Remove data from the STDIN stream buffer of another TTY session

Question

I encountered a problem the other day while running multiple tty sessions in separate terminal windows. I was running a expensive disk access command and was waiting for it to return. Meanwhile I was working in another tty session in a separate window in my graphical desktop environment.

Inadvertently I focused the terminal that was running the disk access command (basically a cp command) and typed the following into the terminal thinking I was going to enter it into my other window:

↑Enter

Based on this question I knew that the data was now sitting in the STDIN data stream buffer waiting to be run immediately upon the return of the first cp command. And as you know I just told the command to run again :/ .

The Question

Since I had some time (this command was taking > 30 mins to return), is there a way I could have flushed these characters out of the STDIN buffer of that tty before bash reran the process it was finishing?

Or is there another way, other than trying to flush data out of the file stream buffer of another bash instance, that I could avoid having the data evaluated as soon as the process that I was waiting on finished?

Answer 1

If your tty has the noflsh flag turned off - you can check with

stty -a < /dev/pts/whatever | grep -e -noflsh

from another tty, but the default is that it's turned off - then typing the interrupt, quit, or suspend character will flush the input queue. So if you type Ctrl-Z, then fg, it will suspend the currently running command, flush the input, then resume the command.

Answer 2

Probably you should send SIGSTOP to bash in order to prevent it from doing anything, and after finishing your task send SIGKILL to bash.

Note that SIGSTOP prevents bash from waiting for your task to be finished, and you'll get zombie. After SIGKILL init will reparent your task, and zombie will disappear.

Answer 3

There's an easier solution that the two proposed ones here, not involving signals.

When you've typed that data and pressed Enter, it's there in the tty line's buffers, waiting for anyone to read it.

The shell, which is running your cp-like command, will read that data in about 30 minutes. Unless someone beats it.

You don't need "special" operations to flush the queue.

Just simply create another reader and read that data from the queue. Be the one who reads it before that shell gets to.

Figure out the tty line's device name, and then cat /dev/pts/xx or cat < /dev/pts/xx will do. Then interrupt (Ctrl+C) that cat. Or, of course, you can do head -1 < /dev/pts/xx to read exactly one line.