Bidirectional Serial Console Setup.md

Bidirectional Serial Console Setup

Configure a serial port (e.g., /dev/ttyS0) to allow both incoming getty login and outbound terminal connections without conflicts.

Prerequisites

apt install util-linux procps

Method 1: agetty with wait-cr (Simplest)

systemd

Create /etc/systemd/system/[email protected]/bidirectional.conf:

[Service]
ExecStart=
ExecStart=-/sbin/agetty --noclear --keep-baud --wait-cr --nohangup --noissue 115200,38400,9600 %I xterm-256color

Enable:

systemctl daemon-reload
systemctl enable [email protected]
systemctl start [email protected]

SysV init

Add to /etc/inittab:

T0:23:respawn:/sbin/agetty -L --wait-cr --nohangup --noissue ttyS0 115200 xterm-256color

Reload:

init q

Method 2: mgetty (Feature-rich)

Install

apt install mgetty mgetty-fax

Configure

Edit /etc/mgetty+sendfax/mgetty.config:

port ttyS0
  speed 115200
  direct y
  fax n
  data y
  blocking n
  port-owner uucp
  port-group dialout
  port-mode 0660
  term xterm-256color

Add to /etc/inittab:

T0:23:respawn:/usr/sbin/mgetty ttyS0

Reload:

init q

Method 3: Custom Script (Full Control)

Create /usr/local/bin/bidir-getty:

#!/bin/bash
TTY=$1
stty -F /dev/$TTY -echo -icanon min 0 time 0
while :;do
  if fuser /dev/$TTY &>/dev/null;then sleep 1;continue;fi
  read -rsn1 -t86400 c </dev/$TTY || continue
  TERM=xterm-256color flock -x -n /var/lock/LCK..$TTY /sbin/agetty --login-options "$c" $TTY 115200 xterm-256color
done

Make executable:

chmod +x /usr/local/bin/bidir-getty

systemd service

Create /etc/systemd/system/[email protected]:

[Unit]
Description=Bidirectional Getty on %I
After=systemd-user-sessions.service

[Service]
ExecStart=/usr/local/bin/bidir-getty %I
Restart=always
StandardInput=tty
StandardOutput=tty

[Install]
WantedBy=multi-user.target

Enable:

systemctl daemon-reload
systemctl enable [email protected]
systemctl start [email protected]

SysV init

Add to /etc/inittab:

T0:23:respawn:/usr/local/bin/bidir-getty ttyS0

Reload:

init q

Testing

Outbound connection

screen /dev/ttyS0 115200
# or
minicom -D /dev/ttyS0

Inbound connection

Connect from another device and press Enter to trigger login prompt.

How It Works

• --wait-cr: Getty waits silently for CR/LF before displaying login prompt
• --nohangup: Doesn't send hangup signal, allows port sharing
• --noissue: Suppresses pre-login banner
• flock: File locking prevents simultaneous access conflicts
• fuser: Checks if port is in use by another process

When outbound program (screen/minicom) opens the port, getty detects it's locked and waits. When released, getty resumes listening for incoming connections.