Dmesg: An old, new, cool tool

[aw3c2]

http://inconsolation.wordpress.com/2013/08/11/dmesg-an-old-new-cool-tool/

[sciurus]

You need to have a recent util-linux for most of these. If you're on Ubuntu, you probably have 2.20.

From the release notes:

2.20: supports new command line options: --clear, --console-on, --console-off, --ctime, --decode, --facility=<list>, --level=<list>, --show-delta, --notime, --kernel and --userspace

2.21: supports new command line options --file to read the log from a file rather than from kernel buffer

2.22:

- reads kernel messages from /dev/kmsg on kernel 3.5

- supports new option --follow to wait for new messages (kernel 3.5 required)

- supports new option --reltime to print human readable deltas

2.23: supports new command line options --color, --human and --nopager, the --human option enables relative times, colors and pager support.

[voltagex_ → sciurus]

I wonder (other than the features requiring kernel 3.5) what dependencies dmesg has - could I just carry a static binary around in my ~/bin for systems that don't have a new util-linux?

[wereHamster → voltagex_]

Check for yourself with `ldd <path/to/dmesg>`

[WestCoastJustin → wereHamster]

Ubuntu 12.04 (32-bit)

   [~]$ ldd /bin/dmesg
   	linux-gate.so.1 =>  (0xb778e000)
   	libc.so.6 => /lib/i386-linux-gnu/libc.so.6 (0xb75c4000)
   	/lib/ld-linux.so.2 (0xb778f000)

CentOS 6.4 (64-bit)

   [~]$ ldd /bin/dmesg
   	linux-vdso.so.1 =>  (0x00007fff115ff000)
   	libc.so.6 => /lib64/libc.so.6 (0x00007ff8077c0000)
   	/lib64/ld-linux-x86-64.so.2 (0x00007ff807b59000)

[WestCoastJustin → sciurus]

On CentOS 6.4 the util-linux-ng v2.17.2 package provide the /bin/dmesg command.

[WestCoastJustin]

On systems that have been up a long time or that spew lots of kernel messages, running dmesg might not give you everything (i.e. just a buffer of the last X messages), you can typically find a complete dmesg log in /var/log/dmesg, which should have a complete dmesg log from boot. I use dmesg a lot, for debugging issue, check if drivers have been loaded, kernel messages, like ops or oom, etc. Very handy!

[ddoolin]

I was kinda hoping to get a description of what dmesg actually is/does...but it's mostly about customizing the output. I could search for it of course, but it was a bit of a surprise to not see any intro to it.

I've been using dmesg for awhile on Arch to debug USB devices mostly, without any clue to what it actually is.

[sciurus → ddoolin]

dmesg reads and displays the contents of /proc/kmsg. /proc/kmsg is an interface to a buffer where the output from calls to printk is stored. printk is the linux kernel's equivalent of libc's printf function.

[mahmud]

First time I learn that dmesg takes arguments!