Create one or more directories. Use -p to also create parent directories automatically.
# Create a single directory
mkdir new_folder
# Create parent directories as needed
mkdir -p ~/projects/app/src
# Create with specific permissions
mkdir -m 755 backups
Create empty files or update their access and modification timestamps.
# Create or update timestamp
touch file1.txt
# Update access time only
touch -a log.txt
# Set modification time to 2025-01-01 00:00
touch -m -t 202501010000 report.md
Copy files or directories. Common flags: -r (recursive), -i (prompt), -v (verbose).
# Copy single file
cp file.txt backup.txt
# Recursive copy with verbose output
cp -rv src/ dest/
# Prompt before overwrite
cp -i config.conf /etc/
Move or rename files and directories. Flags: -n (no-clobber), -i (prompt), -v (verbose).
# Rename a file
mv old.txt new.txt
# Move with verbose output
mv -v doc.pdf ~/Documents/
# Do not overwrite existing files
mv -n temp/ archive/
Remove files or directories. Warning: irreversible! Flags: -r (recursive), -f (force), -i (prompt).
# Delete a single file
rm oldfile.txt
# Recursive + force delete
rm -rf unused_dir/
# Prompt before deleting .bak files
rm -i *.bak
Print the absolute path of the current working directory.
# Show current location
pwd # e.g. /home/user/projects
List directory contents. Common flags: -l (long), -a (all), -h (human sizes).
# Simple listing
ls
# Detailed listing of /etc
ls -l /etc
# All files with human-readable sizes
ls -lah ~/projects
# Colorized output
ls --color=auto ~/downloads
Concatenate and display file contents. Useful to preview or combine files.
# Display a file
cat file.txt
# Display with line numbers
cat -n file.txt
# Combine multiple files
cat a.txt b.txt > merged.txt
Search for patterns in files or input. Flags: -i (ignore case), -R (recursive), -n (show line numbers).
# Search for "error" in a file
grep "error" logfile.txt
# Filter running processes for "ssh" (ignore case)
ps aux | grep -i ssh
# Recursive search with line numbers
grep -Rin "TODO" ~/project/
Search for files and directories by name, type, time, size, and more.
# Find .log files in current directory tree
find . -name "*.log"
# Files modified in the last 7 days
find /var -type f -mtime -7
# Files larger than 100 MB
find /tmp -size +100M
Change file permissions. Modes can be octal (e.g. 755) or symbolic (e.g. u+rwx).
# Owner read/write, group/others read
chmod 644 config.yaml
# Add execute permission for owner
chmod u+x script.sh
# Remove write permission recursively for group/others
chmod -R go-w public/
Change file owner and/or group. Syntax: owner:group.
# Set owner and group
chown user:staff file.txt
# Change only owner
chown user file.txt
# Change only group
chown :staff file.txt
Run a command as another user (default: root). Prompts for password unless timestamp valid.
# Update system packages as root
sudo apt update
# Restart a service
sudo systemctl restart nginx
# Run command as another user
sudo -u www-data whoami
# Open root shell
sudo bash
Print the current effective username.
# Show current user
whoami
# Show effective user with sudo
sudo whoami
Configure or display network interfaces (legacy tool).
# Enable an interface
sudo ifconfig eth0 up
# Assign IP address
sudo ifconfig eth0 192.168.1.10 netmask 255.255.255.0
# Disable interface
sudo ifconfig eth0 down
Show or manipulate IP addresses, interfaces, routing tables (modern replacement for ifconfig).
# Show IP address of eth0
ip addr show eth0
# Bring interface up
ip link set eth0 up
# Add a default route
ip route add default via 192.168.1.1
Send ICMP echo requests to test connectivity. Common flags: -c (count), -i (interval), -W (timeout).
# Ping a hostname until stopped
ping google.com
# Send 4 pings
ping -c 4 8.8.8.8
# Half-second interval, 1s timeout
ping -i 0.5 -W 1 example.com
Dump socket statistics and display network connections (replacement for netstat).
# TCP/UDP listening ports
ss -tuln
# Include process info
ss -tulpn
# Summary statistics
ss -s
Report file system disk space usage. Flags: -h (human-readable), -T (show FS type).
# Human-readable disk usage
df -h
# Show filesystem type
df -T /home
Estimate file and directory space usage. Flags: -h (human), -s (summary).
# Summary of directory
du -sh ~/projects
# Breakdown of /var by depth=1
du -h --max-depth=1 /var
Display the first part of files. Flags: -n (lines), -c (bytes).
# First 10 lines
head file.txt
# First 20 lines
head -n 20 logfile.log
# First 100 bytes
head -c 100 /var/log/syslog
Display the last part of files. Flags: -n (lines), -f (follow).
# Last 10 lines
tail file.txt
# Last 50 lines
tail -n 50 logfile.log
# Follow new log entries
tail -f /var/log/syslog
View file contents one screen at a time. Navigate with j/k, search with /.
# View a README file
less README.md
# Start at end of file
less +G /var/log/syslog
# Start at first occurrence of "Error"
less +/Error log.txt
Report a snapshot of current processes. Common flags: aux (all users, detailed), -ef (full-format).
# Detailed list of processes
ps aux
# Filter processes for sshd
ps -ef | grep sshd
# Sort by memory usage
ps aux --sort=-%mem
Send signals to processes. Common signals: -15 (terminate), -9 (force kill), -HUP (reload).
# Gracefully terminate a process
kill 1234
# Force kill a process
kill -9 1234
# Reload process configuration
kill -HUP 1234
Display memory and swap usage. Common flags: -h (human-readable), -m (MB).
# Human-readable sizes
free -h
# Show values in MB
free -m
Show how long the system has been running, number of users, and load averages.
# Show uptime
uptime # e.g. 12:34:56 up 5 days, 2 users, load average: 0.01, 0.05, 0.07
Print system information. Useful flags: -a (all), -r (kernel release), -m (architecture).
# Complete system info
uname -a
# Kernel release
uname -r
# Machine hardware architecture
uname -m
Archive and extract files. Supports flags: -c (create), -x (extract), -z (gzip), -v (verbose), -f (file).
# Create an archive
tar -cvf archive.tar dir/
# Extract a gzip archive
tar -xvzf archive.tar.gz
# List contents
tar -tvf archive.tar
Control and manage systemd services.
# Check service status
systemctl status nginx
# Restart SSH service
sudo systemctl restart ssh
# Enable Docker at boot
sudo systemctl enable docker
Query and display logs from the systemd journal.
# Show SSH logs
journalctl -u ssh
# Follow new logs
journalctl -f
# Logs from the last hour
journalctl --since "1 hour ago"
Efficiently synchronize files/directories between locations. Common flags: -a (archive), -v (verbose), -z (compress).
# Sync with compression
rsync -avz src/ dest/
# Sync and delete extraneous files
rsync -av --delete src/ remote:/backup/
# Sync over SSH
rsync -e ssh -az src/ user@host:/path/
Trace the path packets take to a host, showing each hop.
# Basic trace
traceroute google.com
# Skip DNS lookups
traceroute -n 8.8.8.8
# ICMP probes on port 80
traceroute -I -p 80 example.com
Capture and display packets on network interfaces. Supports filters and pcap output.
# Live capture on eth0
sudo tcpdump -i eth0
# Capture HTTPS traffic and write to file
sudo tcpdump -i eth0 port 443 -w https.pcap
# Verbose capture without name resolution
sudo tcpdump -nn -vv -i any
Print network connections, routing tables, and interface statistics (legacy tool).
# Listening TCP/UDP ports
netstat -tuln
# Show processes with sockets
netstat -tulpn
# Routing table
netstat -r
# Protocol statistics
netstat -s
Package manager for Debian/Ubuntu. Install, update, remove, and upgrade software.
# Refresh package index
sudo apt update
# Install a package
sudo apt install nginx
# Remove a package
sudo apt remove nano
# Upgrade all upgradable packages
sudo apt upgrade
Download files from the web using HTTP, HTTPS, or FTP.
# Download a file
wget http://example.com/file.zip
# Save with custom filename
wget -O custom.zip http://example.com/file.zip
# Continue partial download
wget -c http://example.com/large.iso
# Recursive download of directory
wget -r -l1 -nd http://site.com/files/
Manage Debian packages (.deb) locally: install, remove, query, and purge.
# Install local package
dpkg -i package.deb
# Remove installed package
dpkg -r package_name
# Purge package and config files
dpkg -P package_name
# List installed packages matching "nginx"
dpkg -l | grep nginx
Transfer data from or to a server using supported protocols (HTTP, HTTPS, FTP, etc.). Great for APIs.
# Simple GET request
curl https://api.github.com
# Save output to file
curl -o image.png http://site.com/img.png
# Fetch headers only
curl -I http://example.com
# Send POST request with data
curl -X POST -d "key=val" http://api/endpoint
Open the manual page for a command. Use sections (1=commands, 5=config files, 8=admin commands).
# Show manual for ls
man ls
# Search manuals for keyword "network"
man -k network
# Show section 5 (config files) for crontab
man 5 crontab
Display or manipulate the shell command history.
# List previous commands
history
# Repeat command #42
!42
# Search history for "ssh"
history | grep ssh
Create shortcuts for commands in your shell (temporary unless added to .bashrc/.zshrc).
# Define ll as ls -lah
alias ll='ls -lah'
# Define gs as git status
alias gs='git status'
# Show all aliases
alias
Generate and manage SSH key pairs for authentication.
# Generate default RSA key
ssh-keygen
# Generate Ed25519 key
ssh-keygen -t ed25519
# Change passphrase for a key
ssh-keygen -f ~/.ssh/id_rsa -p
Query DNS name servers for information about domains.
# A record lookup
dig example.com
# MX records
dig MX example.com
# Use Google DNS for query
dig @8.8.8.8 example.com
Retrieve domain registration and ownership details.
# Basic domain info
whois example.com
# Check Google registration
whois google.com
Read and write data across network connections (aka netcat). Great for testing ports and sockets.
# Listen on port 1234
nc -l 1234
# Connect to a host on port 80
nc host.example.com 80
# Send data
echo "Hello" | nc host 9000
Display running processes and system usage in real time (interactive).
# Show processes sorted by CPU usage
top
Interactive process viewer with colors, tree view, and easier navigation (improved top).
# Start htop (may need sudo apt install htop)
htop