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:
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
So, for example:
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:
The output will appear only on the first device of each defined type.
/dev/consolewill 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
/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,
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
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.
/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
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)
Adjust to kernel flags for the new kernel, again in lilo.conf (kernel section):
append = "console=ttyS1,9600"
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
Sysvinit remembers its stty settings in a file in
/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).
/dev/consoleand X Programs that want to do something with the virtual console usually open
/dev/console. If you have created the new
/dev/consoledevice, 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
/dev/consoleis the same as
/dev/tty0. In that case everything will still work.
Thanks to Geert Uytterhoeven <firstname.lastname@example.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 <email@example.com>, 11-Jun-2000