Cheat Sheet #day30 - dmesg

dmesg Cheatsheet

Basic Usage

• Display the kernel ring buffer messages:

    dmesg
  

Filtering Output

• Filter messages using grep:

    dmesg | grep <pattern>
  

• Show messages with timestamps:

    dmesg -T
  

Controlling Output

• Clear the ring buffer (requires root):

    sudo dmesg -C
  

• Limit the number of lines displayed:

    dmesg | tail -n <number_of_lines>
  

• Show messages in real-time:

    dmesg -w
  

Specific Message Types

• Show only kernel messages:

    dmesg -k
  

• Show only userspace messages:

    dmesg -u
  

• Show messages by facility (e.g., auth, daemon, kern, etc.):

    dmesg -f <facility>
  

• Show messages by level (e.g., emerg, alert, crit, err, etc.):

    dmesg -l <level>
  

Time and Date Formatting

• Show human-readable timestamps:

    dmesg -H
  

• Show messages with nanosecond timestamps:

    dmesg -L
  

Kernel Log Buffer Size

• Display the size of the kernel log buffer:

    dmesg -s <buffer_size>
  

Advanced Usage

• Decode devicetree boot messages:

    dmesg -D
  

• Display raw message buffer data:

    dmesg -r
  

Redirecting Output

• Save output to a file:

    dmesg > /path/to/file
  

• Append output to a file:

    dmesg >> /path/to/file
  

Example Commands

• Filter for USB-related messages:

    dmesg | grep -i usb
  

• Show only error messages:

    dmesg -l err
  

• Follow new kernel messages in real-time:

    dmesg -wH
  

• Clear the ring buffer and display messages:

    sudo dmesg -C && dmesg
  

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This cheatsheet provides a quick reference to the most commonly used dmesg commands and options. dmesg is a powerful tool for system administrators and developers to troubleshoot and monitor system activity by accessing kernel messages.