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Fundamentals/Kernel/Shell12
Files & Filesystem6
Permissions10
Users & Groups9
Processes11
Env & I/O7
Text Processing10
Scripting11
Packages7
Services/Boot13
Storage25
Networking27
Security16
Monitoring17
Troubleshooting9
1 Linux/Unix Fundamentals The open-source, Unix-like OS family built around a shared kernel and POSIX-influenced toolset.
Full page → 2 Linux Kernel The core program managing hardware, memory, processes, and system calls for everything else on the box.
Full page → 3 Linux Distributions A complete OS built around the kernel — Ubuntu, RHEL, Debian, Fedora — each bundling its own package manager and defaults.
Full page → 4 Shell The command interpreter that reads what you type and asks the kernel to act on it.
Full page → 5 Bash The Bourne Again SHell — the default interactive shell and scripting language on most Linux distros.
Full page → 6 Terminal & CLI The text interface for typing commands to the shell, as opposed to a graphical desktop.
Full page → 7 Linux Commands The executable programs (ls, cp, grep...) that make up the day-to-day vocabulary of the CLI.
Full page → 88 Init The first process (PID 1) started by the kernel at boot; on modern distros, that's systemd.
Full page → + Everything else on the box is a descendant of PID 1.
89 Kernel Modules Loadable pieces of kernel code (drivers, filesystems) added or removed without a reboot.
Full page → + 90 /proc A virtual filesystem exposing live kernel and process information as readable files.
Full page → + 91 /sys A virtual filesystem exposing kernel device and driver state, used for low-level tuning.
Full page → + ❯ cat /sys/class/net/eth0/mtu
92 sysctl The command-line interface for reading and writing kernel parameters exposed under /proc/sys.
Full page → + 8 Files & Directories The two basic units of a filesystem — directories are just files that list other files.
9 Linux Filesystem Hierarchy The FHS standard defining what belongs in /etc, /var, /usr, /opt and friends.
10 File Types Regular files, directories, symlinks, block/char devices, sockets, and pipes — the 7 types ls -l shows.
+ 11 Inodes The data structure holding a file's metadata — the name is just a directory entry pointing to one.
+ 12 Hard Links A second directory entry pointing to the same inode; data isn't freed until every hard link is gone.
+ 13 Symbolic Links A pointer file containing a path to another file; breaks if the target moves.
+ 14 File Permissions The read/write/execute bits for owner, group, and others that gate access to a file.
15 Ownership Every file has exactly one owning user and one owning group, both of which affect permission checks.
16 chmod Changes a file's permission bits, numerically (750) or symbolically (u+x).
+ 17 chown Changes a file's owning user (and optionally group).
+ 18 chgrp Changes only a file's owning group.
+ 19 umask The default permission mask subtracted from new files/directories at creation time.
+ 20 ACL Access Control Lists — permissions for specific extra users/groups beyond owner/group/other.
+ ❯ setfacl -m u:alice:rx file
21 SUID Set-User-ID bit — makes an executable run as its owning user, not the caller (e.g. passwd).
22 SGID Set-Group-ID bit — new files in a directory inherit the directory's group.
23 Sticky Bit On a directory, restricts deletion to a file's own owner even with group write (e.g. /tmp).
24 Users Named accounts the kernel tracks by UID, each with their own permissions and home directory.
25 Groups A named collection of users, tracked by GID, used to share permissions across accounts.
26 UID/GID The numeric IDs the kernel actually checks — usernames/group names are just labels for humans.
+ 27 Root User UID 0, the account exempt from standard permission checks — Linux's superuser.
28 sudo Runs a single command as another user (usually root), logged and governed by /etc/sudoers.
+ ❯ sudo systemctl restart nginx
29 su Switches to another user's full login shell, prompting for that user's password.
+ 30 /etc/passwd The world-readable file listing every user account, UID, home directory, and login shell.
31 /etc/shadow The root-only file holding hashed passwords and password-aging policy, separate from passwd.
32 /etc/group The file listing every group, its GID, and its member usernames.
33 Processes A running instance of a program, tracked by the kernel with its own memory space and PID.
34 PID/PPID A process's own ID and its parent's ID — every process except PID 1 has a parent.
35 Process States Running (R), sleeping (S), uninterruptible sleep (D), stopped (T), or zombie (Z).
36 Foreground/Background Processes A foreground job holds the terminal; a background job (cmd &) runs without blocking it.
37 Daemons Long-running background processes with no controlling terminal (sshd, nginx, cron).
38 Signals Asynchronous notifications sent to a process — SIGTERM asks nicely, SIGKILL doesn't.
+ 39 Job Control Suspending, resuming, and switching between foreground/background jobs in a shell.
+ 40 ps Snapshots the current process table.
+ 41 top Live, auto-refreshing view of processes ranked by resource usage.
42 htop A friendlier, colorized, interactive alternative to top.
43 kill Sends a signal to a process by PID — default signal is SIGTERM, not SIGKILL.
+ 44 Environment Variables Named values (like PATH or HOME) inherited by every child process of a shell.
+ 45 PATH The environment variable listing directories the shell searches, in order, to find a command.
46 Standard Input/Output The three default I/O channels every process gets: input, output, and errors.
47 stdin/stdout/stderr File descriptors 0, 1, and 2 — stdout and stderr are separate streams even though both print to your terminal.
48 Redirection Sending a stream to a file instead of the terminal.
+ 49 Pipes Connects one command's stdout directly to the next command's stdin.
+ 50 tee Splits a stream, writing it to a file and passing it through to stdout at the same time.
+ 51 Text Processing The Unix philosophy of small tools chained together to transform text streams.
52 grep Finds lines matching a pattern.
+ 53 sed Stream editor — finds and transforms text, most often via find-and-replace.
+ ❯ sed -i 's/old/new/g' file
54 awk Pattern-scanning language built around columns — extracts and computes on fields.
+ ❯ awk '{print $1,$5}' access.log
55 cut Extracts columns from delimited text by position.
+ 56 sort Sorts lines of text, numerically or alphabetically.
+ 57 uniq Collapses adjacent duplicate lines, usually paired with sort first.
+ 58 tr Translates or deletes individual characters in a stream.
+ 59 wc Counts lines, words, or bytes.
+ 60 xargs Builds and runs commands from piped input, one invocation per batch of arguments.
+ ❯ find . -name "*.log" | xargs rm
61 Shell Scripting Writing sequences of shell commands into a reusable, executable file.
62 Variables Named storage in a script, no type declaration needed.
+ ❯ NAME="vishal"; echo $NAME
63 Operators Comparison (-eq, -lt) and logical (&&, ||) operators used in conditionals and chaining.
64 Conditions Branching logic in a script.
+ ❯ if [[ -f file ]]; then ...; fi
65 Loops Repeating a block of commands.
+ ❯ for f in *.log; do gzip "$f"; done
66 Functions Named, reusable blocks of shell logic.
+ ❯ greet() { echo "hi $1"; }
67 Arrays Ordered lists of values in Bash.
+ ❯ arr=(a b c); echo ${arr[1]}
68 Arguments Values passed into a script or function, available as $1, $2, ... and $@.
69 Exit Codes The 0-255 number a command returns; 0 means success, anything else means failure.
+ 70 Error Handling Catching and reacting to failures instead of silently continuing.
+ 71 Command Substitution Capturing a command's output into a variable.
+ 72 Package Management The system for installing, updating, and removing software along with its dependencies.
73 RPM Red Hat Package Manager — the .rpm package format used by RHEL, Fedora, and derivatives.
74 YUM The older dependency-resolving package manager for RPM-based distros, mostly superseded by DNF.
75 DNF Dandified YUM — the modern package manager on RHEL 8+/Fedora.
+ 76 DEB The .deb package format used by Debian, Ubuntu, and derivatives.
77 APT Advanced Package Tool — the dependency-resolving package manager for .deb-based distros.
+ 78 Repositories Remote servers hosting packages and their index metadata that apt/dnf pull from.
79 Services Long-running background processes managed and supervised by the init system.
80 Systemd The modern init system and service manager used by most major distros, PID 1.
81 systemctl The main command for controlling systemd units and their state.
+ 82 journalctl Queries systemd's structured, binary system log.
+ ❯ journalctl -u nginx --since "1 hour ago"
83 Systemd Units The building blocks systemd manages — .service, .socket, .timer, .mount, and more.
84 Systemd Targets Named synchronization points that group units, replacing old-style SysV runlevels.
85 Boot Process Firmware → bootloader → kernel → initramfs → init — power-on to a login prompt.
86 BIOS/UEFI The firmware that initializes hardware and hands control to the bootloader; UEFI is the modern successor.
87 GRUB The bootloader that lets you pick a kernel/OS and pass boot parameters before Linux starts.
171 Cron The classic time-based job scheduler, driven by crontab entries.
172 Crontab The file (and command) listing a user's scheduled cron jobs.
+ 173 at Schedules a command to run once, at a specific future time.
+ ❯ echo "backup.sh" | at 2am
174 Systemd Timers systemd's modern alternative to cron, with dependency awareness and journald-integrated logging.
93 Storage The general term for how a system persists data — disks, partitions, filesystems, volumes.
94 Disks The physical (or virtual) block devices data is written to.
+ 95 Partitions Fixed divisions of a disk, each independently formattable and mountable.
96 Filesystems The format that organizes raw disk blocks into files and directories (ext4, XFS, Btrfs...).
97 Mount/Unmount Attaching (or detaching) a filesystem at a directory so it becomes accessible.
+ 98 /etc/fstab The file listing filesystems to mount automatically at boot.
99 df Reports filesystem-level disk space usage.
+ 100 du Reports actual disk usage of files and directories.
+ 101 lsblk Lists block devices in a tree, showing partitions and mount points.
102 fdisk The classic interactive partition table editor (MBR and basic GPT support).
103 parted A more capable partition editor with native GPT and resize support.
104 LVM Logical Volume Manager — a flexible layer between raw disks and filesystems for online resize/snapshots.
105 PV Physical Volume — a raw disk or partition initialized for use by LVM.
+ 106 VG Volume Group — a storage pool combining one or more PVs.
+ ❯ vgcreate data-vg /dev/sdb
107 LV Logical Volume — the resizable "partition" carved out of a VG, formatted like normal.
+ ❯ lvcreate -L 100G -n data-lv data-vg
108 RAID Combining multiple disks for redundancy (mirroring) and/or performance (striping).
109 Disk Expansion Growing a volume and its filesystem live, without unmounting.
+ ❯ lvextend -L +50G /dev/vg/lv && resize2fs /dev/vg/lv
110 Swap Disk space used as RAM overflow — slower, but keeps the box from OOM-killing immediately.
+ 175 Backup Copies of data kept separately so a failure, mistake, or attack doesn't mean permanent loss.
176 tar Bundles multiple files/directories into a single archive file.
+ ❯ tar -czvf backup.tar.gz /data
177 gzip A fast, widely-supported single-file compression format, often paired with tar.
178 bzip2 A higher-compression-ratio alternative to gzip, slower but smaller output.
179 xz The highest-compression-ratio common option — slowest to compress, best for cold storage.
180 NFS Network File System — mounts a remote directory over the network as if it were local.
181 Samba Implements SMB/CIFS so Linux can share files with (and access shares from) Windows.
111 Networking How machines discover, address, and exchange data with each other.
112 IP Addressing Assigning a unique numeric address to each device on a network.
113 IPv4 The 32-bit address format still dominant today (e.g. 192.168.1.10).
114 IPv6 The 128-bit successor address format designed to outlast IPv4 exhaustion.
115 Subnetting Dividing an IP range into smaller networks using a subnet mask/CIDR prefix (e.g. /24).
116 MAC Address The hardware address assigned to a network interface, used at the link layer.
117 NIC Network Interface Card — the hardware (or virtual) device a machine uses to connect to a network.
118 Gateway The router a device sends traffic to when the destination isn't on its local subnet.
119 DNS Domain Name System — resolves human-readable hostnames into IP addresses.
+ 120 DHCP Dynamically assigns IP addresses and network config to devices as they join a network.
121 Routing Deciding which path traffic takes to reach a destination network.
122 Routing Table The kernel's list of destination networks and the interface/gateway to reach each one.
+ 123 TCP Connection-oriented, reliable, ordered transport protocol — default for most app traffic.
124 UDP Connectionless, no-retransmit transport protocol — used where speed matters more (DNS, video).
125 ICMP The protocol behind ping and network error messages like "destination unreachable."
126 ARP Address Resolution Protocol — maps a known IP address to its MAC address locally.
127 Ports A 16-bit number identifying which application on a host a connection is for (443 = HTTPS).
128 Sockets The kernel's endpoint abstraction combining an IP address and port for a connection.
129 ip The modern command for interfaces, addresses, and routes.
+ 130 ss The modern replacement for netstat, showing socket/connection state.
+ 131 ping Sends ICMP echo requests to test basic reachability.
+ 132 traceroute Shows the hop-by-hop path packets take to a destination.
+ 133 dig Queries DNS servers directly and shows the full response.
+ 134 nslookup An older, simpler DNS lookup tool, still common on minimal systems.
135 curl Makes HTTP(S) and other protocol requests from the command line.
+ ❯ curl -I https://example.com
136 wget Downloads files over HTTP(S)/FTP, with resume and recursive-download support.
+ ❯ wget https://example.com/file.tar.gz
137 nc Netcat — the "Swiss army knife" for raw TCP/UDP connections and quick data transfer.
+ 138 SSH Secure Shell — encrypted remote login and command execution, the standard way to admin Linux boxes.
+ 139 SSH Keys A public/private key pair used to authenticate to SSH without typing a password.
+ 140 SCP Copies files over SSH.
+ ❯ scp file.tar.gz vishal@host:/data/
141 SFTP An interactive, FTP-like file transfer session running entirely over SSH.
142 Rsync Efficiently syncs files/directories, transferring only the differences.
+ ❯ rsync -avz /data/ user@host:/backup/
143 Firewalls Rules that permit or block network traffic based on address, port, or protocol.
144 iptables The legacy Linux firewall/packet-filtering tool built on netfilter rule chains.
145 nftables The modern successor to iptables, with cleaner syntax and better performance.
146 firewalld A dynamic firewall daemon (common on RHEL) built on nftables/iptables, using named "zones."
147 SELinux Mandatory access control on RHEL-family distros — enforces policy beyond file permissions, even for root.
148 AppArmor Ubuntu/Debian's mandatory access control, using per-application profiles instead of SELinux's labeling.
149 PAM Pluggable Authentication Modules — the framework Linux uses to plug in authentication methods uniformly.
150 Authentication Proving who you are (password, key, certificate) — the "who are you" step.
151 Authorization Deciding what an authenticated identity is allowed to do — the "what can you do" step.
152 TLS/SSL The cryptographic protocol that encrypts traffic in transit, the "S" in HTTPS.
153 Certificates Signed credentials proving a server's identity and carrying its public key for TLS.
154 Logs Timestamped records of what a system or application did, essential for post-mortem troubleshooting.
155 /var/log The traditional directory for plain-text application and system log files.
156 Syslog The long-standing standard protocol/daemon for collecting and routing log messages.
157 Journald systemd's structured, binary logging service, queried through journalctl.
158 Log Rotation Automatically archiving, compressing, and deleting old logs so they don't fill the disk.
159 CPU Monitoring Tracking processor utilization and load to catch saturation before it becomes an incident.
160 Memory Monitoring Tracking RAM/swap usage and pressure to catch leaks or OOM risk early.
161 Disk Monitoring Tracking free space, inode usage, and I/O latency per volume.
162 I/O Monitoring Tracking read/write throughput and latency across disks and network interfaces.
163 Network Monitoring Tracking throughput, errors, and connection counts per interface.
164 Load Average The 1/5/15-minute rolling average of processes running or waiting on CPU/I/O.
+ 165 free Reports memory and swap usage.
+ 166 vmstat Reports CPU, memory, swap, and I/O stats in one rolling view.
+ 167 iostat Reports per-disk I/O utilization and latency.
+ 168 sar Historical and live system activity reporting (CPU, memory, network) from one toolkit.
169 lsof Lists open files — including sockets and deleted-but-still-open files eating disk space.
+ 170 strace Traces every syscall a process makes, in real time.
+ 182 Kernel Troubleshooting Diagnosing kernel-level issues via dmesg, kernel panics, and module load failures.
+ 183 Performance Tuning Adjusting kernel parameters, resource limits, and config based on measured bottlenecks, not guesses.
184 System Troubleshooting Reproduce, isolate the layer (CPU/mem/disk/net), check logs, form a hypothesis, verify the fix.
185 Boot Troubleshooting Diagnosing a box that won't come up cleanly — GRUB, fstab, initramfs, failed units.
186 Disk Troubleshooting Diagnosing full disks, exhausted inodes, and slow I/O.
187 Network Troubleshooting Working the stack in order — DNS, routing, firewall, then application — to isolate failures.
188 Process Troubleshooting Finding runaway, zombie, or stuck (D-state) processes and their root cause.
189 Service Troubleshooting Diagnosing why a systemd service won't start or keeps restarting.
+ 190 Security Troubleshooting Diagnosing access-denied errors from permissions, SELinux/AppArmor, or firewall rules — in that order.
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