.. _serial_console: Linux Serial Console==================== To use a serial port as console you need to compile the support into yourkernel - by default it is not compiled in. For PC style serial portsit's the config option next to menu option: :menuselection:`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 candefine a new kernel command line option to select which device(s) touse 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. Andthe 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 foregroundvirtual console, and kernel messages will appear on both the VGAconsole and the 2nd serial port (ttyS1 or COM2) at 9600 baud. The behavior is more complicated when the same device type is defined moretimes. 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 commandlines 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 kerneltries to register graphical consoles before serial ones. It does itbecause 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 lastdefined ``tty1`` as the login console. If no console device is specified, the first device found capable ofacting as a system console will be used. At this time, the systemfirst looks for a VGA card and then for a serial port. So if you don'thave a VGA card in your system the first serial port will automaticallybecome the console, unless the kernel is configured with theCONFIG_NULL_TTY_DEFAULT_CONSOLE option, then it will default to using thettynull device. 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