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EN_Linux
A summary of Linux concepts for DevOps engineers. Click each link for full details.
- Monolithic Kernel: Kernel structure where filesystem, drivers, and memory management are compiled into a single binary (Linux)
- Kernel Space / User Space: Memory area where the kernel runs (direct hardware access) / restricted area for user applications
- System Call: Interface for user-space apps to request kernel services (open, read, write, fork)
- bash / zsh / sh: Most widely used shell / enhanced autocomplete and plugins / POSIX standard
- TTY: Physical or virtual terminal device
- ext4: Default Linux journaling filesystem
- XFS: High-performance filesystem optimized for large files
- inode: Data structure storing file metadata (permissions, owner, size, timestamps)
- fstab: Auto-mount configuration at boot (/etc/fstab)
- Page Cache: Kernel cache that keeps file contents read from disk in memory
- APT: Debian-based package management (apt install / apt update)
- DNF/YUM: Red Hat-based package management (dnf install / yum install)
- RPM / DEB: Red Hat / Debian package formats
- PID / PPID: Unique process number / parent process number
- Daemon: Process running in the background (sshd, httpd)
- Zombie / Orphan: Process that exited but parent hasn't collected / process whose parent exited first
- Nice Value: Process scheduling priority (-20 to 19, lower = higher priority)
- Copy-on-Write: Defers copying until someone actually writes, then copies only then
- /etc/passwd / /etc/shadow: User account info / encrypted password storage
- chmod / chown / umask: Change file permissions / change owner / default permission mask for new files
- sudo / sudoers: Run commands as root / sudo permission configuration
- systemd: Init system and service manager in modern Linux distributions
- systemctl: Command to control systemd services
- Unit: Entity managed by systemd (Service, Socket, Timer, etc.)
- Target: systemd's equivalent of runlevels (multi-user.target, graphical.target)
- SELinux: Mandatory Access Control (MAC) security module for Red Hat-based systems
- AppArmor: MAC security framework for Debian/Ubuntu systems
- firewalld / UFW: Dynamic firewall wrapping iptables (RHEL) / simple firewall tool (Ubuntu)
- ip / ss: Network interface and routing tool (iproute2) / socket statistics (replaces netstat)
- nmcli: NetworkManager CLI tool
- /etc/hosts / /etc/resolv.conf: Static hostname mapping / DNS configuration
- conntrack: Kernel table that records the state of every connection passing through (basis for NAT and stateful firewalls)
Covers the Linux monolithic kernel architecture compared with a microkernel, the separation of user space and kernel space, and how Red Hat-based (RHEL, CentOS, Fedora) and Debian-based (Ubuntu, Debian) distributions differ in package management, service management, and directory structure.
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Covers file permissions (chmod, chown, umask) and user management (/etc/passwd, /etc/shadow, /etc/skel), systemd service management (systemctl, units, targets), network commands (ip, ss, nmcli), and process monitoring (ps, top, htop, kill, nice), with a focus on day-to-day practice.
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Covers kernel module management (lsmod, modprobe), SELinux/AppArmor security settings, and kernel parameter tuning with sysctl (network performance, security, memory, file systems, and Kubernetes/Docker environments), with practical examples.
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