LinuC Level 1 Exam 101 — knowledge map
The 73 core concepts of LinuC Level 1 Exam 101 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 (73)
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)、PATH
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.
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)
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)
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
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)
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: Filesystems and mkfs、Locating commands (which/whereis/type)
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
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: Signals and terminating processes (kill / pkill / killall / pgrep)
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)
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
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)
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
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)
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
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)
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)
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)
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)
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
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)
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
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)
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)
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: Regular expression
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.
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.
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
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
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.
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
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.
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)
Timestamp
The last-modified/accessed/changed time of a file; touch updates it, and find -mtime can search by modification date.
Prerequisites: Basic file operations (cp / mv / rm / mkdir)、find
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
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)
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
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
Command substitution
A notation embedding a command output inline: $(command) or `command` (backquotes), e.g., FILES=$(ls *.log) captures the result into a variable.
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: Persisting jobs after disconnect (nohup/screen/tmux)、Shell and environment variables、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.
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
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
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.
man (manual)
Displays a command online manual (man ls); man -k keyword (= apropos) finds related commands by keyword.
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
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.
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)
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
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.
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)
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
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).
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)
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)
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
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
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)
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: dpkg (direct deb handling)、Packages and repositories
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)
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)
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
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
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)
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.
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
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
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
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