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sysfs(5) File Formats 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 struc‐
tures. (More precisely, the files and directories 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 manually 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, allow‐
ing 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 dis‐
covered 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. In‐
side each of these directories are two subdirectories:

devices
This subdirectory contains symbolic links to entries in /sys/devices that cor‐
respond 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 de‐
vices, block devices, graphics devices, sound devices, and so on). Inside each of
these subdirectories 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, respec‐
tively, to the block and character devices on the system. Inside each of these subdi‐
rectories are symbolic links with names of the form major-ID:minor-ID, where the ID
values correspond 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 explicitly create the subdirectory.

/sys/fs/cgroup
This directory conventionally is used as a mount point for a tmpfs(5) filesystem con‐
taining 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 in‐
formation 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 indicates 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 Documentation/ad‐
min-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 subdi‐
rectories, 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 con‐
taining the value of the corresponding parameter. 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]

STANDARDS
Linux.

HISTORY
Linux 2.6.0.

NOTES
This manual page is incomplete, possibly inaccurate, and is the kind of thing that needs to
be updated very often.