What is the mknod command used for?

Question

I just started using Ubuntu as my main OS and I wanted to learn about things I should not do, and learn by the bad things people have done in the past. I came across these email about horror stories that UNIX & Linux sys admins had done on their own system when they where new. Many of them involved the use of the mknod command to both distory and fix a problem. I've never heard of this command before and the man page within Ubuntu is not very helpful. So my question is, what is this command used for, and what are some examples where it is useful in day to day use?

Solution 1

mknod was originally used to create the character and block devices that populate /dev/. Nowadays software like udev automatically creates and removes device nodes on the virtual filesystem when the corresponding hardware is detected by the kernel, but originally /dev was just a directory in / that was populated during install.

So yes, in case of a near complete disaster causing the /dev virtual filesystem not to load and/or udev failing spectacularly, using mknod to painstakingly repopulate at least a rudimentary device tree to get something back up can be done... But yeah, that's sysadmin horror story time. Personally, I recommend a rescue USB stick or CD.

Aside from creating named pipes, I can't think of a single possible day-to-day use for it that an end user would need to concern themselves with -- and even that is stretching the definition of 'day to day use'.

Solution 2

You can make a named pipe with it.

I use it with one program to read from it, and another one to write into it.

Makes it easier to communicate between processes.

Otherwise, you may create device files, for devices that aren't present.

Also: http://publib.boulder.ibm.com/infocenter/aix/v6r1/index.jsp?topic=/com.ibm.aix.cmds/doc/aixcmds3/mknod.htm

Solution 3

As oracle DBAs working on raw devices to create Oracle ASM diskgroups, we regularly use mknod to link devices.

The replies above were very helpful to me as we are not system admins. I just wanted to point out that it might be rare from storage admins' point of view to use mknod but from Oracle ASM point of view its a common use. (May be someone could come up with a better way for us?)

Solution 4

IMHO, even for making named pipes, the mkfifo command be better than mknod. One, it's self descriptive in it's name, Second, it permits -m option to override umask settings.

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Comments

[Mark Tomlin]

So it's a VERY specialized way of creating a virtual file that is really used by devices connected to the system as a way of communicating with the system without having to learn how to communicate with the device through it's driver?

[Alen Milakovic]

But udev is Linux-specific, is it not? Perhaps other Unix-like systems still manually create devices?

[Shadur]

@mark Er, no. Creating virtual device files is how software is supposed to communicate with the device through its driver. The driver creates the hook in the kernel; mknod creates the device file that links the hook to a device file that software subsequently can connect to.

[Shadur]

@Faheem I'm not sure; FreeBSD seems to use devfs and/or devd but I have no personal experience with either. I suspect other unices have worked out their own ways to automate the process of making device nodes though.

[Shadur]

@mark For example, Alsa generates a series of nodes in /dev/snd corresponding to the various elements of each bit of sound hardware it detected and supports. Software that uses sound can use those devices to generate sound without having to know precisely what kind of sound card they're talking to.

[Mark Tomlin]

While you answer was correct, Shadur provided an answer that was clear for all levels of readers so I feel it's the best for people looking for an answer for this question.

[polemon]

I know, I just tried to explain from a practical approach as I do it. Using mknod for something else than making named pipes is more than rare.

[derobert]

I'm running ASM on Linux and haven't had any reason to run mknod. I have some udev scripts that make symlinks, but not sure what you need mknod for... Unless you're not on Linux. of course.

[user3438085]

...and it didn't exist until a decade after named pipes; a lot of us learned mknod foo p when it was the only way.

[Chris Wolfe]

I had never even heard of mknod until recently when I was looking through coreutils. I've always just used mkfifo...

[k6mrm]

Please excuse me typing mistakes. iPhone and Parkinson’s don’t play well together. C<FORTRAN<COBOL<PL/1, leftis better.

[telcoM]

@JayJee Be advised that on modern Linux systems, if you make changes or additions to /dev with mknod, those changes will go away the next time the system is rebooted. You would need to write a script or an udev rule file that would make the changes/additions for you at boot time to make them persistent.

[JdeBP]

None of this actually addresses the question asked.