dmesg

Show kernel messages

$ sudo dmesg

$ sudo dmesg [[-l|--level]] err

$ sudo dmesg [[-w|--follow]]

$ sudo dmesg | grep [[-i|--ignore-case]] memory

$ sudo dmesg | less

$ sudo dmesg [[-T|--ctime]]

$ sudo dmesg [[-H|--human]]

$ sudo dmesg [[-L|--color]]

dmesg [options]

-C, --clear
    Clear ring buffer after printing

-c, --read-clear
    Read buffer and clear it

-D, --console-level LEVEL
    Set console logging level

-d, --show-delta
    Show delta timestamps

-E, --dissect
    Dissect structured messages

-e, --reltime
    Show relative timestamps

-F, --file FILE
    Read from file instead of kernel buffer

-H, --human
    Human-readable format

-h, --help
    Display help

-K
    Decode kernel facilities/priorities (alias for -x)

-k, --kernel
    Kernel messages only

-L, --color[=WHEN]
    Colorize output

-l, --level LEVEL_LIST
    Filter by priority levels

-N, --live
    Wait for new messages without blocking

-n, --console-level LEVEL
    Set console log level

-P, --nopager
    Disable pager

-p, --priority PREFIX
    Prefix priority names

-r, --raw-priorities
    Print raw priority values

-S, --since TIME
    Messages since specified time

-s, --buffer-size SIZE
    Set buffer size

-T, --ctime
    Human-readable timestamps

-t, --notime
    Omit timestamps

-u, --userspace
    Userspace messages only

-U, --notruncate
    Don't truncate lines

-V, --version
    Show version

-w, --follow
    Follow new messages (tail -f)

-x, --decode
    Decode facilities and priorities

dmesg is a powerful Linux command used to examine or control the kernel ring buffer, which stores boot-time and runtime kernel messages. These include hardware detection, driver initialization, device status, and error reports. The ring buffer is circular, so old messages are overwritten when full, making dmesg essential for troubleshooting boot issues, hardware problems, or kernel panics.

Without options, it prints the entire buffer in chronological order. Timestamps show boot-relative or wall-clock time. It's invaluable for debugging, as logs persist across reboots until cleared. Modern systems with systemd use journalctl, but dmesg remains key for raw kernel data.

Run as root for write operations like clearing or setting levels. Buffer size is configurable via /proc/sys/kernel/dmesg_restrict or boot params. Output can be filtered by level, time, or content, aiding analysis in large logs.

Requires root or CAP_SYSLOG for clear/set operations; ring buffer overwrites old messages; restricted by /proc/sys/kernel/dmesg_restrict on some systems.

dmesg | grep -i error for errors; dmesg -w to monitor live; dmesg -T for readable times.

Increase size with boot param dmesg=8M or echo 8M > /proc/sys/kernel/dmesg_restrict (requires root).

Originated in early Linux kernels (1990s) for accessing kernel log buffer; integrated into util-linux package around 2000; enhanced in util-linux 2.28+ with colors, deltas, dissection.

journalctl(1), syslogd(8), klogd(8)