• Function in the Linux Kernel is the function called printk(). Similar to C’s printf().
    • The message is then sent to a RAM buffer and the system console.
    • Important messages are displayed on the system console.
    • Logging daemon may send to a file as well.
  • Kernel is written in C with a little bit of Assembly.
  • Command dmesg shows the RAM buffer messages from the Kernel.
    • Shows only messages since the system has booted.
  • dmesg is only a finite size and older messages are removed.
  • systemd journals all messages in /var/log/messages
  • Can also check Kernel messages with journalctl -t kernel. See messages that come directly from the Kernel and not those being stored in log files on the disk.
  • Can also do journalctl -t kernel -f and it will follow any new messages that are printed by the Kernel.
  • Example dmesg check is dmesg | grep command
  • Will show: 
    [    0.035138] Kernel command line: BOOT_IMAGE=/boot/vmlinuz-6.5.0-25-generic root=UUID=ca2eb2d6-2fc2-4c9e-8001-11190efd85ee ro quiet splash radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1 amdgpu.dc=1 vt.handoff=7
[    0.035300] Unknown kernel command line parameters "splash BOOT_IMAGE=/boot/vmlinuz-6.5.0-25-generic", will be passed to user space.
  • Shows the path name to the Kernel file itself.
  • root=UUID identifies the disk and partition for the root filesystem.
  • These are passed to the Kernel from the bootloader.
  • Bootloader can pass multiple parameters to the Kernel.
    • ro read-only. When the Kernel boots, it will mount the filesystem read-only to begin with. Then is checked for consistency. If you don’t have a corrupted disk or anything like that, it will mount it rw. We don’t want things to change, whilst checking the Kernel, hence ro.
  • For Journal Control (journalctl). The journalctl command does its own paging (no need for less)
  • Running journalctl -t kernel | grep command can show the amount of times the machine has booted: 
     3月 18 08:58:44 explosion kernel: Kernel command line: BOOT_IMAGE=/boot/vmlinuz-6.5.0-25-generic root=UUID=ca2eb2d6-2fc2-4c9e-8001-11190efd85ee ro quiet splash radeon.cik_support=0 radeon.si_support=0 amdgpu.cik_support=1 amdgpu.si_support=1 amdgpu.dc=1 vt.handoff=7
 3月 18 08:58:44 explosion kernel: Unknown kernel command line parameters "splash BOOT_IMAGE=/boot/vmlinuz-6.5.0-25-generic", will be passed to user space.
  • Can check the journal command with in real-time with: 
    journalctl -t kernel -f

Updated: