.\" Copyright (c) 2017 by Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .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\e: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 .SH COLOPHON This page is part of release 5.04 of the Linux .I 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/.