how to change the console tty after booting?

Question

the command dmesg | grep console returns:

Kernel command line: console=ttys6,115200 root=/dev/mmcblk1p2 
rootwait rw
console [ttys6] enabled

I want to change this to ttys3 after booting. Is it possible?

Answer 1

The man page for tty_ioctl lists the ioctl TIOCCONS. When applied to an open file descriptor of a tty it will redirect future output intended for /dev/console to that tty.

You can use this in a simple perl script. Create and chmod +x a file mysetconsole holding the following:

#!/usr/bin/perl
# https://unix.stackexchange.com/a/397790/119298
# see man tty_ioctl for TIOCCONS
# and perldoc IO::Tty::Constant
require "sys/ioctl.ph";
use IO::Tty::Constant qw(TIOCCONS);
ioctl(STDIN,TIOCCONS,0) or die $!;

Assuming you can open the wanted device, use it simply as

sudo ./mysetconsole </dev/ttys3

You will not be able to use it again until you set the console back to /dev/console, with

sudo sh -c './mysetconsole </dev/console'

You may get perl warnings about _FORTIFY_SOURCE which can be ignored. You will need rpm package perl-IO-Tty or debian package libio-pty-perl. If you prefer you can just look for the definition of TIOCCONS in the system include files. I found mine in:

/usr/include/asm-generic/ioctls.h: #define TIOCCONS      0x541D

Your perl script can then just be

#!/usr/bin/perl
# https://unix.stackexchange.com/a/397790/119298
sub TIOCCONS{ return 0x541D; }
ioctl(STDIN,TIOCCONS(),0) or die $!;

Answer 2

Short anwser: use chvt Long anwser: How Linux configures the TTYs at boot depends on distribution. It could be /etc/inittab or it could be /etc/rc.local On systemd systems it's automatic based on configurations in logind.conf