SYSFS(5) Linux Programmer's Manual SYSFS(5)
NAME
sysfs - a filesystem for exporting kernel objects
DESCRIPTION
The sysfs filesystem is a pseudo-filesystem which provides an interface to kernel data structures. (More precisely, the files and directo-
ries in sysfs provide a view of the kobject structures defined internally within the kernel.) The files under sysfs provide information
about devices, kernel modules, filesystems, and other kernel components.
The sysfs filesystem is commonly mounted at /sys. Typically, it is mounted automatically by the system, but it can also be mounted manu-
ally using a command such as:
mount -t sysfs sysfs /sys
Many of the files in the sysfs filesystem are read-only, but some files are writable, allowing kernel variables to be changed. To avoid
redundancy, symbolic links are heavily used to connect entries across the filesystem tree.
Files and directories
The following list describes some of the files and directories under the /sys hierarchy.
/sys/block
This subdirectory contains one symbolic link for each block device that has been discovered on the system. The symbolic links point
to corresponding directories under /sys/devices.
/sys/bus
This directory contains one subdirectory for each of the bus types in the kernel. Inside each of these directories are two subdi-
rectories:
devices
This subdirectory contains symbolic links to entries in /sys/devices that correspond to the devices discovered on this bus.
drivers
This subdirectory contains one subdirectory for each device driver that is loaded on this bus.
/sys/class
This subdirectory contains a single layer of further subdirectories for each of the device classes that have been registered on the
system (e.g., terminals, network devices, block devices, graphics devices, sound devices, and so on). Inside each of these subdi-
rectories are symbolic links for each of the devices in this class. These symbolic links refer to entries in the /sys/devices
directory.
/sys/class/net
Each of the entries in this directory is a symbolic link representing one of the real or virtual networking devices that are visible
in the network namespace of the process that is accessing the directory. Each of these symbolic links refers to entries in the
/sys/devices directory.
/sys/dev
This directory contains two subdirectories block/ and char/, corresponding, respectively, to the block and character devices on the
system. Inside each of these subdirectories are symbolic links with names of the form major-ID:minor-ID, where the ID values corre-
spond to the major and minor ID of a specific device. Each symbolic link points to the sysfs directory for a device. The symbolic
links inside /sys/dev thus provide an easy way to look up the sysfs interface using the device IDs returned by a call to stat(2) (or
similar).
The following shell session shows an example from /sys/dev:
$ stat -c "%t %T" /dev/null
1 3
$ readlink /sys/dev/char/1:3
../../devices/virtual/mem/null
$ ls -Fd /sys/devices/virtual/mem/null
/sys/devices/virtual/mem/null/
$ ls -d1 /sys/devices/virtual/mem/null/*
/sys/devices/virtual/mem/null/dev
/sys/devices/virtual/mem/null/power/
/sys/devices/virtual/mem/null/subsystem@
/sys/devices/virtual/mem/null/uevent
/sys/devices
This is a directory that contains a filesystem representation of the kernel device tree, which is a hierarchy of device structures
within the kernel.
/sys/firmware
This subdirectory contains interfaces for viewing and manipulating firmware-specific objects and attributes.
/sys/fs
This directory contains subdirectories for some filesystems. A filesystem will have a subdirectory here only if it chose to explic-
itly create the subdirectory.
/sys/fs/cgroup
This directory conventionally is used as a mount point for a tmpfs(5) filesystem containing mount points for cgroups(7) filesystems.
/sys/fs/smackfs
The directory contains configuration files for the SMACK LSM. See the kernel source file Documentation/admin-guide/LSM/Smack.rst.
/sys/hypervisor
[To be documented]
/sys/kernel
This subdirectory contains various files and subdirectories that provide information about the running kernel.
/sys/kernel/cgroup/
For information about the files in this directory, see cgroups(7).
/sys/kernel/debug/tracing
Mount point for the tracefs filesystem used by the kernel's ftrace facility. (For information on ftrace, see the kernel source file
Documentation/trace/ftrace.txt.)
/sys/kernel/mm
This subdirectory contains various files and subdirectories that provide information about the kernel's memory management subsystem.
/sys/kernel/mm/hugepages
This subdirectory contains one subdirectory for each of the huge page sizes that the system supports. The subdirectory name indi-
cates the huge page size (e.g., hugepages-2048kB). Within each of these subdirectories is a set of files that can be used to view
and (in some cases) change settings associated with that huge page size. For further information, see the kernel source file Docu-
mentation/admin-guide/mm/hugetlbpage.rst.
/sys/module
This subdirectory contains one subdirectory for each module that is loaded into the kernel. The name of each directory is the name
of the module. In each of the subdirectories, there may be following files:
coresize
[to be documented]
initsize
[to be documented]
initstate
[to be documented]
refcnt [to be documented]
srcversion
[to be documented]
taint [to be documented]
uevent [to be documented]
version
[to be documented]
In each of the subdirectories, there may be following subdirectories:
drivers
[To be documented]
holders
[To be documented]
notes [To be documented]
parameters
This directory contains one file for each module parameter, with each file containing the value of the corresponding parame-
ter. Some of these files are writable, allowing the
sections
This subdirectories contains files with information about module sections. This information is mainly used for debugging.
[To be documented]
/sys/power
[To be documented]
VERSIONS
The sysfs filesystem first appeared in Linux 2.6.0.
CONFORMING TO
The sysfs filesystem is Linux-specific.
NOTES
This manual page is incomplete, possibly inaccurate, and is the kind of thing that needs to be updated very often.
SEE ALSO
proc(5), udev(7)
P. Mochel. (2005). The sysfs filesystem. Proceedings of the 2005 Ottawa Linux Symposium.
The kernel source file Documentation/filesystems/sysfs.txt and various other files in Documentation/ABI and Documentation/*/sysfs.txt
Linux 2018-04-30 SYSFS(5)