LinuC Level 1 Exam 102 — knowledge map

The 42 core concepts of LinuC Level 1 Exam 102 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 (42)

• 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: PATH

• 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.

• 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

• ICMP

  A protocol for control and diagnostic messages rather than data transfer. It powers ping (reachability checks) and traceroute (path checks), and carries unreachable/TTL-exceeded notifications—essential for diagnosing network connectivity issues.

  Prerequisites: PATH

  Related: ping / traceroute (reachability and path)、Port number

• 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

• 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)

• 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).

• Port number

  A 16-bit number identifying which service is running on a given IP address (e.g., HTTP uses 80, HTTPS uses 443, SSH uses 22). Carried in TCP/UDP headers, and firewall/security-group rules allow or deny traffic based on source/destination port.

  Prerequisites: IP address (IPv4 / IPv6)、source (dot command)

  Related: ICMP

• 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).

• 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)

• 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)

• 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

  Related: ICMP

• 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)

• CIDR notation

  A notation expressing an IP address range by prefix length, like “/24” (256 addresses) or “/16” (65,536 addresses)—the smaller the number, the larger the range. It underlies network design across every cloud: sizing VPCs/VNets and subnets, longest-prefix-match routing, and scoping firewall-rule allows.

  Prerequisites: IP address (IPv4 / IPv6)

• 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)

• 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)

• 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.

• 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)

• 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

• 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.

• 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)

• Well-known ports

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

  Prerequisites: Port number

• Subnet mask

  A value marking the boundary between the network and host portions of an IP address (e.g., 255.255.255.0)—the dotted-decimal counterpart to a CIDR prefix length, still used in on-prem network gear and some cloud configuration screens.

  Prerequisites: IP address (IPv4 / IPv6)、CIDR notation

• 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).

• 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)

• 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

• 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

• 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: Copyleft and permissive

• 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: Shell (bash)

• 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)

• 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)

• 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: Shell (bash)

• 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: systemd / systemctl

• 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: Shell (bash)

• 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.

• 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

• 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)

• 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: systemd / systemctl

• 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.

• 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)

• 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.