LINUX

Last reviewed: 2026-06-16

Purpose: Quick-reference knowledge base for essential Linux commands — file system navigation, file operations, permissions, process management, networking, disk usage, archives, text processing, package management, and system information.

Command	Description	Example
`pwd`	Print working directory	`pwd` → `/home/user`
`ls [opts] [path]`	List directory contents	`ls -lah /var/log`
`cd [dir]`	Change directory	`cd ~/Documents`
`tree [dir]`	Show directory tree	`tree -L 2 /etc`
`realpath [file]`	Resolve symlink/relative path	`realpath script.sh`
`pushd` / `popd`	Directory stack navigation	`pushd /tmp && popd`

Common ls flags: -l (long), -a (hidden), -h (human-readable), -t (sort by time), -S (sort by size).

File Operations

Command	Description	Example
`cp [src] [dst]`	Copy files/directories	`cp -r backup/ /mnt/disk/`
`mv [src] [dst]`	Move or rename	`mv old.txt new.txt`
`rm [file]`	Remove files	`rm -rf tempdir/`
`mkdir [dir]`	Create directory	`mkdir -p a/b/c`
`rmdir [dir]`	Remove empty directory	`rmdir emptydir/`
`touch [file]`	Create empty file / update timestamp	`touch config.yml`
`ln -s [target] [link]`	Create symbolic link	`ln -s /usr/bin/python3 python`
`find [path] [expr]`	Search for files	`find /home -name '*.log' -size +1M`
`file [path]`	Determine file type	`file /bin/bash`
`stat [file]`	Detailed file metadata	`stat document.pdf`

Permissions

Command	Description	Example
`chmod [mode] [file]`	Change file mode bits	`chmod 755 script.sh`
`chown [user][:group] [file]`	Change owner/group	`chown paul:admin data.txt`
`chgrp [group] [file]`	Change group ownership	`chgrp www-data index.html`
`umask [mask]`	Set default permission mask	`umask 0022`

Permission notation: rwx = 7, rw- = 6, r-x = 5, r-- = 4, -wx = 3, -w- = 2, --x = 1, --- = 0.

Recursive flags: chmod -R, chown -R.

Symbolic mode	Equivalent	Effect
`u+x`	`chmod +x` for owner	Add execute for owner
`g-w`	—	Remove write for group
`o=r`	—	Set others to read-only
`a+rx`	`chmod +rx`	Add read+execute for all

Process Management

Command	Description	Example
`ps [opts]`	Snapshot of current processes	`ps aux --sort=-%mem`
`top` / `htop`	Interactive process viewer	`top -u paul`
`kill [pid]`	Send signal to process	`kill -9 1234`
`pkill [name]`	Kill by process name	`pkill -f "python server.py"`
`pgrep [name]`	Find PID by name	`pgrep -u www-data nginx`
`jobs`	List background jobs	`jobs -l`
`bg` / `fg`	Resume job in bg/fg	`fg %1`
`nohup [cmd] &`	Run immune to hup	`nohup ./server &`
`systemctl [action] [unit]`	Systemd service control	`systemctl restart nginx`
`journalctl [opts]`	Query systemd journal	`journalctl -u sshd -n 50`

Common Signals

Signal	Number	Meaning
`SIGHUP`	1	Hangup (reload config)
`SIGINT`	2	Interrupt (Ctrl+C)
`SIGKILL`	9	Force kill (cannot trap)
`SIGTERM`	15	Graceful termination

systemctl Actions

Action	Effect
`start`	Start a unit
`stop`	Stop a unit
`restart`	Restart a unit
`reload`	Reload config without stopping
`enable`	Start at boot
`disable`	Do not start at boot
`status`	Show unit status
`daemon-reload`	Reload systemd manager config

Networking

Command	Description	Example
`ip addr`	Show/manipulate network interfaces	`ip addr show eth0`
`ip link`	List/configure network links	`ip link set eth0 up`
`ip route`	Show routing table	`ip route show default`
`ss [opts]`	Socket statistics (modern netstat)	`ss -tulpn`
`netstat [opts]`	Network connections (legacy)	`netstat -tulpn`
`curl [url]`	Transfer data from/to server	`curl -I https://example.com`
`wget [url]`	Non-interactive downloader	`wget -q -O data.zip url`
`ping [host]`	ICMP echo test	`ping -c 4 google.com`
`traceroute [host]`	Trace route to host	`traceroute 8.8.8.8`
`nslookup [host]`	DNS lookup	`nslookup example.com`
`dig [host]`	DNS lookup (verbose)	`dig +short example.com`
`ssh [user@host]`	Secure shell	`ssh -p 2222 user@10.0.0.5`
`scp [src] [dst]`	Secure copy over SSH	`scp file.txt user@host:/tmp/`
`ufw [action]`	Uncomplicated firewall	`ufw allow 80/tcp`

ss / netstat Common Flags

Flag	Meaning
`-t`	TCP sockets
`-u`	UDP sockets
`-l`	Listening sockets only
`-p`	Show process/PID
`-n`	Numeric (no name resolution)
`-a`	All sockets (listening + established)

Disk Usage

Command	Description	Example
`df [opts]`	Filesystem disk space usage	`df -hT`
`du [opts] [dir]`	Directory/file disk usage	`du -sh /home/paul/*`
`lsblk [opts]`	List block devices	`lsblk -o NAME,SIZE,FSTYPE,MOUNTPOINT`
`mount [opts]`	Mount filesystem	`mount /dev/sdb1 /mnt/data`
`umount [path]`	Unmount filesystem	`umount /mnt/data`
`blkid`	Show block device UUIDs/labels	`blkid /dev/sda1`
`fdisk [device]`	Partition table manipulator	`sudo fdisk -l`
`parted [device]`	Partition editor	`sudo parted /dev/sda print`

Common df flags: -h (human-readable), -T (filesystem type), -i (inodes).

Common du flags: -s (summary), -h (human-readable), -a (all files), --max-depth=N.

Archives & Compression

Command	Description	Example
`tar [opts] [archive] [files]`	Tape archiver	`tar -czvf archive.tar.gz /path/`
`tar -xf [archive]`	Extract tar archive	`tar -xvf archive.tar.gz -C /target/`
`gzip [file]`	Compress file (`.gz`)	`gzip file.log`
`gunzip [file.gz]`	Decompress `.gz`	`gunzip file.log.gz`
`bzip2 [file]`	Compress (`.bz2`, slower/better)	`bzip2 large.log`
`xz [file]`	Compress (`.xz`, best ratio)	`xz bigfile.img`
`zip [out] [in]`	Zip archive	`zip -r backup.zip ./data/`
`unzip [file]`	Extract zip	`unzip backup.zip -d ./restore/`
`zcat [file.gz]`	Read gzip'd file to stdout	`zcat log.gz \| grep error`
`zless` / `zgrep`	Read/search compressed files	`zgrep fail messages.gz`

tar Operation Flags

Flag	Meaning
`-c`	Create archive
`-x`	Extract archive
`-t`	List contents
`-v`	Verbose
`-f [file]`	Archive filename
`-z`	Filter through gzip
`-j`	Filter through bzip2
`-J`	Filter through xz
`-C [dir]`	Change to directory

Text Processing

Command	Description	Example
`cat [file]`	Concatenate and print files	`cat file1 file2 > combined`
`less [file]`	Paginated file viewer	`less +F /var/log/syslog`
`head [-n] [file]`	First N lines	`head -20 data.csv`
`tail [-n] [file]`	Last N lines	`tail -f /var/log/nginx/access.log`
`wc [opts] [file]`	Word/line/byte count	`wc -l script.py`
`sort [opts] [file]`	Sort lines	`sort -t, -k2 -n grades.csv`
`uniq [opts]`	Remove/report duplicate lines	`sort words.txt \| uniq -c`
`diff [a] [b]`	Compare files line by line	`diff -u old.conf new.conf`
`cut [opts]`	Extract columns from lines	`cut -d: -f1,3 /etc/passwd`
`tr [set1] [set2]`	Translate/delete characters	`echo "HELLO" \| tr A-Z a-z`
`tee [file]`	Split stdout to file and terminal	`echo "log" \| tee -a log.txt`

grep (Global Regular Expression Print)

Usage	Description	Example
`grep [pattern] [file]`	Search for pattern	`grep -i "error" /var/log/syslog`
`-r`	Recursive directory search	`grep -r "TODO" ./src/`
`-v`	Invert match	`grep -v "^#" config.conf`
`-E`	Extended regex	`grep -E "(http\|https)://" urls.txt`
`-c`	Count matches	`grep -c "failed" auth.log`
`-l`	List matching filenames only	`grep -l "main" *.py`
`-A[N]` / `-B[N]`	After / Before context	`grep -A2 -B2 "panic" dmesg`
`-o`	Show only matched part	`grep -oP "\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}\\.\\d{1,3}" log`

sed (Stream Editor)

Usage	Description	Example
`sed 's/old/new/'`	Replace first occurrence per line	`sed 's/foo/bar/' file.txt`
`sed 's/old/new/g'`	Replace all occurrences (global)	`sed 's/old/new/g' file.txt`
`sed 's/old/new/g' -i`	In-place edit	`sed -i 's/Old/New/g' config.yml`
`sed -n '/pattern/p'`	Print matching lines	`sed -n '/ERROR/p' log.txt`
`sed '/pattern/d'`	Delete matching lines	`sed '/^#/d' config.conf`
`sed '3,5d'`	Delete lines 3–5	`sed '3,5d' file.txt`

awk (Pattern Scanning & Processing)

Usage	Description	Example
`awk '{print $1}'`	Print first field	`awk '{print $1, $3}' /proc/loadavg`
`awk -F',' '{print $2}'`	Custom field separator	`awk -F: '{print $1}' /etc/passwd`
`awk '/pattern/'`	Filter by pattern	`awk '/404/ {print $7}' access.log`
`awk '{sum+=$1} END{print sum}'`	Sum a column	`awk '{s+=$5} END{print s}' sizes.txt`
`awk 'NR>1 {print}'`	Skip header line	`awk 'NR>1' data.tsv`

jq (JSON Query)

Usage	Description	Example
`jq '.' file.json`	Pretty-print JSON	`jq '.' data.json`
`jq '.key'`	Access key	`curl api.example.com \| jq '.results[0].name'`
`jq '.[] \| select(.status=="active")'`	Filter array	`jq '.[] \| select(.age > 18)' users.json`
`jq -r '.key'`	Raw output (no quotes)	`jq -r '.ip_address' config.json`
`jq 'keys'`	List object keys	`jq 'keys' package.json`
`jq 'length'`	Array/object length	`jq 'length' data.json`

Package Management (apt)

Command	Description	Example
`apt update`	Update package index	`sudo apt update`
`apt upgrade`	Upgrade all upgradable packages	`sudo apt upgrade -y`
`apt full-upgrade`	Upgrade with dependency changes	`sudo apt full-upgrade`
`apt install [pkg]`	Install package(s)	`sudo apt install htop neofetch`
`apt remove [pkg]`	Remove package (keep config)	`sudo apt remove nginx`
`apt purge [pkg]`	Remove package + config files	`sudo apt purge nginx`
`apt autoremove`	Remove orphaned dependencies	`sudo apt autoremove`
`apt list --installed`	List installed packages	`apt list --installed \| grep python`
`apt search [term]`	Search packages	`apt search "web server"`
`apt show [pkg]`	Show package details	`apt show curl`
`apt edit-sources`	Edit sources.list	`sudo apt edit-sources`
`dpkg -i [.deb]`	Install local .deb file	`sudo dpkg -i package.deb`
`dpkg -l`	List installed .deb packages	`dpkg -l \| grep mysql`

System Information

Command	Description	Example
`uname -a`	All system info (kernel, arch, hostname)	`uname -a`
`uname -r`	Kernel release	`uname -r`
`lscpu`	CPU architecture details	`lscpu`
`lspci`	List PCI devices	`lspci -v`
`lsusb`	List USB devices	`lsusb -t`
`lsblk`	List block devices	`lsblk -o NAME,SIZE,TYPE`
`lshw`	Full hardware inventory	`sudo lshw -short`
`dmidecode`	DMI/BIOS/System info	`sudo dmidecode -t memory`
`free -h`	Memory usage	`free -h`
`uptime`	System uptime and load	`uptime`
`dmesg`	Kernel ring buffer messages	`dmesg \| tail -20`
`hostnamectl`	Hostname and OS info	`hostnamectl`
`timedatectl`	System time/date/zone	`timedatectl list-timezones`
`who` / `w`	Who is logged in	`who -a`
`last`	Last logged-in users	`last -10`

Quick Reference — Combined Shortcuts

Shortcut	Effect
`Ctrl+C`	Kill foreground process (SIGINT)
`Ctrl+D`	EOF / exit shell
`Ctrl+Z`	Suspend foreground process (SIGTSTP)
`Ctrl+L`	Clear terminal
`Ctrl+R`	Reverse search command history
`Ctrl+W`	Delete word before cursor
`Ctrl+U`	Delete from cursor to start of line
`!!`	Repeat last command
`!$`	Last argument of previous command
`!n`	Repeat nth command from history
`~/.bashrc`	Bash init script (user)
`~/.bash_aliases`	Common alias storage
`~/.profile`	Login shell init
`/etc/environment`	System-wide env vars

This is a living reference. Add sections or commands as needed. Always verify with man [command] or [command] --help for the latest options on your system.