/sys/

The /sys directory is an essential part of the Linux filesystem, especially related to the system's hardware and kernel. Here’s a detailed explanation of /sys:

Understanding the /sys Directory

The /sys directory, also known as sysfs, is a virtual filesystem in Linux that provides information about the kernel, devices, and other system-related attributes. It was introduced in Linux kernel version 2.6. The /sys directory is used to export kernel data structures, their attributes, and relationships to userspace. It is mounted automatically by the system at boot time.

Key Characteristics of /sys:

1. Virtual Filesystem: Similar to /proc, the files in /sys do not exist on disk; they are created dynamically by the kernel.

2. Hierarchy: The structure of /sys mirrors the internal kernel structures. It provides a hierarchical view of devices and their attributes.

3. Read and Write: Unlike /proc, which is mostly read-only, many files in /sys can be written to, allowing for dynamic configuration of hardware parameters.

Key Subdirectories in /sys:

1. /sys/class/:

   • Contains directories for each class of device. Device classes include block, net, tty, etc.

   • Example: /sys/class/net contains directories for each network interface (e.g., eth0, lo).

2. /sys/block/:

   • Contains directories for each block device. Block devices include hard drives and other storage devices.

   • Example: /sys/block/sda for the first SCSI disk.

3. /sys/bus/:

   • Contains directories for each bus type in the system (e.g., pci, usb).

   • Example: /sys/bus/usb contains directories for USB devices.

4. /sys/devices/:

   • Contains directories for all devices, organized by their physical hierarchy.

   • Example: /sys/devices/pci0000:00 for PCI devices.

5. /sys/firmware/:

   • Contains directories for firmware-related information.

   • Example: /sys/firmware/efi for EFI firmware variables.

Examples of Using /sys:

1. Listing Network Interfaces:

   ls /sys/class/net/

   Output might include:

   eth0  lo  wlan0

2. Getting Device Information:

   cat /sys/class/net/eth0/address

   Output might be the MAC address of eth0:

   01:23:45:67:89:ab

3. Modifying Device Attributes:

   • Example: Changing the brightness of a laptop screen.

   echo 100 > /sys/class/backlight/acpi_video0/brightness

Practical Use Cases:

1. Monitoring Hardware Health:

   • You can read sensor data like CPU temperature:

     cat /sys/class/thermal/thermal_zone0/temp

2. Managing Power:

   • You can manage power settings for devices, like enabling or disabling USB ports:

     echo 'auto' > /sys/bus/usb/devices/usb1/power/control

3. Configuring Kernel Parameters:

   • You can configure various kernel parameters dynamically without needing a reboot:

     echo 1 > /sys/class/net/eth0/device/sriov_numvfs

Conclusion

The /sys directory is a powerful tool for Linux system administrators, providing a way to query and control kernel and hardware attributes dynamically. Understanding and utilizing /sys can greatly enhance your ability to manage and optimize your Linux system.

Feel free to ask for explanations of other objectives or for more examples and details!