Prevent the console from clearing the screen?
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/dmesg and /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 tty1
to
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/[email protected]
cat >/etc/systemd/system/[email protected]/noclear.conf <<EOF
[Service]
TTYVTDisallocate=no
EOF
systemctl daemon-reload
Verify the result with systemctl cat [email protected]
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.
First, add 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 tty1.conf, tty2.conf, tty3.conf, tty4.conf, tty5.conf, tty6.conf or console.conf. I edited them all adding --noclear option to the getty command. For example, editing tty1.conf:
#> sudo vi /etc/init/tty1.conf
you'll have to replace:
respawn
exec /sbin/getty -8 38400 tty1
with:
respawn
exec /sbin/getty -8 38400 --noclear tty1
That's all, now your system should boot in a single tty without clearing it.