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A while ago I noticed this directory that I had not ever seen before, /sys. I researched a bit and read that "modern Linux systems" often have this directory, and that it manages devices. I thought that was what /dev was for. I can't seem to find a whole lot of information about this directory, other than what I mentioned, and this, quoted from this page:

/sys is a virtual file system that can be accessed to set or obtain information about the kernel's view of the system.

I have been running Trusty for a while now, and never noticed it before, which is why I find it a little strange. Would someone please fill me in? What is the difference between /sys and /dev? When did Ubuntu start using this directory, and why? Thanks.

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  • /sys has been around for over a decade now.
    – muru
    Jan 13, 2016 at 4:21
  • 2
    Sidenote , perhaps will be useful to other users: /proc and /sys directories ,being virtual filesystems, will disappear once the the computer is shut down. If you ever mounted a hard drive with Linux on another OS you'll see that those directories are empty. Jan 13, 2016 at 5:44

1 Answer 1

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/sys is old. It was introduced before the Linux kernel reached 2.6 (back when there was a 2.4/2.5 split). Since the first Ubuntu release used a 2.6 kernel, every version of Ubuntu has had a /sys.

/dev contains the actual device files. It does not provide access to all devices that the kernel knows of (such as ethernet devices, for one - Why are network interfaces not in /dev like other devices?, Why do Ethernet devices not show up in "/dev"?). It is an interface to the device itself - you write to the device, read from it, etc.

/sys is an interface to the kernel. Specifically, it provides a filesystem-like view of information and configuration settings that the kernel provides, much like /proc. Writing to these files may or may not write to the actual device, depending on the setting you're changing. It isn't only for managing devices, though that's a common use case.

More information can be found in the kernel documentation:

Top Level Directory Layout
~~~~~~~~~~~~~~~~~~~~~~~~~~

The sysfs directory arrangement exposes the relationship of kernel
data structures. 

The top level sysfs directory looks like:

block/
bus/
class/
dev/
devices/
firmware/
net/
fs/

devices/ contains a filesystem representation of the device tree. It maps
directly to the internal kernel device tree, which is a hierarchy of
struct device. 

bus/ contains flat directory layout of the various bus types in the
kernel. Each bus's directory contains two subdirectories:

    devices/
    drivers/

devices/ contains symlinks for each device discovered in the system
that point to the device's directory under root/.

drivers/ contains a directory for each device driver that is loaded
for devices on that particular bus (this assumes that drivers do not
span multiple bus types).

fs/ contains a directory for some filesystems.  Currently each
filesystem wanting to export attributes must create its own hierarchy
below fs/ (see ./fuse.txt for an example).

dev/ contains two directories char/ and block/. Inside these two
directories there are symlinks named <major>:<minor>.  These symlinks
point to the sysfs directory for the given device.  /sys/dev provides a
quick way to lookup the sysfs interface for a device from the result of
a stat(2) operation.

For example:

  • One way of setting the brightness of a laptop monitor is:

    echo N > /sys/class/backlight/acpi_video0/brightness
    
  • To get the a network card's MAC address:

    cat /sys/class/net/enp1s0/address
    
  • To get the current CPU scaling governors:

    cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor
    

And so on...

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  • @muru: What if I want to remove one of these files which is consuming a lot of memory? I wrote a device driver which was buggy(may be) which went into a loop and continuously wrote to some file in /sys/devices directory. Also, if I accidentally delete a wrong file what would be the impact? Apr 14, 2016 at 5:13
  • 3
    @BhavikShah you don't remove anything from here - these files don't actually exist. You'll have to remove the module involved.
    – muru
    Apr 14, 2016 at 6:01
  • @muru Can one remove/modify/add any file from/to/in /sys directory? Hope so you'll answer because I'm in some trouble.
    – Error404
    Jan 2 at 10:48

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