sysfs

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  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  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 redun-
       dancy,  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 de-
              vice 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 de-
                     vice driver that is loaded on this bus.

       /sys/class
              This subdirectory contains a single layer of further subdirecto-
              ries 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  repre-
              senting  one  of the real or virtual networking devices that are
              visible in the network namespace of the process that is  access-
              ing  the  directory.  Each of these symbolic links refers to en-
              tries 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  de-
              vice.   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 struc-
              tures within the kernel.

       /sys/firmware
              This subdirectory contains interfaces for viewing and manipulat-
              ing 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 ex-
              plicitly 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 subsys-
              tem.

       /sys/kernel/mm/hugepages
              This subdirectory contains one subdirectory for each of the huge
              page sizes that the system supports.  The subdirectory name  in-
              dicates  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  subdirec-
              tories:

              drivers
                     [To be documented]

              holders
                     [To be documented]

              notes  [To be documented]

              parameters
                     This  directory contains one file for each module parame-
                     ter, with each file containing the value  of  the  corre-
                     sponding  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 de-
                     bugging.

              [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 Ot-
       tawa Linux Symposium.

       The kernel source file Documentation/filesystems/sysfs.txt and  various
       other files in Documentation/ABI and Documentation/*/sysfs.txt

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2018-04-30                          SYSFS(5)