Linux Serial Console

To use a serial port as console you need to compile the support into your kernel - by default it is not compiled in. For PC style serial ports it's the config option next to menu option:

Character devices ‣ Serial drivers ‣ 8250/16550 and compatible serial support ‣ Console on 8250/16550 and compatible serial port

You must compile serial support into the kernel and not as a module.

It is possible to specify multiple devices for console output. You can define a new kernel command line option to select which device(s) to use for console output.

The format of this option is:

console=device,options

device:         tty0 for the foreground virtual console
                ttyX for any other virtual console
                ttySx for a serial port
                lp0 for the first parallel port
                ttyUSB0 for the first USB serial device

options:        depend on the driver. For the serial port this
                defines the baudrate/parity/bits/flow control of
                the port, in the format BBBBPNF, where BBBB is the
                speed, P is parity (n/o/e), N is number of bits,
                and F is flow control ('r' for RTS). Default is
                9600n8. The maximum baudrate is 115200.

You can specify multiple console= options on the kernel command line.

The behavior is well defined when each device type is mentioned only once. In this case, the output will appear on all requested consoles. And the last device will be used when you open /dev/console. So, for example:

console=ttyS1,9600 console=tty0

defines that opening /dev/console will get you the current foreground virtual console, and kernel messages will appear on both the VGA console and the 2nd serial port (ttyS1 or COM2) at 9600 baud.

The behavior is more complicated when the same device type is defined more times. In this case, there are the following two rules:

1. The output will appear only on the first device of each defined type.

2. /dev/console will be associated with the first registered device. Where the registration order depends on how kernel initializes various subsystems.

   This rule is used also when the last console= parameter is not used for other reasons. For example, because of a typo or because the hardware is not available.

The result might be surprising. For example, the following two command lines have the same result:

console=ttyS1,9600 console=tty0 console=tty1
console=tty0 console=ttyS1,9600 console=tty1

The kernel messages are printed only on tty0 and ttyS1. And /dev/console gets associated with tty0. It is because kernel tries to register graphical consoles before serial ones. It does it because of the default behavior when no console device is specified, see below.

Note that the last console=tty1 parameter still makes a difference. The kernel command line is used also by systemd. It would use the last defined tty1 as the login console.

If no console device is specified, the first device found capable of acting as a system console will be used. At this time, the system first looks for a VGA card and then for a serial port. So if you don't have a VGA card in your system the first serial port will automatically become the console.

You will need to create a new device to use /dev/console. The official /dev/console is now character device 5,1.

(You can also use a network device as a console. See Documentation/networking/netconsole.rst for information on that.)

Here's an example that will use /dev/ttyS1 (COM2) as the console. Replace the sample values as needed.

1. Create /dev/console (real console) and /dev/tty0 (master virtual console):

   cd /dev
   rm -f console tty0
   mknod -m 622 console c 5 1
   mknod -m 622 tty0 c 4 0
   

2. LILO can also take input from a serial device. This is a very useful option. To tell LILO to use the serial port: In lilo.conf (global section):

   serial  = 1,9600n8 (ttyS1, 9600 bd, no parity, 8 bits)
   

3. Adjust to kernel flags for the new kernel, again in lilo.conf (kernel section):

   append = "console=ttyS1,9600"
   

4. Make sure a getty runs on the serial port so that you can login to it once the system is done booting. This is done by adding a line like this to /etc/inittab (exact syntax depends on your getty):

   S1:23:respawn:/sbin/getty -L ttyS1 9600 vt100
   

5. Init and /etc/ioctl.save

   Sysvinit remembers its stty settings in a file in /etc, called /etc/ioctl.save. REMOVE THIS FILE before using the serial console for the first time, because otherwise init will probably set the baudrate to 38400 (baudrate of the virtual console).

6. /dev/console and X Programs that want to do something with the virtual console usually open /dev/console. If you have created the new /dev/console device, and your console is NOT the virtual console some programs will fail. Those are programs that want to access the VT interface, and use /dev/console instead of /dev/tty0. Some of those programs are:

   Xfree86, svgalib, gpm, SVGATextMode
   

   It should be fixed in modern versions of these programs though.

   Note that if you boot without a console= option (or with console=/dev/tty0), /dev/console is the same as /dev/tty0. In that case everything will still work.

7. Thanks

   Thanks to Geert Uytterhoeven <geert@linux-m68k.org> for porting the patches from 2.1.4x to 2.1.6x for taking care of the integration of these patches into m68k, ppc and alpha.

Miquel van Smoorenburg <miquels@cistron.nl>, 11-Jun-2000