Linux

Linux is a free, Unix-like operating system kernel that powers most servers, embedded devices, and supercomputers. Daily work happens in a shell — usually bash or zsh — and through systemd-managed services.

Overview

A “Linux system” bundles the kernel with userland tools (GNU coreutils, BusyBox), a service manager (systemd, OpenRC), a package manager, and optional desktop. Distros differ mostly in package manager, release cadence, and defaults.

Filesystem & Layout

• /etc — config files.
• /var — logs, caches, spool.
• /usr — installed binaries & libraries.
• /home — user data.
• /opt — third-party / vendor software.
• /proc, /sys — kernel and device interfaces.
• /tmp — ephemeral, cleared on reboot.

Essential Commands

• Navigation: ls, cd, pwd, find, tree.
• Files: cp, mv, rm, ln -s, chmod, chown.
• Inspection: cat, less, head, tail -f, grep, awk, sed, jq.
• Processes: ps, top, htop, kill, nice, renice.
• Network: ip a, ss -tlnp, ping, curl, dig, nmap.
• Storage: df -h, du -sh, lsblk, mount, fdisk, parted.
• Archives: tar czf, tar xzf, zstd, gzip.

Services & systemd

• systemctl status / start / stop / restart / enable / disable name.
• journalctl -u name -f — follow logs.
• Unit files live in /etc/systemd/system/.
• Timers replace most cron jobs.
• systemd-analyze blame finds slow boot units.

Users & Permissions

• chmod 755 file — owner rwx, group rx, world rx.
• Setuid/setgid/sticky bits (4/2/1) — special permissions.
• sudo -i, su - — elevate privileges.
• useradd, groupadd, usermod -aG group user.
• SSH keys in ~/.ssh/authorized_keys beat passwords.

Distros

• Debian / Ubuntu — apt; widely deployed.
• RHEL / Rocky / AlmaLinux / Fedora — dnf; enterprise.
• Arch — pacman; rolling.
• Alpine — apk; small; popular in containers.
• Yocto / Buildroot — custom embedded builds.