TopolearnTools like ps, top and free are just readers of /proc; going direct answers questions they cannot.
/proc is a virtual filesystem where the kernel publishes live information as text files. Nothing is stored on disk; each read is generated on demand.
System-wide files: /proc/cpuinfo, /proc/meminfo, /proc/loadavg, /proc/uptime, /proc/mounts, /proc/cmdline (kernel boot parameters) and /proc/pressure/* (pressure stall information: how long tasks waited for CPU, memory or I/O).
Per-process directories /proc/<PID>/ (and /proc/self for the reader):
cmdline(arguments separated by NUL bytes),environ,cwdandexe(symlinks),status(state, memory such asVmRSS, threads, UIDs),limits(ulimits),fd/(one symlink per open file descriptor, including sockets),cgroup,ns/(namespaces).
/sys (sysfs) exposes the device and driver model: network interfaces in /sys/class/net, block devices in /sys/block, CPU frequency, power settings and the cgroup tree in /sys/fs/cgroup.
Kernel tunables live in /proc/sys and are managed with sysctl: sysctl net.ipv4.ip_forward reads one, sysctl -w changes it until reboot, and files in /etc/sysctl.d/*.conf make settings permanent (sysctl --system applies them). Common examples: enabling IP forwarding for routers and containers, vm.swappiness, fs.inotify.max_user_watches for file watchers, and network buffer sizes. Change tunables deliberately and document why.
Example
grep -m1 'model name' /proc/cpuinfo
grep -E 'MemTotal|MemAvailable' /proc/meminfo
cat /proc/loadavg # 0.15 0.10 0.05 1/312 4567
tr '\0' ' ' < /proc/$$/cmdline; echo # this shell's command line
readlink /proc/$$/cwd # its working directory
grep -E '^(State|VmRSS|Threads)' /proc/$$/status
ls -l /proc/$$/fd | tail -n +2 | head -n 3 # open file descriptors
cat /sys/class/net/lo/mtu # 65536
cat /proc/sys/net/ipv4/ip_forward # 0 or 1, same as: sysctl net.ipv4.ip_forward
# sudo sysctl -w vm.swappiness=10 # until reboot
# echo 'vm.swappiness = 10' | sudo tee /etc/sysctl.d/99-local.conf # permanent
# sudo sysctl --system # apply all filesRemember
- /proc/<PID>/ holds cmdline, status, fd, cwd and more for each process
- /sys exposes devices, drivers and cgroups
- sysctl -w for now, /etc/sysctl.d/*.conf for permanent
Watch out for
- Changing a tunable with sysctl -w and losing it at the next reboot.