How to reset tty after exec-ed program crashes? - linux

Question

I am writing a Ruby wrapper around Docker and nsenter. One of the command my tool provides is to start a Bash shell within a container. Currently, I am doing it like this:
payload = "sudo nsenter --target #{pid(container_name)} --mount --uts --ipc --net --pid -- env #{env} /bin/bash -i -l;"
Kernel.exec(payload)
In Ruby, Kernel#exec relies on the exec(2) syscall, hence there is no fork.
One issue is that the container sometime dies prematurely which effectively kills my newly created Bash prompt. I then get back the prompt originally used to run my Ruby tool, but I cannot see what I am typing anymore, the tty seems broken and running reset effectively solves the issue.
I'd like to conditionally run reset if the program I exec-ed crashes. I found that the following works well:
$ ./myrubytool || reset
Except I'd like to avoid forcing people using my tool to append || reset every time.
I have tried the following:
payload = "(sudo nsenter --target #{pid(container_name)} --mount --uts --ipc --net --pid -- env #{env} /bin/bash -i -l) || reset;"
But this surprisingly puts reset in the background (i.e. I can run reset by entering fg). One benefit is that the tty is working properly, but it's not really ideal.
Would you have any idea to solve this issue?

Answer 1

If terminal echo has been disabled in a terminal, then you can run the command stty echo to re-enable the terminal echo. (Conversely, stty -echo disables terminal echo, and stty -a displays all terminal settings.)
This is safe to run even if terminal echo is already enabled, so if you want to play it safe, you can do something like ./myrubytool ; stty echo which will re-enable terminal echo if it is disabled regardless of the exit status of your Ruby program. You can put this in a shell script if you want to.
It might be that there is a way to execute a command when the Ruby program exits (often referred to as a "trap"), but I'm not familiar enough with Ruby to know whether such capabilities exist.
However, if you are creating a script for general use, you probably should look into more robust techniques and not rely on workarounds.

Answer 2

How about this? It should do exactly what you want.
It runs the command in a separate process, waits on it, and if, when it finishes, the return value is not 0, it runs the command reset.
payload = "sudo nsenter --target #{pid(container_name)} --mount --uts --ipc --net --pid -- env #{env} /bin/bash -i -l;"
fork { Kernel.exec(payload) }
pid, status = Process.wait2
unless status.exitstatus == 0
system("reset")
end
EDIT
If all you want to do is turn echo back on, change the system("reset") line to system("stty echo").