Plotnick, Dec 17 '16 at 12:24
Suggested reading: How do keyboard input and text output work?, what is meant by connecting STDOUT and STDIN?, How can a command have more than one output? — Gilles 'SO- stop being evil', Dec 18 '16 at 22:58

score 3 · Accepted Answer · answered Dec 17 '16 at 12:13

3

Does that mean we can write to the terminal using the input file descriptor?

Sure. You can write to a terminal (indeed to any file or pipe or device or socket that supports and authorizes writing) using whatever open file descriptor you have for it. A simpler version of your code would be this:

echo hello >&0

which, as you'd expect, sends "hello\n" to whatever file descriptor 0 points to. If that's your terminal, so be it.

answered Dec 17 '16 at 12:13

Celada

44,132

but the file descriptor '0' is associated with stdin which is the keyboard, so how is it possible to write to the terminal using stdin. – Pankaj Pandey Dec 17 '16 at 12:15
1

"the file descriptor '0' is associated with stdin which is the keyboard": false. File descriptor 0 points to your terminal, not to your keyboard. The terminal is a device that can be both read and written. – Celada Dec 17 '16 at 12:18
thnks for the explanation.. @Celeda – Pankaj Pandey Dec 17 '16 at 12:24
@Celeda , when you google about file descriptor , it describes file descriptor 0 i.e. stdin connected to the keyboard(default input device), so when we redirect the output to stdin how does it gets printed on the console(monitor) – Pankaj Pandey Dec 17 '16 at 13:39
@PankajPandey, the Google search result you found is not correct. The stdio file descriptors 0, 1, and 2, are typically connected (by default) to the terminal, not to a keyboard (for input) or video card (for output) or anything else of the sort. A "terminal" is a device which is historically a serial port connected to a (real, physical) terminal (perhaps through a modem), but nowadays is much more commonly an abstraction, connected to, say, a terminal application that displays itself as a window in a GUI system or connected to the server-end of an SSH connection. – Celada Dec 18 '16 at 14:04

score 1 · Answer 2 · edited May 23 '17 at 12:39

This is a copy of my answer to a similar question on stackoverflow last year.

You can write to your terminal device's standard input due to historical custom. Here's what's happening:

When a user logs into a terminal on a Unix-like system, or opens a terminal window under X11, file descriptors 0, 1, and 2 are connected to a terminal device, and each of them is opened for both reading and writing. This is the case despite the fact that one normally only reads from fd 0 and writes to fd 1 and 2.

Here is the code from 7th edition init.c:

open(tty, 2);
dup(0);
dup(0);
...
execl(getty, minus, tty, (char *)0);

And here is how ssh does it:

ioctl(*ttyfd, TCSETCTTY, NULL);
fd = open("/dev/tty", O_RDWR);
if (fd < 0)
    error("%.100s: %.100s", tty, strerror(errno));
close(*ttyfd);
*ttyfd = fd;
...
/* Redirect stdin/stdout/stderr from the pseudo tty. */
if (dup2(ttyfd, 0) < 0) 
    error("dup2 stdin: %s", strerror(errno));
if (dup2(ttyfd, 1) < 0) 
    error("dup2 stdout: %s", strerror(errno));
if (dup2(ttyfd, 2) < 0) 
    error("dup2 stderr: %s", strerror(errno));

(The dup2 function dups arg1 into arg2, closing arg2 first if necessary.)

And here is how xterm does it:

if ((ttyfd = open(ttydev, O_RDWR)) >= 0) {
    /* make /dev/tty work */
    ioctl(ttyfd, TCSETCTTY, 0);
...
/* this is the time to go and set up stdin, out, and err
 */
{
/* dup the tty */
for (i = 0; i <= 2; i++)
    if (i != ttyfd) {
    IGNORE_RC(close(i));
    IGNORE_RC(dup(ttyfd));
    }
/* and close the tty */
if (ttyfd > 2)
    close_fd(ttyfd);

Can I duplicate an input file descriptor and use it for writing data?

2 Answers2