Linux

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Category	Environment / Scope	Name / Utility	Syntax / Combo	Common Flags / Options	Real Usage Example	Description & Functionality
8. Services & Packages	Debian/Ubuntu	apt / dpkg	`apt update && apt install`	update, upgrade, install, purge, dpkg -i	`sudo apt update && sudo apt install -y htop`	Package management suite for Debian and Ubuntu based distributions.
8. Services & Packages	POSIX / All	at / nohup	`at [time] / nohup cmd &`	nohup.out redirection	`nohup python3 script.py > app.log 2>&1 &`	Executes one-time delayed jobs (at) or runs tasks detached from terminal SIGHUP disconnects (nohup).
3. Text Processing	POSIX / All	awk	`awk 'program' file`	-F [delimiter], print $N, BEGIN/END blocks	`awk -F':' '{print $1, $7}' /etc/passwd`	Pattern scanning language optimized for parsing columnated log files and structured text.
5. Process & Memory	POSIX / All	bg / fg / jobs	`bg [%job] / fg [%job] / jobs`	-l (show PIDs)	`jobs -l && fg %1`	Lists suspended jobs and manages foreground/background process execution in current shell.
3. Text Processing	POSIX / All	cat	`cat [options] file`	-n (line numbers), -A (show non-printing)	`cat -n /etc/hosts`	Concatenates file contents and prints directly to standard output.
2. File & Directory	POSIX / All	cd	`cd [dir]`	- (previous dir), ~ (home)	`cd /etc/nginx && cd -`	Changes active directory location. `cd -` toggles back to previous path.
4. Permissions & Users	POSIX / All	chgrp	`chgrp group file`	-R (recursive)	`chgrp -R developers /opt/project`	Changes group ownership of target files and directories.
4. Permissions & Users	POSIX / All	chmod	`chmod [mode] file`	755, 644, +x, -R (recursive)	`chmod +x deploy.sh && chmod 600 id_rsa`	Modifies file mode bits for Owner, Group, and Others using octal (755) or symbolic (+x) notation.
4. Permissions & Users	POSIX / All	chown	`chown owner:group file`	-R (recursive), --reference	`chown -R www-data:www-data /var/www`	Changes target file or directory system user ownership and group assignment.
3. Text Processing	POSIX / All	cmp	`cmp [options] file1 file2`	-l (list byte numbers), -s (silent mode)	`cmp -s file1.bin file2.bin`	Compares two files byte by byte to check for identical binary data.
2. File & Directory	POSIX / All	cp	`cp [options] src dst`	-r (recursive), -p (preserve permissions), -a (archive)	`cp -a /etc/nginx /etc/nginx.bak`	Copies files and directories. Option `-a` preserves ownership, mode, and timestamps.
8. Services & Packages	POSIX / All	crontab	`crontab [options]`	-e (edit), -l (list), -r (remove), -u [user]	`crontab -e / crontab -l`	Schedules automated recurring background tasks using cron timing syntax.
1. Terminal Keys	Shortcut	Cut Line / Kill Buffer	`Ctrl + U / Ctrl + K`	Readline Cut Buffer	`Ctrl + U (Cut to start)`	Cuts line from cursor to beginning (Ctrl+U) or cursor to end (Ctrl+K). Paste with Ctrl+Y.
7. Networking	POSIX / All	curl / wget	`curl [opts] URL / wget [opts] URL`	-I (headers), -X (POST/PUT), -O, -c (continue download)	`curl -Is https://api.github.com`	Downloads web assets and interacts with REST APIs over HTTP/HTTPS/FTP.
3. Text Processing	POSIX / All	cut	`cut [options] file`	-d [delimiter], -f [fields], -c [characters]	`cut -d',' -f1,3 data.csv`	Extracts specific character ranges or delimited fields from lines in text files.
6. System & Storage	POSIX / All	df	`df [options]`	-h (human-readable), -T (fs type), -i (inodes)	`df -hT`	Displays total disk space usage and free capacity for all mounted filesystems.
3. Text Processing	POSIX / All	diff	`diff [options] file1 file2`	-u (unified output), -y (side-by-side), -r (recursive)	`diff -u config.orig config.new`	Compares two text files line by line and outputs differences (used to generate patch files).
7. Networking	POSIX / All	dig / nslookup	`dig [domain] [type] / nslookup`	+short, @server, MX, TXT, AAAA	`dig example.com MX +short`	Queries DNS servers for record resolution (A, CNAME, MX, TXT) and troubleshooting.
6. System & Storage	POSIX / All	dmesg	`dmesg [options]`	-T (human timestamps), -l (level: err,warn)	`dmesg -T -l err,crit`	Prints kernel ring buffer messages to diagnose boot errors, drive failures, and hardware faults.
8. Services & Packages	RHEL/Fedora	dnf / rpm	`dnf install [pkg]`	install, check-update, remove, rpm -qa	`sudo dnf install -y nginx`	Package management suite for Red Hat Enterprise Linux, Fedora, and Rocky Linux.
2. File & Directory	POSIX / All	du	`du [options] [path]`	-s (summary), -h (human readable), -d [depth]	`du -sh /var/log/* \| sort -hr`	Estimates and prints disk space consumption for specified directory paths.
6. System & Storage	POSIX / All	fdisk / parted	`fdisk [device] / parted`	-l (list partitions)	`sudo fdisk -l /dev/sda`	Low-level partition manipulation utilities for creating and resizing MBR and GPT disk tables.
2. File & Directory	POSIX / All	file	`file [options] filename`	-b (brief mode), -i (mime type string)	`file -i archive.tar.gz`	Inspects file magic headers to determine true file type (binary, text, encoding, archive).
2. File & Directory	POSIX / All	find	`find [path] [expression]`	-name, -type (f/d), -mtime, -size, -exec	`find /var/log -name "*.log" -mtime +30 -delete`	Recursively searches directory trees based on name, age, size, permissions, or executes actions.
5. Process & Memory	POSIX / All	free	`free [options]`	-h (human-readable), -m (MB), -g (GB), -s [delay]	`free -h`	Displays total, used, free, shared, buffer/cache, and available RAM memory and Swap space.
6. System & Storage	POSIX / All	fsck	`fsck [options] device`	-y (auto repair), -f (force check)	`sudo fsck -f -y /dev/sda1`	Checks and repairs corrupted filesystem blocks and inode structure errors. Drive must be unmounted.
3. Text Processing	POSIX / All	grep	`grep [options] pattern file`	-i (ignorecase), -r (recursive), -n (line num), -v (invert)	`grep -rnw "Failed password" /var/log/auth.log`	Searches files or stdin for lines matching regular expression patterns.
4. Permissions & Users	POSIX / All	groupadd	`groupadd [options] groupname`	-g (custom GID), -r (system group)	`groupadd -r sysadmins`	Creates new user group definitions inside `/etc/group`.
3. Text Processing	POSIX / All	head	`head [options] file`	-n [lines], -c [bytes]	`head -n 20 /var/log/auth.log`	Prints top N lines (default 10) of input files or pipeline stream.
6. System & Storage	POSIX / All	hostname / hostnamectl	`hostnamectl [cmd]`	status, set-hostname	`sudo hostnamectl set-hostname node-01`	Queries and sets machine network hostname and system metadata.
1. Terminal Keys	Shortcut	Insert Last Argument	`Alt + . or Esc + .`	Readline Argument	`mkdir my_dir -> cd [Alt+.]`	Cycles and pastes the final argument from previous commands. Saves typing long paths.
1. Terminal Keys	Shortcut	Interrupt Signal	`Ctrl + C`	SIGINT Signal	`Ctrl + C`	Sends SIGINT to immediately stop and terminate the foreground process.
7. Networking	POSIX / All	ip	`ip [object] [command]`	addr, link, route, -c (color)	`ip -c addr show eth0 / ip route`	Modern utility to view/configure network interfaces, IP addresses, and routing tables (replaces ifconfig).
8. Services & Packages	Systemd	journalctl	`journalctl [options]`	-u [unit], -f (follow), -p [err], --since "1 hour ago"	`journalctl -u docker -f --since "10 min ago"`	Queries and filters systemd binary journal log files.
5. Process & Memory	POSIX / All	kill / killall	`kill [signal] PID / killall name`	-15 (SIGTERM), -9 (SIGKILL), -1 (SIGHUP)	`kill -15 1234 / killall -9 nginx`	Sends specified termination signals to processes by process PID or binary name string.
3. Text Processing	POSIX / All	less	`less [options] file`	-N (line numbers), -S (chop long lines), +F (live tail)	`less -N /var/log/syslog`	Interactive file pager supporting backward navigation and keyword searching.
1. Terminal Keys	Shortcut	Line Start / End Jump	`Ctrl + A / Ctrl + E`	Readline Shortcut	`Ctrl + A (Start) / Ctrl + E (End)`	Moves cursor instantly to beginning (Ctrl+A) or end (Ctrl+E) of command line.
2. File & Directory	POSIX / All	ln	`ln [options] target link`	-s (symbolic link), -f (force override)	`ln -s /etc/nginx/sites-avail/app /etc/nginx/sites-enabled/`	Creates soft (symbolic) or hard filesystem links to target files/directories.
2. File & Directory	POSIX / All	locate	`locate [options] pattern`	-i (ignore case), -r (regex search)	`locate nginx.conf`	Fast indexed search using internal mlocate database. Requires running `updatedb` to refresh.
2. File & Directory	POSIX / All	ls	`ls [options] [path]`	-l (long), -a (all), -h (human), -t (time sort)	`ls -la --color=auto /var/log`	Lists directory contents including permissions, owner, size, and mod dates.
6. System & Storage	POSIX / All	lsblk / blkid	`lsblk [opts] / blkid [dev]`	-f (filesystems), -p (full path)	`lsblk -f && sudo blkid`	Displays block devices tree hierarchy, drive partition layout, filesystems, and device UUIDs.
6. System & Storage	POSIX / All	lscpu / lsusb / lspci	`lscpu / lsusb / lspci`	-v (verbose), -t (tree mode)	`lscpu \| grep "Model name"`	Enumerates hardware details for CPU sockets/cores, USB controller buses, and PCI express cards.
7. Networking	POSIX / All	lsof	`lsof [options]`	-i:[port], -u [user], -p [PID], +D [path]	`sudo lsof -i :80`	Lists all open files, directory locks, and associated network sockets held by processes.
2. File & Directory	POSIX / All	mkdir	`mkdir [options] dir`	-p (parents), -m (mode)	`mkdir -p /var/www/site/public`	Creates directories. Option `-p` creates parent subdirectories recursively if missing.
6. System & Storage	POSIX / All	mkfs	`mkfs.[type] [device]`	-t (type: ext4, xfs, btrfs), -L (label)	`sudo mkfs.ext4 -L "DATA" /dev/sdb1`	Formats specified disk partition with a Linux filesystem (ext4, xfs, btrfs, vfat).
6. System & Storage	POSIX / All	mount / umount	`mount [dev] [dir] / umount`	-a (all fstab), -t (type), -o (options: ro,rw)	`sudo mount -o ro /dev/sdb1 /mnt/data`	Attaches or unmounts storage device filesystems into designated mount directories.
2. File & Directory	POSIX / All	mv	`mv [options] src dst`	-i (interactive prompt), -n (no clobber), -v (verbose)	`mv config.tmp config.yaml`	Moves or renames files and directories within or across filesystems.
7. Networking	POSIX / All	nc (netcat)	`nc [options] host port`	-z (zero-I/O port scan), -v (verbose), -l (listen)	`nc -zv 192.168.1.50 22-80`	Swiss-army knife for TCP/UDP socket testing, banner grabbing, and port connectivity verification.
5. Process & Memory	POSIX / All	nice / renice	`nice -n [val] cmd / renice -n PID`	Priority range -20 (highest) to +19 (lowest)	`nice -n 19 tar -czf backup.tar.gz /data`	Sets or alters process CPU scheduling priority to balance resource usage against foreground tasks.
7. Networking	POSIX / All	nmap	`nmap [options] target`	-sS (SYN scan), -p [ports], -A (aggressive/OS detection)	`nmap -sV -p 22,80,443 192.168.1.0/24`	Network exploration security scanner for host discovery, open port detection, and OS fingerprinting.
4. Permissions & Users	POSIX / All	passwd	`passwd [options] [username]`	-l (lock), -u (unlock), -e (expire password)	`sudo passwd devuser`	Updates local user account password authentication credentials.
5. Process & Memory	POSIX / All	pgrep / pkill	`pgrep [opts] pattern / pkill`	-f (full command line), -u (user), -9 (SIGKILL)	`pkill -9 -f "python3 server.py"`	Looks up processes by regex name (pgrep) or sends termination signals matching full command string.
7. Networking	POSIX / All	ping	`ping [options] host`	-c [count], -i [interval], -4 (IPv4), -6 (IPv6)	`ping -c 4 1.1.1.1`	Sends ICMP ECHO_REQUEST packets to verify reachability and measure round-trip latency.
5. Process & Memory	POSIX / All	ps	`ps [options]`	aux, -ef, --sort=-%cpu, --sort=-%mem	`ps aux --sort=-%mem \| head -n 10`	Takes a static snapshot of active system processes, showing PID, CPU/RAM usage, and TTY.
2. File & Directory	POSIX / All	pwd	`pwd [options]`	-P (physical), -L (logical)	`pwd -P`	Prints current working directory absolute path, resolving symlinks with -P.
1. Terminal Keys	Shortcut	Reverse History Search	`Ctrl + R`	Bash / Zsh History	(reverse-i-search)`ssh': ...	Interactively searches command history backwards as you type characters.
2. File & Directory	POSIX / All	rm	`rm [options] path`	-r (recursive), -f (force), -i (interactive)	`rm -rf /tmp/cache_*`	Deletes files or directories recursively. Use extreme caution with `-rf`.
2. File & Directory	POSIX / All	rmdir	`rmdir [options] dir`	-p (parents), --ignore-fail-on-non-empty	`rmdir -p old_dir/sub_dir`	Removes empty directories from the filesystem hierarchy.
7. Networking	POSIX / All	scp / rsync	`rsync [opts] src user@host:dst`	-a (archive), -v (verbose), -z (compress), -P (progress)	`rsync -avzP ./build/ user@remote:/var/www/html/`	Transfers and synchronizes files over SSH, updating only modified block deltas.
3. Text Processing	POSIX / All	sed	`sed [options] 'script' file`	-i (in-place write), -e (expression), s/pattern/replace/g	`sed -i 's/Port 22/Port 2222/g' /etc/ssh/sshd_config`	Stream editor for inline text parsing, string search and replace, and line filtering.
8. Services & Packages	POSIX / All	shutdown / reboot	`shutdown [opts] [time]`	-h (halt now), -r (reboot), +N (delay minutes)	`sudo shutdown -h +15 "Server maintenance"`	Safely signals processes and shuts down or reboots the OS kernel.
3. Text Processing	POSIX / All	sort	`sort [options] file`	-n (numeric), -r (reverse), -k [column], -u (unique)	`sort -t: -k3 -n /etc/passwd`	Sorts lines of text files alphabetically or numerically across designated key columns.
7. Networking	POSIX / All	ss / netstat	`ss [options] / netstat [options]`	-t (tcp), -u (udp), -l (listen), -p (process), -n (numeric)	`ss -tulpn \| grep :80`	Displays active listening sockets, open network connections, and associated PIDs.
7. Networking	POSIX / All	ssh	`ssh [options] user@host`	-i [key], -p [port], -L (local tunnel), -D (SOCKS proxy)	`ssh -i ~/.ssh/id_ed25519 user@10.0.0.5 -p 2222`	Connects securely to remote host command lines over encrypted channels.
2. File & Directory	POSIX / All	stat	`stat [options] file`	-f (filesystem), -c (format specification)	`stat /etc/passwd`	Displays detailed inode metadata, file size, access permissions, owner UID, and timestamps.
4. Permissions & Users	POSIX / All	su / sudo	`su - [user] / sudo [cmd]`	-i (login shell), -u (user target), -v (validate)	`sudo -i / su - postgres`	Switches target user shell environment or elevates command privileges using sudo policies.
1. Terminal Keys	Shortcut	Suspend Job	`Ctrl + Z`	SIGTSTP Signal	`Ctrl + Z (Then run 'bg')`	Suspends active job and pushes it to background. Resume with `fg` or `bg`.
8. Services & Packages	Systemd	systemctl	`systemctl [cmd] [service]`	status, start, stop, restart, enable, disable, reload	`sudo systemctl restart nginx`	Controls systemd init services, daemons, and target runlevels.
3. Text Processing	POSIX / All	tail	`tail [options] file`	-f (follow live stream), -n [lines], --retry	`tail -100f /var/log/nginx/error.log`	Outputs trailing lines of files. Option `-f` monitors file growth in real-time.
8. Services & Packages	POSIX / All	tar	`tar [options] archive.tar.gz`	-c (create), -x (extract), -z (gzip), -v, -f (file)	`tar -czvf backup.tar.gz /var/www`	Bundles files into tape archives and compresses/extracts using gzip or bzip2.
7. Networking	POSIX / All	tcpdump	`tcpdump [options] [filter]`	-i [interface], -nn (no resolve), -w [file.pcap], port [N]	`sudo tcpdump -i eth0 -nn port 80 -w http.pcap`	Command-line packet analyzer that captures network traffic for Wireshark inspection.
3. Text Processing	POSIX / All	tee	`tee [options] file`	-a (append mode), -i (ignore signals)	`echo "data" \| sudo tee -a /etc/config`	Reads standard input and writes simultaneously to standard output and disk files.
5. Process & Memory	POSIX / All	top / htop	`top / htop`	-u [user], -p [PID], -d [delay]	`htop -u www-data`	Dynamic real-time monitoring interface for CPU utilization, RAM consumption, and process trees.
2. File & Directory	POSIX / All	touch	`touch [options] file`	-a (access time), -m (mod time), -c (no create)	`touch /var/www/app/index.php`	Creates empty file if missing or updates file access and modification timestamps.
3. Text Processing	POSIX / All	tr	`tr [options] set1 [set2]`	-d (delete), -s (squeeze repeats)	`cat file.txt \| tr 'a-z' 'A-Z'`	Translates, squeezes, or deletes characters from standard input stream.
7. Networking	POSIX / All	traceroute / mtr	`traceroute host / mtr host`	-n (numeric IP), -T (TCP packets)	`mtr --report 8.8.8.8`	Traces network packet hop paths and pinpoints latency bottlenecks or packet loss.
2. File & Directory	POSIX / All	tree	`tree [options] [path]`	-L [depth], -d (dirs only), -a (all files)	`tree -L 2 /var/www`	Displays directory tree structure visually in depth-indented format.
7. Networking	POSIX / All	ufw / iptables	`ufw status / iptables -L`	allow [port], deny, -t nat, -A INPUT	`sudo ufw allow 22/tcp && sudo ufw enable`	Manages Linux kernel netfilter packet filtering rules and firewall policies.
4. Permissions & Users	POSIX / All	umask	`umask [mask]`	-S (symbolic output)	`umask 022`	Sets default access permission masking mode for newly created files and directories.
6. System & Storage	POSIX / All	uname	`uname [options]`	-a (all details), -r (kernel release), -m (arch)	`uname -a`	Prints kernel name, hostname, OS version, architecture (x86_64/aarch64), and build details.
3. Text Processing	POSIX / All	uniq	`uniq [options] [input]`	-c (count), -d (duplicates only), -u (unique only)	`sort access.log \| uniq -c \| sort -nr`	Filters adjacent duplicate lines from sorted text streams and reports occurrence counts.
2. File & Directory	POSIX / All	updatedb	`updatedb [options]`	--output, --prunepaths	`sudo updatedb`	Rebuilds mlocate search index database across mounted local filesystems.
5. Process & Memory	POSIX / All	uptime	`uptime [options]`	-p (pretty format), -s (since timestamp)	`uptime -p`	Shows running duration, active users, and 1, 5, 15 minute system load averages.
4. Permissions & Users	POSIX / All	useradd	`useradd [options] username`	-m (create home), -s (shell), -g (group), -G (groups)	`useradd -m -s /bin/bash -G sudo devuser`	Creates a new user account, setting up home directories and initial group memberships.
4. Permissions & Users	POSIX / All	userdel	`userdel [options] username`	-r (remove home directory and mail spool)	`userdel -r olduser`	Deletes user account and optionally wipes home directory files with `-r`.
4. Permissions & Users	POSIX / All	usermod	`usermod [options] username`	-aG (append groups), -s (change shell), -L (lock)	`usermod -aG docker devuser`	Modifies user account parameters, secondary group affiliations, or login shells.
3. Text Processing	POSIX / All	vim / nano	`vim [file] / nano [file]`	+line, -R (read-only mode)	`sudo vim /etc/fstab`	Terminal text editors for modifying configuration files directly on headless servers.
4. Permissions & Users	POSIX / All	visudo	`visudo [options]`	-c (check syntax), -f [file]	`sudo visudo`	Safely opens `/etc/sudoers` in an editor with syntax validation checks to prevent lockouts.
5. Process & Memory	POSIX / All	vmstat	`vmstat [delay] [count]`	-s (event counters), -d (disk stats)	`vmstat 2 5`	Reports virtual memory, swap-in/swap-out paging, process queues, I/O blocks, and CPU states.
4. Permissions & Users	POSIX / All	w / who / last	`w / who / last`	-n [lines], -f [file]	`w && last -n 10`	Inspects active SSH sessions, logged-in terminal users, system load, and recent boot history.
2. File & Directory	POSIX / All	wc	`wc [options] [file]`	-l (lines), -w (words), -c (bytes)	`wc -l /etc/passwd`	Prints line, word, character, and byte count totals from input files or pipes.
4. Permissions & Users	POSIX / All	whoami / id	`whoami / id [user]`	-u (UID), -g (GID), -G (all groups)	`id -u root`	Prints effective username (whoami) or full numeric UID/GID and group assignments (id).
8. Services & Packages	POSIX / All	zip / unzip	`zip archive.zip file / unzip`	-r (recursive), -q (quiet), -d (destination)	`unzip archive.zip -d /tmp/extracted`	Compresses and extracts standard zip archive files.

Command Structure & Syntax Anatomy

$ command -flags --long-option [argument_source] [argument_target] | pipeline_next

1. Binary / Alias

The executable file located in $PATH (e.g. /usr/bin/rsync) or custom shell alias.

2. Short / Long Flags

Short options use single dash (-a -v or combined -av). Long options use double dashes (--archive).

3. Arguments

Operands passed to the program, such as file targets, string regex patterns, or network host IP addresses.

4. Redirection / Pipe

Chains stdout output into stdin of the next process using | or redirects streams to disk with >, >>.

Standard I/O Streams

stdin (0) stdout (1) stderr (2) 2>&1 /dev/null

File Mode Bits (Octal)

4 = Read (r) 2 = Write (w) 1 = Execute (x) 755 (rwxr-xr-x) 644 (rw-r--r--)

Process Signals

SIGHUP (1) SIGINT (2) SIGTERM (15) SIGKILL (9)

System Administrator Pro-Tips

Re-run Last Command as Root

Forgot to prepend sudo to a privileged system command? Use history substitution: sudo !! Executes the exact last command line with elevated privileges.

Fast Directory Navigation

Quickly toggle back to your previous working directory without typing full paths: cd - Swaps instantly between your current path and previous $OLDPWD.

Monitor Active Connections Live

Combine process and network socket monitors with continuous update interval tracking: watch -n 1 "ss -tulpn" Refreshes open listening port status output every 1.0 second.

System Administration Golden Rules

Safety First CLI Principles

Dry-Run Verification: Always run non-destructive flags first (e.g., rsync --dry-run or rm -i) before executing bulk operations.
Avoid Blind chmod 777: Never grant world-writable permissions to fix permission errors; use exact group grants (e.g., 755 or 644).
Use Absolute Paths in Scripts: Cron jobs and systemd service units do not inherit your user shell's $PATH environment. Always specify full binary paths (e.g. /usr/bin/python3).

Production Log Extraction Pipeline Example

High-efficiency pipeline combining process filtering, sorting, and field selection:

ps aux --sort=-%mem | grep -v "grep" | head -n 10 | awk '{print $2, $4, $11}'