/proc and /sys: Reading Kernel State at Runtime

Need to inspect or tune kernel parameters at runtime without rebooting, or diagnose hardware and process state without specialized tools.

Read from /proc for process and kernel info; read/write /sys for hardware and driver parameters.

# CPU info
cat /proc/cpuinfo

# Memory info
cat /proc/meminfo

# Loaded kernel modules
cat /proc/modules
lsmod

# Running process info (PID 1234)
ls /proc/1234/
cat /proc/1234/cmdline | tr '\0' ' '
cat /proc/1234/status
ls -la /proc/1234/fd/   # open file descriptors

# Kernel parameters (sysctl)
sysctl vm.swappiness
sysctl -a | grep net.ipv4

# Change kernel parameter at runtime
sysctl -w vm.swappiness=10

# Persist across reboots
echo 'vm.swappiness=10' >> /etc/sysctl.conf
sysctl -p

# /sys: hardware tuning (e.g., CPU governor)
cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor

/proc is a virtual filesystem — files have no disk size but reading them queries the kernel. /sys exposes the device model and driver attributes. sysctl is the canonical interface for kernel tunable parameters.