Intro To 'mknod' Command In Linux

2023-05-31 - By Robert Elder

I use the 'mknod' command to create special block files, character files, and pipe files:

mknod -m 666 my_special_file c 1 8

Creating An Equivalent To '/dev/zero' Using 'mknod'

The file located at '/dev/zero' is a special file that always returns an infinite number of zeros:

head -c 100 /dev/zero | xxd

00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000020: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000030: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000040: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000050: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000060: 0000 0000                                ....

I can use the 'mknod' command to create my own version of '/dev/zero' directly on the desktop:

sudo mknod -m 666 my_custom_dev_zero c 1 5

The '-m' sets the permissions for the file and the letter 'c' indicates that this will be a character device. The number '1' indicates the major number of this device and the number '5' indicates the minor number. You can also view the corresponding system file related to this character device at this location:

cat /sys/dev/char/1:5/uevent

MAJOR=1
MINOR=5
DEVNAME=zero
DEVMODE=0666

Now, I can use 'my_custom_dev_zero' just like the real '/dev/zero':

head -c 10 my_custom_dev_zero | xxd

00000000: 0000 0000 0000 0000 0000                 ..........

Creating Other Special Files Using 'mknod'

As you might expect, you can also use the 'mknod' command to create other special device nodes, like '/dev/random':

sudo mknod -m 666 my_dev_random c 1 8

or '/dev/urandom':

sudo mknod -m 666 my_dev_urandom c 1 9

or '/dev/full':

sudo mknod -m 666 my_dev_full c 1 7

A full listing of the major and minor numbers for these special files can be found in the kernel documentation here.

Creating A Block Device File Using 'mknod'

I can also use the 'mknod' command to create a special file that represents a block device like a storage drive. For example, here's the block device file representing a solid state drive in my system:

ls -l /dev/sdb

brw-rw---- 1 root disk 8, 16 May 21 23:42 /dev/sdb

If I use the 'b' character and the major and minor number of an existing block device, I'll get a second block device node that represents the same drive:

sudo mknod -m 666 my_custom_block_device b 8 16

And here is a file listing of both the original and the new block devices representing this drive:

ls -l my_custom_block_device /dev/sdb

brw-rw---- 1 root disk 8, 16 May 21 23:42 /dev/sdb
brw-rw-rw- 1 root root 8, 16 May 28 18:04 my_custom_block_device

It will now be possible to view system information about the new block device node with the 'fdisk' command:

sudo fdisk -l my_custom_block_device

Disk my_custom_block_device: 1.82 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: RTL9210 NVME
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
Disklabel type: dos
Disk identifier: 0x57607dd4

Device                  Boot Start        End    Sectors  Size Id Type
my_custom_block_device1      65535 3907029167 3906963633  1.8T 83 Linux

This works just like with the original '/dev/sdb' block device:

sudo fdisk -l /dev/sdb

Disk /dev/sdb: 1.82 TiB, 2000398934016 bytes, 3907029168 sectors
Disk model: RTL9210 NVME
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 33553920 bytes
Disklabel type: dos
Disk identifier: 0x57607dd4

Device     Boot Start        End    Sectors  Size Id Type
/dev/sdb1       65535 3907029167 3906963633  1.8T 83 Linux

Creating A Named Pipe File Using 'mknod'

The 'mknod' command can also create named pipes just like the 'mkfifo' command does:

mknod other-hexdump-fifo1 p

mkfifo other-hexdump-fifo2

ls -l other-hexdump-fifo1 other-hexdump-fifo2

prw-rw-r-- 1 robert robert 0 May 28 18:19 other-hexdump-fifo1
prw-rw-r-- 1 robert robert 0 May 28 18:19 other-hexdump-fifo2

And that's why the 'mknod' command is my favourite Linux command.

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