How to use the command 'dmesg' (with examples)

The dmesg command is a diagnostic tool that prints messages from the kernel’s message buffer. This buffer includes messages about hardware components, driver initialization, system errors, and other system operations logged during the boot process and system runtime. It is an essential tool for system administrators and developers to monitor and troubleshoot system hardware and kernel operation issues.

Use case 1: Show kernel messages

Code:

dmesg

Motivation:
The primary use of the dmesg command is to display kernel messages, which are critical for understanding how your system is interacting with its hardware and software components. This information can be invaluable when diagnosing a poorly functioning piece of hardware, or understanding how hardware drivers interact during the boot process. The kernel messages include various system warnings, errors, and other informational messages that can provide a comprehensive view of system behavior.

Explanation:

• dmesg: By using the command dmesg without any additional arguments, you retrieve all kernel messages available in the buffer. This buffer accumulates messages, representing a timeline of interactions between the operating system and the hardware components. Whether you are dealing with hardware initialization or runtime issues, this serves as a centralized log for understanding how underlying processes are handled by the kernel.

Example output:

[    0.000000] Linux version 5.4.0-72-generic (buildd@lcy01-amd64-025) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04))
[    0.000000] Command line: BOOT_IMAGE=/vmlinuz-5.4.0-72-generic root=/dev/mapper/vgubuntu-root ro quiet splash
[    0.100000] PCI: Using configuration type 1 for base access
[    0.200000] ACPI: Added _OSI(Module Device)
[    1.500000] x86/MCE: CPU supports 6 MCE banks
[    2.000000] EDAC MC0: Giving out device to module i7core_edac controller i7core_mc0: DEV 0000:ff:00.0

Use case 2: Show how much physical memory is available on this system

Code:

dmesg | grep -i memory

Motivation:
Understanding the physical memory setup of your system is vital, particularly for performance optimization and problem diagnosis. This command is used when you need to quickly check the available physical memory as recognized by the kernel. This could be particularly important when dealing with memory issues, optimizing your server, or configuring virtual machines where memory allocation is critical.

Explanation:

• dmesg: Extracts and displays the kernel log messages.
• |: The pipe symbol is used to filter the output of dmesg by passing it to another command. It allows for refining large sets of data into more specific results that the user is interested in.
• grep: This command searches for specified patterns within text. It outputs lines from the input data that match the supplied pattern.
• -i: This flag with grep tells the command to perform a case-insensitive search, ensuring it captures all variations of the word “memory”, regardless of capitalization.

Example output:

[    0.000000] Memory: 16338428K/16711680K available (14339K kernel code, 2300K rwdata, 4296K rodata, 2732K init, 5608K bss, 373252K reserved, 0K cma-reserved)
[    0.000000] PM: Registered nosave memory: [mem 0x0009d800-0x000dffff]
[    0.000000] total_memory: 16245656K

Use case 3: Show kernel messages 1 page at a time

Code:

dmesg | less

Motivation:
Kernel logs can be lengthy and complex, making it difficult to analyze all the information efficiently. The rationale behind this command is to allow users to go through the kernel logs at their own pace, making it easier to identify and scrutinize specific details that might be relevant to troubleshooting or system analysis. Viewing these logs one page at a time alleviates the risk of overloading the user with data and can be more user-friendly, especially when dealing with error logs during boot processes or hardware malfunction inspections.

Explanation:

• dmesg: Outputs the current kernel log messages.
• |: This operator pipes the output of dmesg into the subsequent command, allowing for an organized and manageable review process.
• less: A program used to view output one screen at a time. It permits scrolling back and forth through the text, unlike some pagers that only go forward, thus providing a more interactive experience for large outputs.

Example output:
To demonstrate this process, the output appears in sections, allowing users to navigate using keyboard controls such as arrow keys or spacebar to progress through each page. This creates a scrollable environment that can be exited by pressing q.

Conclusion:

The dmesg command is a powerful tool that aids system administrators and developers in troubleshooting and maintaining system stability by accessing valuable logs provided by the kernel. Whether you’re looking to analyze full kernel messages, inspect physical memory specifics, or sift through logs line-by-line, dmesg provides the functionality to handle these tasks efficiently. Understanding how to harness this command allows users to gain deeper insights into their systems’ hardware and software interactions, facilitating better maintenance, optimization, and problem-resolution capabilities.