/sys Filesystem

/sys is a virtual filesystem where the Linux kernel exports its internal state as files. Every piece of hardware, every kernel subsystem, and every device driver has a directory here. Read a file to query a setting. Write to it to change behavior — no reboot required, no config file needed.

What Is sysfs?

How is /sys different from /proc? /proc was the original Linux "kernel info as files" interface — it's messier, less organized, and mixes process information with hardware info. /sys (sysfs) was introduced with the 2.6 kernel as a clean replacement for hardware/device information. /sys is strictly hierarchical and organized by kernel subsystem. Both exist today, but /sys is the right place for device and driver configuration.

$ ls /sys/
block/   bus/   class/   dev/   devices/   firmware/
fs/      kernel/  module/  power/

# Key directories:
# /sys/block/       ← block devices (sda, nvme0n1)
# /sys/bus/         ← buses (pci, usb, i2c, platform)
# /sys/class/       ← device classes (net, input, power_supply)
# /sys/devices/     ← all devices in hardware tree
# /sys/module/      ← loaded kernel modules and their params

/sys/block — Disk Configuration

# Every block device has a directory
ls /sys/block/
# sda  sdb  nvme0n1  loop0  loop1 ...

# Disk queue settings (most important for performance)
ls /sys/block/sda/queue/
# scheduler         ← I/O scheduler
# nr_requests       ← queue depth
# read_ahead_kb     ← read-ahead size
# rotational        ← 1=HDD, 0=SSD
# max_sectors_kb    ← max I/O size per request

# Read scheduler
cat /sys/block/sda/queue/scheduler
# mq-deadline [bfq] none

# Check if SSD or HDD
cat /sys/block/sda/queue/rotational
# 1 = spinning disk (HDD)
# 0 = solid state (SSD)

# Disk size in 512-byte sectors
cat /sys/block/sda/size
# 1953525168   ← * 512 = ~1TB

# Partition info
ls /sys/block/sda/
# dev  queue  sda1  sda2  sda3  size  stat ...

cat /sys/block/sda/stat
# reads  read_merges  read_sectors  read_ms  writes  write_merges ...
# ← I/O statistics (same data as /proc/diskstats)

/sys/class/net — Network Interface Config

# List network interfaces
ls /sys/class/net/
# eth0  lo  wlan0  docker0  ...

# Interface speed
cat /sys/class/net/eth0/speed
# 1000  ← 1Gbps

# MAC address
cat /sys/class/net/eth0/address
# 00:11:22:33:44:55

# TX queue length
cat /sys/class/net/eth0/tx_queue_len
# 1000

# Carrier (link up/down)
cat /sys/class/net/eth0/carrier
# 1 = link up, 0 = link down

# Bring interface up/down via /sys:
echo 1 > /sys/class/net/eth0/flags   # (use 'ip link' instead in practice)

/sys/class/power_supply — Battery & Power

# Laptop battery info
ls /sys/class/power_supply/
# AC  BAT0  BAT1

cat /sys/class/power_supply/BAT0/capacity
# 87  ← 87% charge

cat /sys/class/power_supply/BAT0/status
# Discharging

cat /sys/class/power_supply/BAT0/energy_full
# 50000000  ← microwatt-hours

cat /sys/class/power_supply/BAT0/energy_now
# 43500000  ← current charge in microwatt-hours

# CPU frequency scaling
ls /sys/devices/system/cpu/cpu0/cpufreq/
# scaling_governor  scaling_cur_freq  scaling_min_freq  scaling_max_freq

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
# powersave

# Switch to performance mode:
echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

/sys/module — Kernel Module Parameters

# See loaded modules
ls /sys/module/
# bluetooth  ext4  usbcore  nvidia  ...

# Module parameters (can change without unloading module!)
ls /sys/module/usbcore/parameters/
# autosuspend  usbfs_memory_mb  ...

cat /sys/module/usbcore/parameters/autosuspend
# 2   ← USB autosuspend timeout in seconds

# Change parameter live:
echo -1 > /sys/module/usbcore/parameters/autosuspend  # disable autosuspend

# NVMe module params
cat /sys/module/nvme/parameters/io_timeout
# 30   ← I/O timeout in seconds

Controlling Hardware Directly

sysfs lets you control hardware without any program — just write to files:

# Control LEDs (Raspberry Pi, servers with front panel)
ls /sys/class/leds/
# input2::capslock  input2::numlock  ...

cat /sys/class/leds/input2::capslock/brightness
# 0

echo 1 > /sys/class/leds/input2::capslock/brightness  # turn on

# Raspberry Pi GPIO (older kernels):
echo 17 > /sys/class/gpio/export            # claim GPIO 17
echo out > /sys/class/gpio/gpio17/direction # set as output
echo 1 > /sys/class/gpio/gpio17/value       # set high (3.3V)

# CPU core online/offline (disable a core):
echo 0 > /sys/devices/system/cpu/cpu3/online   # take cpu3 offline
echo 1 > /sys/devices/system/cpu/cpu3/online   # bring it back