The first thing that loads in OS X is the kernel. The kernel is how users interface with hardware and sets the stage for interaction by probing for each driver that needs to be loaded and tracking what is found. The presence of everything about the system is tracked when the kernel loads as well as pertinent boot parameters. Even if you’re booting in verbose mode, most of this probably happens too fast to notice. You might be able to pause it, but you’re still trying to react to things too quickly in many cases. That’s where the dmesg command comes into play, which lets you review and control the messages since the system has booted.
The dmesg command also looks at some diagnostic messages after the system is due booting, such as when a new interface is detected. The dmesg command allows for more than what you can see in console, except explicitly for I/O related kernel messages after boot, rather than all the pesky OS stuff you might otherwise see, such as snarky little “spotlight can’t index this or that” kind of errors. Instead, dmesg output is only a few screens, easily readable that focuses on the system hardware & I/O interfaces.
Using dmesg is pretty easy. Even if you’ve su’d, you need to run sudo in front of it each time, but there’s no need for any parameters:
sudo dmesg
You can use -n for the level of errors or -s to constrain the buffer size, but in Mac OS X you likely don’t need to do either as there’s not a ton of information. If you’re looking for something specific, such as information on an AirPort interface, just grep it out:
sudo dmesg | grep AirPort
Overall, dmesg is helpful for stuff like this:
vnic0: promiscuous mode enable failed
Apparently, Rajesh Koothrappali couldn’t find any booze…
Or this:
USBF: 23952.350 AppleUSBEHCI[0xffffff800b7dc000]::Found a transaction which hasn't moved in 5 seconds on bus 0xfd, timing out! (Addr: 4, EP: 2)
I had a bad drobo on the system… Which I might have known by the fact that the Finder froze when plugged in. But I always like to see it in the logs. Swap out one drive, message goes away, drobo loads just fine. Have I mentioned recently I’m liking the low end drobos less and less…
