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.TH SYSFS 5 2018-04-30 "Linux" "Linux Programmer's Manual"
.SH NAME
sysfs \- a filesystem for exporting kernel objects
.SH DESCRIPTION
The
.B sysfs
filesystem is a pseudo-filesystem which provides an interface to
kernel data structures.
(More precisely, the files and directories in
.B sysfs
provide a view of the
.IR kobject
structures defined internally within the kernel.)
The files under
.B sysfs
provide information about devices, kernel modules, filesystems,
and other kernel components.
.PP
The
.B sysfs
filesystem is commonly mounted at
.IR /sys .
Typically, it is mounted automatically by the system,
but it can also be mounted manually using a command such as:
.PP
.in +4n
.EX
mount \-t sysfs sysfs /sys
.EE
.in
.PP
Many of the files in the
.B 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.
.\"
.SS Files and directories
The following list describes some of the files and directories under the
.I /sys
hierarchy.
.TP
.IR /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
.IR /sys/devices .
.TP
.IR /sys/bus
This directory contains one subdirectory for each of the bus types
in the kernel.
Inside each of these directories are two subdirectories:
.RS
.TP
.IR devices
This subdirectory contains symbolic links to entries in
.IR /sys/devices
that correspond to the devices discovered on this bus.
.TP
.IR drivers
This subdirectory contains one subdirectory for each device driver
that is loaded on this bus.
.RE
.TP
.IR /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 subdirectories are symbolic links for each of the
devices in this class.
These symbolic links refer to entries in the
.IR /sys/devices
directory.
.TP
.IR /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
.IR /sys/devices
directory.
.TP
.IR /sys/dev
This directory contains two subdirectories
.IR block /
and
.IR 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
.IR 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
.B sysfs
directory for a device.
The symbolic links inside
.IR /sys/dev
thus provide an easy way to look up the
.B sysfs
interface using the device IDs returned by a call to
.BR stat (2)
(or similar).
.IP
The following shell session shows an example from
.IR /sys/dev :
.IP
.in +4n
.EX
$ \fBstat \-c "%t %T" /dev/null\fP
1 3
$ \fBreadlink /sys/dev/char/1\\:3\fP
\&../../devices/virtual/mem/null
$ \fBls \-Fd /sys/devices/virtual/mem/null\fP
/sys/devices/virtual/mem/null/
$ \fBls \-d1 /sys/devices/virtual/mem/null/*\fP
/sys/devices/virtual/mem/null/dev
/sys/devices/virtual/mem/null/power/
/sys/devices/virtual/mem/null/subsystem@
/sys/devices/virtual/mem/null/uevent
.EE
.in
.TP
.IR /sys/devices
This is a directory that contains a filesystem representation of
the kernel device tree,
which is a hierarchy of
.I device
structures within the kernel.
.TP
.IR /sys/firmware
This subdirectory contains interfaces for viewing and manipulating
firmware-specific objects and attributes.
.TP
.IR /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.
.TP
.IR /sys/fs/cgroup
This directory conventionally is used as a mount point for a
.BR tmpfs (5)
filesystem containing mount points for
.BR cgroups (7)
filesystems.
.TP
.IR /sys/fs/smackfs
The directory contains configuration files for the SMACK LSM.
See the kernel source file
.IR Documentation/admin-guide/LSM/Smack.rst .
.TP
.IR /sys/hypervisor
[To be documented]
.TP
.IR /sys/kernel
This subdirectory contains various files and subdirectories that provide
information about the running kernel.
.TP
.IR /sys/kernel/cgroup/
For information about the files in this directory, see
.BR cgroups (7).
.TP
.IR /sys/kernel/debug/tracing
Mount point for the
.I tracefs
filesystem used by the kernel's
.I ftrace
facility.
(For information on
.IR ftrace ,
see the kernel source file
.IR Documentation/trace/ftrace.txt .)
.TP
.IR /sys/kernel/mm
This subdirectory contains various files and subdirectories that provide
information about the kernel's memory management subsystem.
.TP
.IR /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.,
.IR 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
.IR Documentation/admin-guide/mm/hugetlbpage.rst .
.TP
.IR /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:
.RS
.TP
.I coresize
[to be documented]
.TP
.I initsize
[to be documented]
.TP
.I initstate
[to be documented]
.TP
.I refcnt
[to be documented]
.TP
.I srcversion
[to be documented]
.TP
.I taint
[to be documented]
.TP
.I uevent
[to be documented]
.TP
.I version
[to be documented]
.RE
.IP
In each of the subdirectories, there may be following subdirectories:
.RS
.TP
.I drivers
[To be documented]
.TP
.I holders
[To be documented]
.TP
.I notes
[To be documented]
.TP
.I parameters
This directory contains one file for each module parameter,
with each file containing the value of the corresponding parameter.
Some of these files are writable, allowing the
.TP
.I sections
This subdirectories contains files with information about module sections.
This information is mainly used for debugging.
.TP
.I
[To be documented]
.RE
.TP
.IR /sys/power
[To be documented]
.SH VERSIONS
The
.B sysfs
filesystem first appeared in Linux 2.6.0.
.SH CONFORMING TO
The
.B sysfs
filesystem is Linux-specific.
.SH NOTES
This manual page is incomplete, possibly inaccurate, and is the kind
of thing that needs to be updated very often.
.SH SEE ALSO
.BR proc (5),
.BR udev (7)
.PP
P. Mochel. (2005).
.IR "The sysfs filesystem" .
Proceedings of the 2005 Ottawa Linux Symposium.
.\" https://www.kernel.org/pub/linux/kernel/people/mochel/doc/papers/ols-2005/mochel.pdf
.PP
The kernel source file
.I Documentation/filesystems/sysfs.txt
and various other files in
.IR Documentation/ABI
and
.IR Documentation/*/sysfs.txt