How to start agetty on a serial port that is not the primary console

Solution Unverified - Updated 15 Apr 2020

Environment

Red Hat Enterprise Linux 6

Issue

For RHEL6, upstart (the init process) starts an agetty process automatically if the primary console is the serial port (recall that the last console= parameter is the primary).

If we want to start agetty on a serial port that is not the primary console, How should we do?

Resolution

create a new /etc/init/serial-ttyS1.conf file with the following contents:

```
start on stopped rc RUNLEVEL=[2345]
stop on runlevel [S016]

respawn
exec /sbin/agetty /dev/ttyS1 115200 vt100-nav
```

Then start the new agetty by rebooting or by running
```
[root@localhost ~]# initctl start serial-ttyS1
```

Note: if the keyboard is not working correctly on the new agetty, you may need to remove console=ttyS1 from the kernel parameters.

Some background information:

As the system boots, it starts udev to handle hardware devices. The /lib/udev/rules.d/10-console.rules file tells udev to check a number of devices, including /dev/console, and run the /lib/udev/console_check helper program against each device:

        # Console initialization - keyboard, font, etc.
        KERNEL=="tty0",         RUN+="/lib/udev/console_init %k"

        # Check and set up serial and serial-like consoles if necessary
        KERNEL=="console",      RUN+="/lib/udev/console_check %k"
        KERNEL=="ttySG*",       RUN+="/lib/udev/console_check %k"
        KERNEL=="xvc*",         RUN+="/lib/udev/console_check %k"
        KERNEL=="hvsi*",        RUN+="/lib/udev/console_check %k"
        KERNEL=="hvc*",         RUN+="/lib/udev/console_check %k"

If console_check determines that /dev/console is a serial console (by running some ioctl() system calls on it), then it runs this command:
```
/sbin/initctl emit --no-wait fedora.serial-console-available DEV=ttyS0 SPEED=115200
```

This command sends a fedora.serial-console-available event to the init daemon. The init daemon is configured for this event by /etc/init/serial.conf:

        start on fedora.serial-console-available DEV=* and stopped rc RUNLEVEL=[2345]
        stop on runlevel [S016]

        instance $DEV
        respawn
        pre-start exec /sbin/securetty $DEV
        exec /sbin/agetty /dev/$DEV $SPEED vt100-nav

The init daemon launches agetty by running the command from the exec line in serial.conf:
```
/sbin/agetty /dev/ttyS1 115200 vt100-nav
```
And this shows the login prompt on the serial console.

To check the status of the serial console, use the initctl command to query the init daemon's jobs:
```
  [root@localhost ~]# initctl list | grep serial
  serial (ttyS0) start/running, process 1254
```

Or:

  [root@localhost ~]# initctl status serial DEV=ttyS0
  serial (ttyS0) start/running, process 1254

Note the DEV=ttyS1 parameter above. Without this parameter, initctl returns a not-very-clear error message:
```
  [root@localhost ~]# initctl status serial
  initctl: Unknown parameter: DEV
```
The DEV=ttyS1 parameter is necessary because of the job definition in /etc/init/serial.conf as shown above.

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