How do I host a shell for a serial terminal?

Question

I am building a dumb terminal with an arduino and I can now see the output of serial connections. How do I get a login/shell? I have tried screen /dev/ttyACM0 and that lets me type on the host computer and see the results on the terminal screen, but no shell. Using screen's :exec /bin/sh command starts a shell on that controlling xterm window but no longer outputs to the serial line. various attempts with agetty -L /dev/ttyACM0 9600 just sit and hang for a minute then exits without ever having put anything on the screen. I have tried cu -l /dev/ttyACM0 -s 9600 and again it hangs with no screen interaction. I have connected minicom to echo keys onto the arduino similar to screen but could not figure out how to get a shell. To be clear I am trying to access my main computer via the arduino based terminal, and I can not find a way to make my main computer offer a shell session to the arduino over a USB serial connection.

Answer 1

agetty seems to be fairly quiet about problems. I tried:

$ strace -e open agetty -I 'hello\012' -L /dev/ttyS0 9600

and it opened /dev//dev/ttyS0, which failed of course. It then hung around a few seconds and stopped.

The man page does say it wants a relative filename, so I then tried:

$ strace -e open agetty -I 'hello\012' -L ttyS0 9600

and it opened the right device this time, but failed again:

open("/dev/ttyS0", O_RDWR|O_NOCTTY|O_NONBLOCK) = -1 EACCES (Permission denied)

Finally, this worked and wrote something on the tty:

$ sudo strace -e open agetty -I 'hello\012' -L ttyS0 9600

You should also add the setsid mentioned in the comments. You can also use systemd (if you have it) to start the login with:

$ sudo systemctl start serial-getty@ttyACM0

This runs agetty with --keep-baud 115200,38400,9600. If you systemctl enable the service, it might start automatically whenever device ttyACM0 is created.