LinuC Level 2 Exam 202 — knowledge map

The 156 core concepts of LinuC Level 2 Exam 202 and how they connect. Click a node in the map above to explore related terms and prerequisites; the list below indexes every concept with its definition and links to its prerequisites and related concepts.

Concepts (156)

• Locating commands (which/whereis/type)

  Three ways to locate a command: which finds the executable on PATH, whereis adds man pages and source, and type also detects aliases and shell builtins (type cd -> shell builtin).

  Prerequisites: Alias / shell builtin、locate / updatedb、man (manual)、source (dot command)

• Shell (bash)

  The interactive environment that interprets user commands and passes them to the kernel; the Linux default is bash. Its mechanics (variables, quoting, history, path resolution) underpin scripting and text processing.

  Prerequisites: Command history (history)、Quoting (single / double / backquote)

• Kernel

  The core of the OS; it manages hardware (CPU, memory, devices) and provides services to processes. VMs each have their own kernel, while containers share the host kernel.

• Packages and repositories

  Linux software is managed as packages fetched from distribution servers (repositories); a package manager auto-resolves dependencies and handles install/update/remove centrally, working from a local index (package catalog).

  Related: Distribution

• Filesystems and mkfs

  Making a partition usable (formatting) means creating a filesystem, done with the mkfs family. ext4 is the general Linux standard (predecessor ext3), XFS excels at large/parallel workloads (RHEL default), VFAT interoperates with Windows, Btrfs is new-generation, and tmpfs lives in memory and vanishes on reboot.

  Prerequisites: Shutdown and reboot commands (shutdown/reboot/halt/poweroff)

• Distribution

  A packaged form distributing the Linux kernel together with package management, tools, and configuration; the Debian/Ubuntu family (apt) and the RHEL/CentOS family (yum/dnf) differ in management commands and defaults.

  Prerequisites: Kernel

  Related: Packages and repositories

• Filesystem mounting (/etc/fstab)

  Mounting attaches a filesystem at a point in the directory tree (a mount point); use mount/umount (umount fails while busy). Boot-time mounts are defined in /etc/fstab (device, point, type, options, dump, fsck order) and verified with mount -a. Removable media mount under /media.

  Prerequisites: Control flow (if / case / for / while)、Filesystems and mkfs

• Filters (text-processing commands)

  Single-purpose commands that transform stdin to stdout: view with cat/less (pager)/head/tail (-f follows), transform with tr/nl, join with paste/join, split with split, format with expand/unexpand/fmt/pr, dump binary with od. Combined via pipes.

  Prerequisites: Pipe (|)、Standard streams (stdin / stdout / stderr)

• iptables / firewalld (firewall)

  A host packet filter: iptables drives the kernel netfilter directly via rule chains (INPUT/OUTPUT/FORWARD); firewalld manages by zones and service names (rules vanish on reboot without --permanent). The policy is default-deny, allowing only what is needed.

  Prerequisites: Filters (text-processing commands)、Kernel、Shutdown and reboot commands (shutdown/reboot/halt/poweroff)

  Related: Firewall

• Web server

  A server that accepts client requests over HTTP/HTTPS, returning static content or forwarding dynamic requests to an application server. Apache and nginx are common examples.

  Related: Application server (AP server)、Apache HTTP Server

• Apache HTTP Server

  A widely used Linux web server (httpd / apache2). It is configured via httpd.conf for virtual hosts, authentication, and access control, and controlled with apachectl / apache2ctl.

  Related: Virtual host、Web server

• IP address (IPv4 / IPv6)

  A number identifying a host on a network: IPv4 is 32 bits in dotted quads (192.168.1.10), IPv6 is 128 bits in colon-hex, structurally solving address exhaustion; an fe80:: link-local address auto-configures for on-link traffic.

• Redundancy

  Providing multiple components with the same role so the overall service continues even if one fails. A basic building block of HA configurations, realized in combination with clustering and replication.

  Prerequisites: Cluster、Replication

  Related: HA (high-availability) configuration

• systemd / systemctl

  The modern Linux init and service manager; it runs as PID 1 and starts services (units) in parallel by dependency (successor to legacy SysV init). systemctl is its control command (start/stop/enable/disable/status).

• Access log

  A log recording requests a web server has received (source, timestamp, URL, status code, etc.). Apache writes it to access_log by default.

  Prerequisites: Apache HTTP Server、source (dot command)、Timestamp、Web server

• Control flow (if / case / for / while)

  Branching and looping in scripts: if…fi is a conditional ([ ] is test), case…esac branches a value by patterns, and for/while…done loop. The key is not to confuse the closers (fi/esac/done).

• find

  Walks a path in real time to find matching files: filter with -name, -type (f/d), -size, -mtime (modified; -7 within 7 days), -user, and act in bulk with -exec command {} \;.

  Prerequisites: Filters (text-processing commands)、PATH、Locating commands (which/whereis/type)

• Firewall

  A mechanism that controls traffic according to allow/deny rules. On Linux it is implemented via iptables / firewalld, and in cloud environments it is offered as constructs like security groups.

  Related: iptables / firewalld (firewall)

• HA (high-availability) configuration

  A configuration designed to minimize downtime of servers or services, combining redundancy, clustering, and load balancing to eliminate single points of failure (SPoF) and raise availability.

  Prerequisites: Cluster、Load balancing (load balancer)

  Related: Redundancy

• NAT (Network Address Translation)

  A mechanism that translates private IP addresses to public IP addresses and back, running on a router or gateway so many internal hosts can share a small number (or just one) of public IPs to reach the internet (NAPT / IP masquerading). On Linux this is implemented via the iptables MASQUERADE target, among others.

  Prerequisites: iptables / firewalld (firewall)、IP address (IPv4 / IPv6)、Private / public IP address、systemd targets (default target / single-user mode)

• Persisting jobs after disconnect (nohup/screen/tmux)

  Ways to keep work running after logout or a dropped SSH: nohup ignores hangup (SIGHUP); screen/tmux are terminal multiplexers you can detach and reattach.

  Prerequisites: Session (stateless architecture)、Signals and terminating processes (kill / pkill / killall / pgrep)

• PATH

  An environment variable listing directories searched for command executables; the current directory is not included by convention, so run local scripts as ./script.sh (relative) or by absolute path.

  Prerequisites: Shell and environment variables

  Related: Absolute and relative paths

• Permissions and ownership

  Access control per file for three classes (owner, owning group, others) with read (r), write (w), execute (x); shown in the first 10 chars of ls -l. Change the owner with chown and the group with chgrp.

• Resilience

  The property of a system to keep functioning, or to recover quickly, even when a failure occurs; strengthened through redundancy, clustering, and geographic distribution.

  Prerequisites: Cluster、Distribution、Redundancy

  Related: Geographic distribution

• Application server (AP server)

  The middle tier of the three-tier web model, running application/business logic. It receives requests from the web server and exchanges data with the DB server to produce results.

  Related: Three-tier web model、Web server、Database server (DB server)

• Asynchronous data processing

  A pattern in which time-consuming work is handed off via a queue or messaging system to run later, rather than making the requester wait for a response; it decouples processing and improves throughput.

  Prerequisites: Locating commands (which/whereis/type)

  Related: Messaging system、Queue (message queue)

• Cluster

  A configuration of multiple servers working in coordination as a single system, built with tools such as Pacemaker and Corosync, raising availability through failover on failure.

• Console / terminal emulator

  The text screen for operating the shell; you enter commands via a physical text console or a terminal emulator that provides it on the desktop.

  Prerequisites: Persisting jobs after disconnect (nohup/screen/tmux)、Shell (bash)

• Fixed IP

  An IP address bound to a specific instance (server) that does not change. This contrasts with a floating IP, which can be carried over even when the underlying instance is replaced.

  Prerequisites: IP address (IPv4 / IPv6)、Locating commands (which/whereis/type)

  Related: Floating IP

• Floating IP

  A public IP address that can be dynamically reassigned between instances, enabling fast failover by moving it to a healthy instance when one fails. This contrasts with a fixed IP, which stays bound to a specific instance.

  Prerequisites: IP address (IPv4 / IPv6)、Locating commands (which/whereis/type)

  Related: Fixed IP

• Hard and symbolic links (ln)

  Multiple names for one entity: a hard link (ln) is an equal name for the same inode—same filesystem only, no directories, and data survives deletion. A symbolic link (ln -s) is a file holding a path—it can cross filesystems and point at directories, but dangles if the target is removed.

  Prerequisites: Filesystems and mkfs、PATH

  Related: inode

• Queue (message queue)

  A mechanism that temporarily holds messages or tasks awaiting processing; it absorbs speed differences between sender and receiver and underpins asynchronous data processing.

  Related: Asynchronous data processing

• Reverse proxy

  A proxy positioned on the server side that receives client requests and relays them to one or more backend servers; used for load balancing, SSL termination, and caching. Its direction is the opposite of a forward proxy (used on the client side).

  Prerequisites: Load balancing (load balancer)

  Related: Forward proxy

• Session (stateless architecture)

  The state of an ongoing interaction with a user. Making servers "stateless" (not holding session data locally) means any server can handle a given request, making the system easier to scale out.

  Prerequisites: Scale-out

• Shutdown and reboot commands (shutdown/reboot/halt/poweroff)

  Commands to safely stop or restart the system. shutdown is the canonical one, supporting scheduling (-h halt / -r reboot) and warnings to logged-in users; halt stops, reboot restarts, poweroff powers off.

• source (dot command)

  The . (dot) and source commands read a script into the current shell without creating a new process; used to apply config files (e.g., ~/.bashrc) without re-login.

  Prerequisites: Shell (bash)

• Timestamp

  The last-modified/accessed/changed time of a file; touch updates it, and find -mtime can search by modification date.

  Prerequisites: System and hardware clocks (date / hwclock)、Basic file operations (cp / mv / rm / mkdir)、find、Login status (who/w/last)

• Alias / shell builtin

  An alias is an alternate name for a command (alias ll='ls -l') that shadows a同名 command; a shell builtin (cd, echo) is built into the shell itself with no external executable.

  Prerequisites: Shell (bash)

• Boot loader (GRUB2)

  A program that selects and loads the kernel after firmware; the Linux standard is GRUB2, which offers a menu of kernels and boot options.

  Prerequisites: Kernel、Locating commands (which/whereis/type)

• Database server (DB server)

  The backend tier of the three-tier web model, responsible for persisting and managing data; it reads and writes data in response to queries from the AP server.

  Related: Three-tier web model、Application server (AP server)

• Default route (default gateway)

  The exit for packets destined outside your network; check with ip route (or route -n)—without it nothing leaves the local segment. Fix a wrong routing table with ip route del/add default.

  Prerequisites: Exit status ($? / exit)、Network configuration (ip / ifconfig / nmcli)

• Default editor (EDITOR / nano / emacs)

  Which editor crontab -e and similar launch is set by the EDITOR variable (export EDITOR=nano). nano shows keys on-screen (beginner-friendly), and emacs is another major editor.

  Prerequisites: cron / crontab (periodic scheduling)、Persisting jobs after disconnect (nohup/screen/tmux)、Shell and environment variables、Locating commands (which/whereis/type)

• Error log

  A log recording a web server startup/configuration errors or problems during request handling. Apache writes it to error_log and it is usually the first clue when troubleshooting.

  Prerequisites: Apache HTTP Server、Web server

• Failure pattern

  The categories of failure anticipated in a system (physical failures, logical failures, breakdown of a single point of failure, etc.). HA design considers redundancy so service continues under any of these failure patterns.

  Prerequisites: HA (high-availability) configuration、Redundancy

• Forward proxy

  A proxy positioned on the client side that sends requests to external servers on the client's behalf; used to anonymize clients or to centrally manage and filter outbound traffic from an organization. Its direction is the opposite of a reverse proxy (used on the server side).

  Prerequisites: Filters (text-processing commands)

  Related: Reverse proxy

• Geographic distribution

  Distributing servers or data centers across multiple geographic regions so service can continue even during a wide-area outage such as a regional disaster or power failure; one way to increase resilience.

  Prerequisites: Distribution

  Related: Resilience

• systemd journal (journalctl)

  Binary-format logs collected by systemd, queried with journalctl (-u per unit, -b this boot, -f follow, -p severity, --since time range). It persists if /var/log/journal/ exists, otherwise memory-only (lost on reboot). Configure via journald.conf; trim old data with --vacuum-*.

  Prerequisites: Shutdown and reboot commands (shutdown/reboot/halt/poweroff)、systemd / systemctl

• Load balancing (load balancer)

  A mechanism (or the device/software implementing it) that distributes requests across multiple servers, avoiding concentration on one server and improving availability and scalability. DNS round-robin is one simple way to achieve it.

• Messaging system

  Infrastructure for exchanging messages (events or tasks) between applications; it uses queues to loosely couple sender and receiver, enabling asynchronous data processing.

  Prerequisites: Queue (message queue)

  Related: Asynchronous data processing

• MTA and mail forwarding (aliases / .forward)

  An MTA (postfix, exim; SMTP port 25) receives and delivers mail. System-wide forwarding goes in /etc/aliases and must be applied with newaliases after editing; per-user forwarding uses ~/.forward. Send with mail; check the pending queue with mailq.

  Prerequisites: Queue (message queue)

  Related: SMTP protocol

• OSS community and ecosystem

  The distributed development that sustains OSS: worldwide contributors send patches/pull requests to maintainers (the final gatekeepers). Communication happens on mailing lists (e.g., LKML), forums, and development sites (issue trackers). Distributions integrate upstream projects and deliver them to users.

  Prerequisites: Distribution

• Pipe (|)

  Connects the left command's stdout to the right command's stdin (ps aux | grep nginx); stderr does not enter the pipe (add 2>&1 first to include it).

  Prerequisites: Text search and extract commands (grep/egrep/fgrep/sed)、Redirection (> / >> / 2>&1)

  Related: Standard streams (stdin / stdout / stderr)

• Proxy server

  A general term for a server that sits between clients and servers, relaying requests. Depending on placement, it acts as a forward proxy (client side) or a reverse proxy (server side); Squid is a common implementation.

  Prerequisites: Forward proxy、Reverse proxy

• RPO (Recovery Point Objective)

  A target indicating how far back in time data must be recoverable after a failure, determined by backup frequency; a smaller RPO means less tolerable data loss. Distinct from RTO, which is about recovery time.

  Prerequisites: Filters (text-processing commands)、Locating commands (which/whereis/type)

  Related: RTO (Recovery Time Objective)

• Resource record (RR)

  The individual configuration entries written in a DNS zone file. Types include SOA (zone authority info), NS (name server), A/AAAA (IPv4/IPv6 address), MX (mail server), CNAME (alias), and PTR (reverse lookup).

  Prerequisites: Alias / shell builtin、IP address (IPv4 / IPv6)

  Related: Zone file

• Scale-out

  Increasing processing capacity by adding more servers—horizontal scaling, as opposed to scale-up (vertical scaling), which boosts a single server's specs. It pairs well with stateless servers lined up behind a load balancer.

  Prerequisites: Locating commands (which/whereis/type)

• Shell and environment variables

  A shell variable is set with NAME=value (no spaces around =) and is valid only in that shell; export makes it an environment variable inherited by child processes. List with set (all) / env (environment only), remove with unset, print with echo.

  Prerequisites: Shell (bash)

  Related: Child process

• SMTP protocol

  The protocol used to send mail between servers and from clients to servers (TCP port 25, etc.). MTAs such as Postfix use this protocol to deliver mail.

  Related: MTA and mail forwarding (aliases / .forward)

• Standard streams (stdin / stdout / stderr)

  The three streams every command has: standard input (0), standard output (1), standard error (2). Rewiring them with redirection and pipes is the basis of Linux text processing.

  Related: Pipe (|)、Redirection (> / >> / 2>&1)

• Virtual host

  A mechanism for hosting multiple domains (sites) on a single web server. Apache or nginx separates configuration per domain, IP, or port so each responds as an independent site.

  Prerequisites: Web server

  Related: Apache HTTP Server

• Virtual machine (VM)

  A virtual computer running on a hypervisor with its own independent guest OS (including a kernel); it can run a different OS from the host but boots slower and uses more resources.

  Prerequisites: Hypervisor、Kernel

• Login status (who/w/last)

  Commands to see who is logged in: who shows current logins, w adds activity and load, and last shows login history from /var/log/wtmp.

  Related: Command history (history)

• Zone transfer

  The mechanism by which a primary (master) DNS server replicates zone-file contents to a secondary (slave) DNS server, keeping the secondary up to date with the latest zone information.

  Prerequisites: System and hardware clocks (date / hwclock)、Locating commands (which/whereis/type)、Zone file

• Autoscaler

  A mechanism that automatically increases or decreases the number of servers in response to load; provided as a cloud service feature to automate scale-out and scale-in.

  Prerequisites: Scale-in

• Child process

  A process started by another (its parent). When the shell runs a command it forks a child, and only environment variables are inherited (the reason export is needed).

  Prerequisites: Shell (bash)

  Related: Shell and environment variables

• chmod / umask

  chmod changes permissions: octal mode sums r=4, w=2, x=1 per triplet (chmod 754); symbolic mode uses u/g/o/a with +/-/=. umask sets default bits removed at creation—files start from 666, directories from 777 (umask 022 gives 644/755).

  Prerequisites: Octal (permission notation)、Permissions and ownership

• System and hardware clocks (date / hwclock)

  Linux keeps two clocks: the OS-maintained system clock (date/timedatectl) and the motherboard hardware clock (hwclock). hwclock --systohc writes system->hardware; --hctosys is the reverse.

  Prerequisites: Time zone (timedatectl)

• Copyleft and permissive

  Two families of OSS licenses: copyleft requires derivatives you redistribute to carry the same license (source disclosed), a propagating condition; permissive lets you close modifications if you keep the copyright notice. The trigger for obligations is generally "redistribution".

  Prerequisites: source (dot command)

• Dependencies (packages)

  Other packages a package needs to function; apt/yum resolve them automatically and install together, while dpkg/rpm do not and fail when a dependency is missing.

  Prerequisites: Control flow (if / case / for / while)、dpkg (direct deb handling)、Packages and repositories

• ESP (EFI System Partition)

  A FAT-formatted partition holding the boot loader for UEFI boot, mounted at /boot/efi.

  Prerequisites: Boot loader (GRUB2)、Filesystem mounting (/etc/fstab)、UEFI / BIOS

• FHS (Filesystem Hierarchy Standard)

  A convention defining the standard directory layout: /etc config, /var variable data (logs), /home users, /usr apps, /bin,/sbin core commands, /opt add-ons, /tmp temporary, /boot kernels, /dev devices, /proc,/sys virtual FS. Nearly universal across distributions.

  Prerequisites: Distribution、Filesystems and mkfs、Kernel

• Text search and extract commands (grep/egrep/fgrep/sed)

  grep prints lines matching a regex (-i ignore case, -v non-matching, -n line numbers, -r recursive); egrep (grep -E) uses extended regex, fgrep (grep -F) does fixed-string search with no regex. sed is a stream editor, best known for sed 's/old/new/g' search-and-replace.

  Prerequisites: Control flow (if / case / for / while)、Regular expression

• Host key

  The key pair (/etc/ssh/ssh_host_*_key) an SSH server holds to prove its own identity. On connecting, clients check this key's fingerprint against known_hosts to detect impersonation or man-in-the-middle attacks.

  Related: SSH key files (authorized_keys / known_hosts / id_rsa)

• initramfs

  An initial RAM disk containing the drivers the kernel needs to mount the real root filesystem; it bridges early boot.

  Prerequisites: Filesystems and mkfs、Kernel、Filesystem mounting (/etc/fstab)

• KVM

  Virtualization built into the Linux kernel (Kernel-based Virtual Machine), turning Linux itself into a hypervisor.

  Prerequisites: Hypervisor、Kernel、Virtual machine (VM)

• locate / updatedb

  locate searches the index database built by updatedb, so it is fast but blind to files newer than the last index; refresh with updatedb and configure exclusions in /etc/updatedb.conf. When accuracy matters, use find (a live walk).

  Prerequisites: find、Login status (who/w/last)

• Port and process inspection (nmap/lsof/fuser)

  nmap scans ports from outside to see the real exposure; lsof (lsof -i:80) and fuser identify the process holding a port or file. The difference between ss (inside) and nmap (outside) is the firewall effect.

  Prerequisites: Firewall

• Private / public IP address

  Private IP addresses (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) are numbers used only within a limited scope such as an organization and cannot reach the internet directly. They pair with public IP addresses, which are uniquely reachable on the internet, and a router translates between the two to communicate.

  Prerequisites: IP address (IPv4 / IPv6)、Locating commands (which/whereis/type)

• Redirection (> / >> / 2>&1)

  Re-points stream I/O: > overwrites stdout to a file, >> appends, < feeds a file to stdin. Capture both streams with > file 2>&1 (order matters); discard into /dev/null.

  Related: Standard streams (stdin / stdout / stderr)

• Regular expression

  A language for describing string patterns: ^ line start, $ line end, . any one char, [abc] a set, * zero-or-more of the previous. Basic (BRE) and extended (ERE: + ? | ( )) forms exist; the * differs from a shell wildcard (any string is .*). Full grammar in regex(7).

  Prerequisites: Shell (bash)

  Related: Wildcards (globbing)

• Replication

  A mechanism that copies data (e.g., of a database) from a master to one or more replicas; used to distribute read load and to serve as a failover target if the master fails.

• Scale-in

  Reducing the number of servers when load drops, to cut wasted cost—the counterpart operation to scale-out. In autoscaling, scale-in triggers automatically once a metric (e.g., CPU utilization) falls below a threshold.

• Scale-up / scale-down

  Adjusting processing capacity by increasing or decreasing an individual server's specs (CPU, memory, etc.): scale-up raises capacity, scale-down lowers it. This contrasts with scale-out/in, which changes the number of servers.

  Prerequisites: Locating commands (which/whereis/type)

• Shell scripts and the shebang

  Automating a series of commands in a file; the first-line shebang #!/bin/bash names the interpreter, and chmod +x grants execute permission to run it as ./script.sh.

  Prerequisites: chmod / umask、Permissions and ownership、Shell (bash)

• showmount (list NFS exports)

  A command to check which directories an NFS server exports and which clients have them mounted. `showmount -e servername` lists the exported directories on the target server.

  Prerequisites: Filesystem mounting (/etc/fstab)、Locating commands (which/whereis/type)

• Signals and terminating processes (kill / pkill / killall / pgrep)

  Signals convey control to processes: the default SIGTERM (15) politely asks to terminate, SIGKILL (9) force-kills uncatchably, SIGHUP (1) means hangup/reload. kill takes a PID, pkill/killall take a name, pgrep finds PIDs by name.

• Time zone (timedatectl)

  Regional time settings; the data lives under /usr/share/zoneinfo/, configured by symlinking /etc/localtime to one. On systemd, timedatectl set-timezone Asia/Tokyo persists it, the TZ variable switches temporarily, and tzselect picks interactively.

  Prerequisites: Hard and symbolic links (ln)、systemd / systemctl

• UEFI / BIOS

  Firmware that runs right after power-on; it initializes hardware (POST self-test), selects and prioritizes the boot device, and loads the boot loader. UEFI is the successor to BIOS, supporting GPT disks and large capacities.

  Prerequisites: Boot loader (GRUB2)

• Virtual filesystems (/proc, /sys)

  Information windows with no on-disk存在, generated in memory by the kernel: /proc exposes process/kernel state (e.g., /proc/cpuinfo) and /sys (sysfs) the device tree.

  Prerequisites: Filesystems and mkfs、Kernel、Device management (udev/sysfs/D-Bus)

• Virtual machine image

  An image file packaging an OS and applications in a pre-configured state. Virtual machines with identical configuration can be quickly cloned and started from it, supporting scale-out and machine reprovisioning.

  Prerequisites: Packages and repositories、Virtual machine (VM)

• Three-tier web model

  A system architecture that splits roles across three tiers: web server, application server, and database server. Each tier can scale independently, and the impact of changes stays contained within a tier.

  Prerequisites: Web server

  Related: Application server (AP server)、Database server (DB server)

• Remote GUI (DISPLAY / xauth / X11 forwarding)

  Displaying a remote GUI app on your local screen: the DISPLAY variable points to the target X server and xauth manages authorization. In practice, ssh -X (X11 forwarding) shows it over the SSH tunnel.

  Prerequisites: Persisting jobs after disconnect (nohup/screen/tmux)

  Related: X Window System (X server / X client)

• xargs / tee

  xargs turns incoming text into arguments for the next command (find … | xargs rm)—a bridge to commands that take arguments, not stdin. tee duplicates a stream to both the screen and a file.

  Prerequisites: find、Persisting jobs after disconnect (nohup/screen/tmux)、Standard streams (stdin / stdout / stderr)

• X Window System (X server / X client)

  The server/client foundation of the Linux GUI: the X server runs where the display/input are and provides drawing and input, while X clients are the applications. Where an app runs and where it displays can differ.

  Prerequisites: Control flow (if / case / for / while)

  Related: Remote GUI (DISPLAY / xauth / X11 forwarding)

• apt (Debian family)

  Repository-based package management on Debian/Ubuntu: apt update refreshes the index (not packages), apt upgrade upgrades packages, plus install/remove/purge. apt-cache searches, apt-file reverse-looks-up which package provides a file (including uninstalled). Repositories are defined in /etc/apt/sources.list.

  Prerequisites: Packages and repositories、Locating commands (which/whereis/type)

• Operating Linux in the cloud

  For cloud Linux servers, the provider controls everything outside the OS (power, network entry, physical placement). The management console is internet-reachable, so protect it with multi-factor authentication / one-time passwords (a leak loses every server). Region choice ties to regulation and data sovereignty; ephemeral storage vanishes on stop so keep no durable data there. Design for auto-recovery against provider maintenance (reboots).

  Prerequisites: Console / terminal emulator、X Window System (X server / X client)

• DNS client config (host / dig / resolv.conf)

  Configuring and checking name resolution: nameserver in /etc/resolv.conf sets the DNS servers, /etc/nsswitch.conf the lookup order (usually files->dns), and /etc/hosts holds static entries. host/dig query DNS directly (dig selects a server with @), while getent resolves following nsswitch order (including /etc/hosts).

  Prerequisites: Control flow (if / case / for / while)

• dpkg (direct deb handling)

  The low-level tool beneath apt, handling deb files directly (-i install, -r remove, -P purge). It resolves no dependencies. Query with -l list, -L package->files, -S file->package. dpkg-reconfigure re-runs a package interactive setup.

  Prerequisites: Packages and repositories

• Exit status ($? / exit)

  A number indicating a command result: $? holds the last exit status (0 = success, non-zero = failure), and exit N sets a script own status. It underpins branching with && and ||.

  Prerequisites: Login status (who/w/last)

• Basic file operations (cp / mv / rm / mkdir)

  Core commands to manage files/directories: cp copies, mv moves (renames), rm deletes, mkdir/rmdir create/remove (empty) directories, touch creates empty files or updates timestamps, file identifies type. Directory-wide operations need -r (recursive).

  Prerequisites: Locating commands (which/whereis/type)

• Command history (history)

  A record of previously run commands: history lists them, !number re-runs one, !! repeats the last. Saved to ~/.bash_history on logout.

  Related: Login status (who/w/last)

• Hypervisor

  Software that virtualizes a physical machine CPU/memory/disk and shares them among virtual machines; it uses CPU virtualization extensions (Intel VT-x / AMD-V) for practical speed. On Linux, KVM plays this role.

• inode

  The metadata record managing a file (size, owner, permissions, data location); a filename is just a label on an inode. ls -i shows inode numbers—matching numbers mean hard links.

  Prerequisites: Permissions and ownership

  Related: Hard and symbolic links (ln)

• Kernel modules (modprobe)

  A way to load drivers into the kernel only when needed: lsmod lists loaded modules, modprobe loads/unloads with dependencies (the first choice), and insmod/rmmod load/unload a single module by file path.

  Prerequisites: Kernel、PATH

• Network configuration (ip / ifconfig / nmcli)

  Viewing/changing host networking: the current standard is the ip command (ip addr for addresses, ip route for routes), legacy is ifconfig/route. ip/ifconfig changes are temporary and vanish on reboot; persist via NetworkManager nmcli or config files. ifup/ifdown bring interfaces up/down from config. The hostname is in /etc/hostname.

  Prerequisites: Shutdown and reboot commands (shutdown/reboot/halt/poweroff)

• Octal (permission notation)

  A base-8 numbering system using digits 0-7; Linux permissions are written as three octal digits summing r=4, w=2, x=1 (rwxr-xr-- is 754).

  Prerequisites: Permissions and ownership

• Partition (MBR / GPT)

  The unit into which a disk is divided; the ledger is the partition table, either legacy MBR (2 TB max, four primary partitions with extended as the workaround) or the mainstream GPT (practically unlimited, paired with UEFI).

  Prerequisites: UEFI / BIOS、Locating commands (which/whereis/type)

• Permissive licenses (BSD/MIT/Apache/MPL)

  Permissive licenses and a middle ground: BSD and MIT allow closing modifications with minimal terms; Apache License 2.0 adds explicit patent grants; MPL is file-level copyleft (only modified files disclosed). The public domain claims no copyright.

  Prerequisites: Apache HTTP Server、Copyleft and permissive

• Port redirection

  A mechanism that forwards traffic destined for one port to a different port or host, implemented via constructs such as the iptables NAT table (DNAT); commonly used to route an externally exposed port to an internal service.

  Prerequisites: iptables / firewalld (firewall)、Redirection (> / >> / 2>&1)

• Quoting (single / double / backquote)

  Single quotes '…' keep everything literal (no expansion), double quotes "…" expand $variables, and backquotes `…` (= $(…)) perform command substitution, replaced by the output.

  Related: Command substitution

• RTO (Recovery Time Objective)

  A target for how much time is allowed to restore service after a failure occurs; a smaller RTO means faster recovery is required. Distinct from RPO, which concerns tolerable data loss.

  Prerequisites: Locating commands (which/whereis/type)

  Related: RPO (Recovery Point Objective)

• SMTP authentication (SASL)

  A mechanism that authenticates the sender with a username and password when sending mail, using the SASL (Simple Authentication and Security Layer) framework to prevent unauthorized relaying (open relay) by third parties.

  Prerequisites: SMTP protocol

• SSH key generation and management (ssh-keygen / ssh-agent)

  ssh-keygen generates a key pair (-t ed25519 is the current best; RSA/ECDSA/Ed25519). A passphrase encrypts the private key file itself. ssh-agent holds decrypted keys in memory, and ssh-add registers them so the passphrase is entered only once per session.

  Prerequisites: Session (stateless architecture)

• Shell startup files (login / non-login shells)

  A login shell (e.g., ssh login) reads /etc/profile then only the first of ~/.bash_profile, ~/.bash_login, ~/.profile; a non-login shell (a new terminal tab) reads /etc/bash.bashrc then ~/.bashrc. Put environment variables in the profile files and aliases/functions in .bashrc; on logout ~/.bash_logout runs.

  Prerequisites: Alias / shell builtin、Shell (bash)

• systemd targets (default target / single-user mode)

  Unit groups representing how far to boot: multi-user.target (CUI server standard), graphical.target (GUI), rescue.target (single-user mode). set-default sets the default from next boot; isolate switches now.

  Prerequisites: systemd / systemctl

• Device management (udev/sysfs/D-Bus)

  udev detects hot-plug events and dynamically creates/names device files in /dev; sysfs (/sys) is the device tree the kernel exposes, and D-Bus is the IPC that notifies desktops and services of events.

  Prerequisites: Kernel

• ulimit / TMOUT

  ulimit caps resources for the shell and its children (-n open files, -u processes, -c core size). TMOUT is the idle seconds before auto-logout, set in a profile as a defense against unattended terminals.

  Prerequisites: Child process、Shell (bash)

• UUID / label (blkid)

  Robust ways to identify a filesystem: using UUID= or a label in /etc/fstab survives device-order changes (e.g., /dev/sdb becoming /dev/sdc). Inspect UUIDs with blkid.

  Prerequisites: Filesystems and mkfs、Filesystem mounting (/etc/fstab)

• Wildcards (globbing)

  Shell expansion of filenames by pattern: * matches any run of characters, ? exactly one, [a-c] a set/range. Note the * differs in meaning from a regex *.

  Prerequisites: Shell (bash)

  Related: Regular expression

• Zone file

  A text file listing the resource records (SOA, NS, A, MX, etc.) belonging to a DNS zone. In BIND, named serves domain name-resolution information based on this file.

  Related: Resource record (RR)

• Absolute and relative paths

  An absolute path starts from / (the full location, /etc/hosts); a relative path is from the current location (./script.sh, ../dir). Check the current directory with pwd.

  Related: PATH

• at / anacron

  at schedules a one-time run (at 23:00, at now + 2 hours; atq lists, atrm cancels; control via at.allow/at.deny). anacron manages daily-or-longer periods by interval and runs jobs missed while powered off after boot (config in /etc/anacrontab).

  Prerequisites: Control flow (if / case / for / while)

• Participating in the ecosystem (bug reports)

  Participation can be graduated: use (itself a form of testing) -> advocacy -> bug reports -> docs/translation/patches. A good bug report includes environment (OS/version), expected vs actual behavior, and reproduction steps, filed on the issue tracker. You can contribute without writing code.

  Prerequisites: OSS community and ecosystem

• Character encodings and iconv

  Ways to represent characters as numbers: ASCII is the 7-bit base for letters/symbols, Unicode assigns numbers to every script with UTF-8 (ASCII-compatible, the Linux default) as its main encoding; ISO-8859 is for European languages, ISO-2022-JP the Japanese email tradition. Convert with iconv -f from -t to (rescuing mojibake).

• Command substitution

  A notation embedding a command output inline: $(command) or `command` (backquotes), e.g., FILES=$(ls *.log) captures the result into a variable.

  Related: Quoting (single / double / backquote)

• Container (Linux)

  A lightweight runtime that shares the host OS kernel and isolates processes with namespaces; it starts no guest OS so it is fast and light, but can only run the host OS family.

  Prerequisites: Kernel

• cron / crontab (periodic scheduling)

  Runs tasks periodically at set times: edit with crontab -e using the format "minute hour day month weekday command". User jobs live in /var/spool/cron/, system ones in /etc/crontab (with a user column), /etc/cron.d/, and cron.{hourly,daily,weekly,monthly}/. Access control via cron.allow/cron.deny (allow wins if present).

• Shell functions

  A reusable part naming a routine: define with name() { … } or function name { … }; inside, $1 refers to the function own argument.

  Prerequisites: Shell (bash)

• GnuPG (gpg)

  A public-key tool to encrypt, decrypt, and sign files. Encrypt with the recipient public key (gpg -e -r recipient) so only they can decrypt with their private key; sign with your own private key. The keyring is ~/.gnupg/, with gpg-agent managing passphrases.

  Prerequisites: SSH key generation and management (ssh-keygen / ssh-agent)

• Copyleft licenses (GPL/AGPL/LGPL)

  Copyleft licenses: GPL is the flagship, extending disclosure to the linked whole; AGPL extends it to network provision (SaaS); LGPL is the library relaxation where mere linking does not propagate.

  Prerequisites: Copyleft and permissive

• ISO image

  An image format packing an optical disc into a single file; distributed as Linux install media, written to DVD/USB or mounted directly by a VM to boot.

  Prerequisites: Filesystem mounting (/etc/fstab)

• Job control (bg / fg / jobs)

  Running commands in the foreground/background from the shell: trailing & backgrounds, Ctrl+Z suspends, bg resumes in the background, fg brings to the foreground, jobs lists them.

  Prerequisites: Shell (bash)

• Journaling

  A mechanism that records write operations before performing them, keeping the filesystem consistent through power loss; provided by ext3/ext4/XFS.

  Prerequisites: Filesystems and mkfs

• Legacy service management (inetd / xinetd / chkconfig)

  Pre-systemd service management: inetd/xinetd are super-servers that spawn services on demand (config in /etc/xinetd.d/); chkconfig/service are the legacy RHEL autostart and start/stop tools. Stopping unused services shrinks the attack surface.

  Prerequisites: systemd / systemctl

• Load average

  A system-load metric averaging the number of runnable/running processes; uptime and top show three values (1/5/15 minutes) that you compare against the CPU core count to judge overload.

  Related: Process monitoring (top / ps / pstree / uptime)

• Locale (LANG / LC_ALL)

  A combination of language, region, and encoding (ja_JP.UTF-8); precedence is LC_ALL (forces all) > LC_* (per category) > LANG (default). When scripts parse command output, pin LANG=C (English, byte order) to remove locale variation. Inspect with locale (-a lists). This is internationalization (i18n) and localization (l10n) configuration.

  Prerequisites: Character encodings and iconv

• logrotate

  Automates rotation, compression, and deletion of text logs; the global /etc/logrotate.conf plus per-package /etc/logrotate.d/ define rules like "weekly, keep 4, compress", run periodically.

  Prerequisites: Packages and repositories

• man (manual)

  Displays a command online manual (man ls); man -k keyword (= apropos) finds related commands by keyword.

• Open source (OSS)

  Software whose source code is published with guaranteed freedom to use, modify, and redistribute (free as in freedom, not price). Not a waiver of rights—it is a copyrighted work imposing terms via a license, provided without warranty and developed continuously.

  Prerequisites: source (dot command)

• Point-to-point (VPN connection type)

  A VPN connection type linking a single client to a single site (or server) one-to-one, as configured with tools like OpenVPN. It differs from site-to-site, which links multiple sites together.

  Prerequisites: Locating commands (which/whereis/type)

• Account files (/etc/passwd / shadow / group)

  Three files holding account data: /etc/passwd has basic user info (world-readable, no real passwords), /etc/shadow the hashed passwords and aging (root-only), /etc/group the groups and members. Verify with id (UID/GID/groups) or getent (via NSS, including LDAP); set passwords with passwd.

  Prerequisites: DNS client config (host / dig / resolv.conf)

• ping / traceroute (reachability and path)

  Tools to isolate network faults by layer: ping (ping6) checks reachability via ICMP, and traceroute/tracepath show each router hop to pinpoint where traffic stops; tracepath needs no privileges and probes MTU.

  Prerequisites: PATH

• Package group

  A bundle of packages selectable by purpose at install time (e.g., a minimal server set, a desktop environment); can be added later as a unit.

  Prerequisites: Packages and repositories

• SSH port forwarding

  Tunneling other traffic through an encrypted SSH tunnel: local forwarding ssh -L local-port:target-host:target-port reaches an internal server (e.g., a DB) behind a firewall via a bastion; remote forwarding (-R) goes the other way.

  Prerequisites: Firewall

• Process / PID

  A process is a running instance of a program, identified by a unique PID. The first to start, PID 1, is systemd (init).

  Prerequisites: systemd / systemctl

• Process monitoring (top / ps / pstree / uptime)

  Commands to inspect running processes and load: top shows live CPU/memory leaders, ps a snapshot (ps aux), pstree the parent-child tree, uptime the uptime and load average.

  Related: Load average

• rpm (direct RPM handling)

  The query-rich low-level tool beneath yum: -qa list all, -qi info, -qR requires, -ql package->files, -qf file->package, -V verify tampering, --checksig signature. Add -p to inspect an uninstalled rpm file. It resolves no dependencies.

  Prerequisites: Packages and repositories

• Script debugging (bash -x)

  Ways to diagnose a misbehaving script: bash -x prints each executed command after variable expansion (an execution trace), and bash -v echoes lines as read.

  Prerequisites: Shell (bash)

• Socket and port inspection (ss/netstat/netcat)

  Inspecting sockets and ports: ss lists sockets (netstat successor; ss -tlnp shows TCP LISTEN with the process), and netcat (nc) tests connections to any port (nc -zv host 443)—separating firewall drops from missing services.

  Prerequisites: Firewall

• SSH key files (authorized_keys / known_hosts / id_rsa)

  Three files for SSH public-key authentication. The private key id_rsa and public key id_rsa.pub are created on the client; the public key is registered in the server authorized_keys. The client known_hosts records the server host key fingerprint and warns of a possible man-in-the-middle attack if it changes.

  Related: Host key

• HDD / SSD

  Mass storage: an HDD is a magnetic disk—cheap and large but with moving parts and slower; an SSD uses flash memory—fast and shock-resistant but with limited write cycles. Optical storage (DVDs) is for distribution/archival.

  Prerequisites: Distribution

• Special permission bits (SUID/SGID/sticky bit)

  Special permission bits: SUID (4000) runs as the file owner (e.g., passwd); SGID (2000) runs as the file group for executables, and on a directory makes new files inherit its group; the sticky bit (1000) lets only a file owner delete it within a directory (e.g., /tmp).

  Prerequisites: Permissions and ownership

• virsh

  A CLI to manage virtual machines via libvirt; virsh start / virsh shutdown start and stop VMs.

  Prerequisites: Shutdown and reboot commands (shutdown/reboot/halt/poweroff)

• Well-known ports

  Default port numbers assigned to major services: 22 SSH, 25 SMTP, 53 DNS, 80 HTTP, 123 NTP, 443 HTTPS.

  Prerequisites: SMTP protocol

• X management layers (display manager/window manager/desktop environment)

  The three X management layers: the display manager (e.g., GDM) provides the graphical login, the window manager handles window frames/movement/stacking, and the desktop environment (GNOME/KDE) bundles the full desktop. startx launches X manually without a display manager.

  Prerequisites: Remote GUI (DISPLAY / xauth / X11 forwarding)

• yum / dnf (RHEL family)

  Repository-based package management on RHEL/CentOS (successor dnf): yum update upgrades packages (index auto-refreshes), plus install/remove/search/info. provides reverse-looks-up the package for a file; yumdownloader only downloads. Repositories are .repo files in /etc/yum.repos.d/, with global settings in /etc/yum.conf.

  Prerequisites: Packages and repositories