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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. 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 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/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/hypervisor [To be documented] /sys/kernel [To be documented] /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
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