devtmpfs
is a file system with automated device nodes populated by the kernel. This means you don't have to have udev running nor to create a static /dev
layout with additional, unneeded and not present device nodes. Instead the kernel populates the appropriate information based on the known devices.
On the other hand the standard /dev
handling requires either udev
, an additional daemon running or to statically create device nodes on /dev
.
The files in /dev
are actual devices files which UDEV creates at run time. The directory /sys/class
is exported by the kernel at run time, exposing the hierarchy of the hardware through sysfs
.
From the libudev and Sysfs Tutorial
excerpt
On Unix and Unix-like systems, hardware devices are accessed through special files (also called device files or nodes) located in the /dev directory. These files are read from and written to just like normal files, but instead of writing and reading data on a disk, they communicate directly with a kernel driver which then communicates with the hardware. There are many online resources describing /dev files in more detail. Traditonally, these special files were created at install time by the distribution, using the mknod command. In recent years, Linux systems began using udev to manage these /dev files at runtime. For example, udev will create nodes when devices are detected and delete them when devices are removed (including hotplug devices at runtime). This way, the /dev directory contains (for the most part) only entries for devices which actually exist on the system at the current time, as opposed to devices which could exist.
another excerpt
The directories in Sysfs contain the heirarchy of devices, as they are attached to the computer. For example, on my computer, the hidraw0 device is located under:
/sys/devices/pci0000:00/0000:00:12.2/usb1/1-5/1-5.4/1-5.4:1.0/0003:04D8:003F.0001/hidraw/hidraw0
Based on the path, the device is attached to (roughly, starting from the end) configuration 1 (:1.0) of the device attached to port number 4 of device 1-5, connected to USB controller 1 (usb1), connected to the PCI bus. While interesting, this directory path doesn't do us very much good, since it's dependent on how the hardware is physically connected to the computer.
Fortunately, Sysfs also provides a large number of symlinks, for easy access to devices without having to know which PCI and USB ports they are connected to. In /sys/class there is a directory for each different class of device.
Usage?
In general you use rules in /etc/udev/rules.d
to augment your system. Rules can be constructed to run scripts when various hardware is present.
Once a system is up you can write scripts to work against either /dev
or /sys
, and it really comes down to personal preferences, but I would usually try and work against /sys
and make use of tools such as udevadm
to query UDEV for locations of various system resources.
$ udevadm info -a -p $(udevadm info -q path -n /dev/sda) | head -15
Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.
looking at device '/devices/pci0000:00/0000:00:1f.2/ata1/host0/target0:0:0/0:0:0:0/block/sda':
KERNEL=="sda"
SUBSYSTEM=="block"
DRIVER==""
ATTR{ro}=="0"
ATTR{size}=="976773168"
ATTR{stat}==" 6951659 2950164 183733008 41904530 16928577 18806302 597365181 580435555 0 138442293 622621324"
ATTR{range}=="16"
...
Best Answer
Yes, both accept and discard all input, but their output is not the same:
/dev/null
produces no output./dev/zero
produces a continuous stream of NULL (zero value) bytes.You can see the difference by executing
cat /dev/null
andcat /dev/zero
.Try
cat /dev/null > file
and you will find an emptyfile
.Now try
cat /dev/zero > file
, while watching the size of the file (watch -n 1 du -h file
) continuously increase. This is because reading from/dev/zero
gives an endless stream of\0
(null) characters.Use
dd
to visualize the difference more appropriately:/dev/zero
is used to create dummy files or swap.Also visit: