Filtering dmesg Output

Many administrators just run "dmesg" to check a system for problems and do not bother with its options. But actually it is worth to know about the filtering and output options of the most recent versions (Note: older distros e.g. CentOS5 might not yet ship these options!).

You always might want to use "-T" to show human readable timestamps:

$ dmesg -T
[...]
[Wed Oct 10 20:31:22 2012] Buffer I/O error on device sr0, logical block 0
[Wed Oct 10 20:31:22 2012] sr 1:0:0:0: [sr0]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
[...]

Additionally the severity and source of the messages is interesting (option -x):

$ dmesg -xT
[...]
kern  :err   : [Wed Oct 10 20:31:21 2012] Buffer I/O error on device sr0, logical block 0
kern  :info  : [Wed Oct 10 20:31:21 2012] sr 1:0:0:0: [sr0]  Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
[...]

Now we see that only one of those example message lines was an actual error. But we can even filter for errors or worse ignoring all the boot messages (option -l):

$ dmesg -T -l err,crit,alert,emerg
[...]
[Wed Oct 10 20:31:21 2012] Buffer I/O error on device sr0, logical block 0
[...]

In the same way it is possible to filter the facility (the first column in the -x output). For example this could return:

$ dmesg -T -f daemon
[...]
[Wed Oct 10 19:57:50 2012] udevd[106]: starting version 175
[Wed Oct 10 19:58:08 2012] udevd[383]: starting version 175
[...]

In any case it might be worth remembering:

• -xT for a quick overview with readable timestamps
• -T -l err,crit,alert,emerg to just check for errors