The /proc Filesystem

/proc is one of Linux's most ingenious designs. It's a virtual filesystem — nothing in it is stored on disk. The kernel generates file contents on the fly when you read them. Every tool you use for monitoring (top, ps, free) reads from /proc.

Per-Process Information — /proc/PID/

Every running process has its own directory at /proc/PID/:

ls /proc/1/
# cmdline  cwd  environ  exe  fd/  maps  mem  net/
# ns/  oom_score  smaps  stat  status  task/

# The command that started the process
cat /proc/1/cmdline | tr '\0' ' '
# /sbin/init splash

# Current working directory
ls -la /proc/1/cwd   # symlink to actual directory

# Open file descriptors
ls -la /proc/1/fd/
# 0 -> /dev/null   (stdin)
# 1 -> /dev/null   (stdout)
# 2 -> /dev/console (stderr)

# Memory map — all virtual memory regions
cat /proc/1/maps
# address           perms offset dev inode  pathname
# 7f8a12000000-7f8a16000000 r--p ... /lib/x86_64/libc.so.6

# Environment variables (NUL-separated)
cat /proc/1/environ | tr '\0' '\n'

# Namespaces this process belongs to
ls -la /proc/1/ns/

System-Wide Information

File	What it shows
/proc/cpuinfo	CPU model, cores, flags, clock speed
/proc/meminfo	RAM: total, free, available, cached, swap
/proc/loadavg	1/5/15 min load average + running/total processes
/proc/uptime	Seconds since boot, seconds idle
/proc/stat	CPU time breakdown, context switches, interrupts, boot time
/proc/diskstats	Per-disk I/O counters (reads, writes, time spent)
/proc/net/dev	Per-interface network stats
/proc/interrupts	Per-CPU interrupt counts by IRQ

/proc/sys — Kernel Tuning

How do you tune kernel parameters? By reading and writing files in /proc/sys/. The sysctl command is just a friendly interface to these files.

# Read a kernel parameter
cat /proc/sys/net/ipv4/ip_forward   # 0 or 1

# Change it (takes effect immediately, not persistent)
echo 1 > /proc/sys/net/ipv4/ip_forward

# Same thing with sysctl
sysctl net.ipv4.ip_forward          # read
sysctl -w net.ipv4.ip_forward=1    # write

# Make persistent across reboots
echo "net.ipv4.ip_forward = 1" >> /etc/sysctl.conf
sysctl -p   # apply sysctl.conf

Key tunable areas: /proc/sys/vm/ (memory behavior), /proc/sys/net/ (networking), /proc/sys/kernel/ (scheduler, core dumps, panic behavior).