When booting into Linux, there are sometimes one or two lines that get quickly cleared. I think that some of them don't even appear in dmesg. If nothing else, I want to suppress the clear before the "login:" prompt. Is there a kernel command or sysctl that I can set to prevent this so I can read them on the console screen after booting?
Most of the information you want will be in
/var/log/messages after the system boots, you should check those files first.
Generally linux machines run mingetty for the virtual terminals. If you have a traditional sysv init system, those are controlled by
/etc/inittab. You can add the
--noclear option to mingetty to prevent clearing the screen. To do this, edit
/etc/inittab and change this line:
1:2345:respawn:/sbin/mingetty --noclear tty1
then reboot the machine.
Some newer linux distros use init replacements like Upstart (for example, Ubuntu). These generally don't use /etc/inittab and instead use some other config files. Here's a discussion of how calling mingetty works on Ubuntu.
With systemd things are different. See article Stop Clearing My God Damned Console. In short:
mkdir /etc/systemd/system/getty@.service.d cat >/etc/systemd/system/getty@.service.d/noclear.conf <<EOF [Service] TTYVTDisallocate=no EOF systemctl daemon-reload
Verify the result with
systemctl cat firstname.lastname@example.org
If nothing else helped, you can grab your laptop and capture all the kernel log through serial console by adding something like this to kernel parameters:
console=tty0 console=ttyS0,9600n8 console=tty0
This will cause output to apperar on serial console (in your terminal program) and on standard tty. Sometimes SOL (Serial Over LAN) is available.
After hours of googling, I found the solution in this thread and this question. This procedure works for
Ubuntu 12.04.1 LTS as also described here, but it should not differ too much for other distributions.
console=tty1 to your
GRUB_CMDLINE_LINUX (I also suggest to add
noplymouth to inhibit
plymouth and its useless splashscreen).
#> sudo vi /etc/default/grub GRUB_CMDLINE_LINUX="console=tty1 noplymouth"
This forces the kernel log to be printed on
tty1 instead of
tty7 and avoid the
tty switch before the login prompt.
Then just go into
/etc/init and edit one or more of
console.conf. I edited them all adding
--noclear option to the
getty command. For example, editing
#> sudo vi /etc/init/tty1.conf
you'll have to replace:
respawn exec /sbin/getty -8 38400 tty1
respawn exec /sbin/getty -8 38400 --noclear tty1
That's all, now your system should boot in a single
tty without clearing it.
dmesg here goes back to the very beginning of the kernel, starting with
[ 0.000000] Initializing cgroup subsys cpuset
Perhaps this is some bios message or part of your boot loader? Either way, there are so many different things that can blank the screen, without knowing where exactly in the process it happened it's hard to say what to do about it. Is the only thing left on the screen "Login:"? or is there some other boot-up stuff above it? If it's immediately before the login prompt and nothing else is on the screen, then perhaps
/etc/issue has a screen-blanking command in it? Otherwise, you could be using some kind of console frame buffer that is switching the video mode. On my computer, the screen is blanked when a console font is loaded.