sysfs - a filesystem for exporting kernel objects
The
sysfs filesystem is a pseudo-filesystem which provides an interface
to kernel data structures. (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, allowing kernel variables to be changed. To avoid redundancy,
symbolic links are heavily used to connect entries across the filesystem tree.
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 subdirectories:
- 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 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, 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
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 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 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/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 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]
The
sysfs filesystem first appeared in Linux 2.6.0.
The
sysfs filesystem is Linux-specific.
This manual page is incomplete, possibly inaccurate, and is the kind of thing
that needs to be updated very often.
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