Read input from another tty

Shell Programming and Scripting
1

Hello,

I am trying to find out how I can find the tty number of a terminal a bash script has just created.

Ultimately what I am trying to achieve is a script that starts a new terminal where stderr can be directed to.

ls -l missing_file 2> /dev/pts/X

To find the tty number of the new terminal I have

#!/bin/bash
tty_current=`tty`
echo $tty_current
xterm -e "tty > $tty_current" &

This will output the tty number of both the "parent and child" terminals.

My problem is how can I read in the output from the new terminal?

Are there any other methods people know of to achieve the same thing?

Regards,
Colin

2

You haven't said which OS, etc., you are on, so you might have to adapt the following a bit:

In principle every device in Unix/Linux has a "device file", which resides in /dev . In the case of a terminal this is most probably /dev/ttyX , where "X" is some number and the device file is a character device. For instance, from my Ubuntu system:

# ls -l /dev/tty*
crw-rw-rw- 1 root tty     5,  0 Aug 14 08:35 /dev/tty
crw--w---- 1 root root    4,  0 Aug 14 08:35 /dev/tty0
crw------- 1 root root    4,  1 Aug 14 08:36 /dev/tty1
crw--w---- 1 root tty     4, 10 Aug 14 08:35 /dev/tty10
crw--w---- 1 root tty     4, 11 Aug 14 08:35 /dev/tty11
[...etc. ...]

Reading from such a device will give you whatever a user sitting at this terminal types, writing to such a device will produce the output at that screen. So, after making sure you are allowed to write to and read from the terminals device file you could probably direct your <stderr> output to it by

/some/process 2>/dev/ttyX

I hope this helps.

bakunin

3

Hello bakunin,

Thanks for your reply. The main issue I am having is reading in output from a terminal to a variable in the bash script. I know the following won't work but hopefully it can illustrate what I am trying to achieve.

 new_term_num=`xterm -e "tty > /dev/pts/0`

Regards,
Colin

4

First off, I'd like to warn you: using backticks is a very, very, bad idea: they are outdated, they have a lot of drawbacks, they are only supported for backwards compatibility purposes (with some scripts from the seventies) and you should forget that they exist. Instead of

`command`

use

$(command)

which does the same, only much better.

terminals don't have output - only processes have output. If you start a process you can redirect its output to <stdout> and/or <stderr> (to your terminal, a file or the device another file descriptor points to). There is no way to redirect the output of a process you don't have started.

If you want to start an xterm and display the output of some process there, you have to:

  1. create the terminal window
  2. find out the device file attached to it
  3. start your process and redirect its output to the device file in question.

I hope this helps.

bakunin