Linux Commands

Master the command line with this comprehensive guide to essential Linux commands for beginners and intermediate users.

• pwd — Print working directory (show current path)

pwd
# Output: /home/user/project

• ls — List directory contents

  ls           # Simple listing
  ls -l        # Long format with details
  ls -a        # Show hidden files (starting with .)
  ls -lh       # Human-readable file sizes
  

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• cd — Change directory

  cd /path/to/dir      # Absolute path
  cd ../..             # Go up two levels
  cd ~                 # Home directory
  cd -                 # Previous directory
  

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• mkdir — Create directories

  mkdir new_folder              # Single directory
  mkdir -p a/b/c                # Nested directories (parents)
  

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• rmdir — Remove empty directories

  rmdir old_folder
  

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• touch — Create or update file timestamps

  touch newfile.txt           # Create empty file
  touch -t 202412311200.00 f  # Set specific timestamp
  

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• cp — Copy files/directories

  cp source.txt dest.txt              # File copy
  cp -r src_dir/ dest_dir/            # Directory recursive copy
  cp -i file1 dir/                    # Interactive (prompt before overwrite)
  

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• mv — Move or rename files/directories

  mv oldname.txt newname.txt          # Rename
  mv file.txt /path/to/dir/           # Move to directory
  

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• rm — Remove files/directories

  rm file.txt                         # Delete single file
  rm -r folder_name                   # Recursive delete (directory)
  rm -rf path_to_dir                  # Force recursive delete
  rm -i *                             # Interactive confirmation for all
  

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• ln — Create links

  ln source linkname                 # Hard link
  ln -s target symlink_name          # Symbolic (soft) link
  

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• cat — Concatenate and display files

  cat file.txt                       # Display entire file
  cat > newfile.txt                  # Create from stdin
  cat file1 file2 >> combined.txt    # Append multiple files
  

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• less / more — Page through large files

  less logfile.log                   # Scrollable viewer (q to quit)
  more bigfile.txt                   # Basic pager
  

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• head / tail — View beginning or end of file

  head -20 file.txt                  # First 20 lines
  tail -f logfile.log                # Follow log (real-time updates)
  tail -100 access.log               # Last 100 lines
  

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• nano / vim — Terminal text editors

  nano file.txt                      # Simple editor
  vim file.txt                       # Advanced modal editor
  

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• find — Locate files by name, type, size, etc.

  find . -name "*.txt"               # Find all text files in current dir
  find /home -user john              # Files owned by user 'john'
  find /var/log -size +10M           # Files larger than 10MB
  find . -type f -mtime -7           # Regular files modified in last 7 days
  

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• grep — Search text patterns

  grep "error" logfile.txt           # Find lines containing 'error'
  grep -i "warning" file             # Case-insensitive search
  grep -r "function" src/            # Recursive directory search
  grep -c "pattern" file             # Count matching lines
  

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• locate — Fast filename lookup (uses database)

  locate nginx.conf                  # Quick file location
  sudo updatedb                        # Update the locate database first
  

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• ps — Snapshot of current processes

  ps                                 # Simple process list
  ps aux                             # Full format (BSD style)
  ps -ef                             # Full format (UNIX style)
  ps -p PID                          # Specific process by ID
  

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• top / htop — Real-time process viewer

  top                                # Basic real-time monitor
  htop                               # Enhanced interactive version (if installed)
  

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• kill / killall — Terminate processes

  kill PID                           # Send SIGTERM to process ID
  kill -9 PID                        # Force kill (SIGKILL)
  killall nginx                      # Kill all processes named 'nginx'
  pkill firefox                      # Kill by name pattern
  

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• bg / fg — Job control in terminal

  sleep 100 &                        # Run background job
  bg %1                              # Resume job as background
  fg %1                              # Bring to foreground
  jobs                               # List current jobs
  

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• df — Disk space usage

  df                                 # Basic disk info
  df -h                              # Human-readable sizes (GB/MB)
  df -T                              # Show filesystem types
  

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• du — Directory size analysis

  du -sh /path/to/dir                # Summarize directory size
  du --max-depth=1                   # Limit depth of scan
  du -h | sort -rh | head            # Top largest directories
  

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• free — Memory usage

  free                               # Basic memory info
  free -h                            # Human-readable (MB/GB)
  free -m                            # Megabytes only
  

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• uptime / w — System load and users

  uptime                             # Load average, uptime, user count
  w                                  # Who's logged in with activity
  who                                # Logged-in users list
  

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• vmstat / iostat — Virtual memory & I/O stats

  vmstat 1                           # Report every second
  iostat -x                          # Extended CPU and device stats
  

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• useradd / adduser — Create users

  useradd newuser                    # Basic user creation (Debian/Ubuntu)
  adduser newuser                    # Interactive setup (preferred on Debian-based)
  useradd -m -s /bin/bash username   # With home directory and shell
  

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• usermod — Modify existing users

  usermod -aG sudo john              # Add 'john' to sudo group
  usermod -d /home/john2 john        # Change home directory
  usermod -L username                # Lock account (disable password)
  

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• userdel — Remove users

  userdel username                   # Delete user without home dir
  userdel -r username                # Delete with home and mail spool
  

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• passwd — Manage passwords

  passwd                             # Change own password
  sudo passwd username               # Set another user's password
  passwd -e username                 # Force password change at next login
  chage -l username                  # Show password aging info
  

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• groups / id — Group and identity checks

  groups                             # Groups for current user
  id                                 # UID, GID, and group memberships
  getent passwd                      # View all users from system databases
  

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• chmod — Change file permissions (mode)

  chmod u+rwx file                   # Add read/write/execute for owner
  chmod g-wx file                    # Remove write and execute for group
  chmod o=r file                     # Set others to read-only
  
  # Numeric mode examples:
  chmod 755 script.sh                # rwxr-xr-x (owner: full, others: read+exec)
  chmod 644 document.txt             # rw-r--r--
  chmod +x file                      # Add execute permission
  

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• chown — Change owner and group

  chown user file                    # Change owner only
  chown :group file                  # Change group only
  chown user:group file              # Change both
  
  # Recursive ownership change:
  sudo chown -R www-data:www-data /var/www/html
  

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• chgrp — Change group specifically

  chgrp developers project/          # Set directory to 'developers' group
  

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• ping — Test network connectivity

  ping google.com                    # Send ICMP echo requests
  ping -c 4 google.com               # Limit to 4 packets
  ping -W 2 host                     # Set timeout (seconds)
  

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• traceroute / tracepath — Trace network path

  traceroute example.com             # Show all hops to destination
  tracepath hostname                 # Alternative without root privileges
  

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• nslookup / dig — DNS lookups

  nslookup google.com                # Query DNS server for IP
  dig +short google.com              # Get only the A record (IP)
  dig @8.8.8.8 example.com           # Use specific DNS server
  

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• host — Simple DNS lookup tool

  host example.com                   # Quick hostname to IP resolution
  

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• ip / ifconfig — View and configure interfaces (modern vs legacy)

  ip a                               # Show all addresses (preferred modern)
  ip addr show eth0                  # Specific interface details
  ifconfig                           # Legacy tool (may need installation)
  
  # Configure IP:
  sudo ip addr add 192.168.1.50/24 dev eth0
  

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• netstat / ss — Network connections and sockets

  netstat -tuln                      # List listening TCP ports (legacy)
  ss -tulpn                          # Modern replacement for netstat
  ss -s                              # Socket statistics summary
  

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• curl / wget — Download files from web

  curl https://example.com/file.zip  # Transfer data with URL syntax
  wget http://site.com/largefile.iso # Simple file downloader (resume: -c)
  
  # Advanced usage:
  curl -O http://server.com/file     # Download and save original filename
  

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• ssh — Secure shell remote access

  ssh user@hostname                  # Connect to remote server
  ssh -p 2222 user@host              # Custom port connection
  scp file.txt user@remote:/path/    # Copy files via SSH
  
  # Key-based auth setup:
  ssh-keygen                         # Generate RSA key pair
  ssh-copy-id user@hostname          # Install public key on remote
  

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• scp / rsync — Secure file transfer and sync

  scp local.txt user@remote:/dest/   # Simple secure copy
  rsync -avz ./local_dir/ user@host:/remote/path/
                                    # Sync with compression (-z) and archive mode
  
  # Exclude patterns:
  rsync -av --exclude='*.tmp' src/ dest/
  

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• nc (Netcat) — Network debugging utility

  nc -zv host port                   # Check if port is open
  echo "hello" | nc localhost 80     # Send data to service
  

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• ufw — Uncomplicated Firewall (Ubuntu/Debian)

  ufw status                         # Show firewall state
  ufw allow ssh                      # Allow SSH access
  ufw allow from 192.168.1.0/24      # Allow subnet
  sudo ufw enable                    # Activate firewall (careful!)
  

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• iptables — Low-level packet filter (advanced)

  iptables -L                        # List current rules
  iptables -A INPUT -p tcp --dport 22 -j ACCEPT   # Allow SSH
  sudo iptables-save                 # Persist rules to file
  
  # Flush all rules:
  sudo iptables -F                   # Warning: clears everything!
  

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• firewall-cmd — Firewalld (RHEL/CentOS/Fedora)

  firewall-cmd --list-all            # Show current zones and services
  sudo firewall-cmd --permanent --add-service=http
                                     # Add HTTP service permanently
  sudo firewall-cmd --reload         # Apply changes
  

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• sed — Stream editor (in-place editing, substitutions)

  sed 's/old/new/g' file.txt       # Replace all occurrences of old with new
  sed -i.bak 's/foo/bar/' file     # Edit in place and create backup (.bak)
  sed -n '/pattern/p' log           # Print only lines matching pattern
  

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• awk — Pattern scanning and processing language

  awk '{print $1}' file.txt         # Print first column
  awk '$3 > 100 {sum += $3} END {print sum}' data   # Sum third column if > 100
  awk -F: '{print $1, $7}' /etc/passwd    # Use colon as field separator
  

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• cut — Extract sections from lines (field-based)

  cut -d':' -f1 /etc/passwd        # First field using ':' delimiter
  cut -c5-10 file.txt              # Characters 5 through 10 only
  

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• sort / uniq — Sort and remove duplicates

  sort names.txt                   # Alphabetical sorting
  sort -n numbers.txt              # Numerical sorting
  uniq file.txt                    # Remove adjacent duplicate lines (input must be sorted)
  
  # Combined:
  cat list.txt | sort | uniq       # Unique entries from unsorted input
  

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• wc — Word, line, character count

  wc -l file.txt                   # Count lines only
  wc -w                            # Words (whitespace-separated)
  wc -c                            # Characters/bytes
  wc -m                            # Multibyte characters
  

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• tr — Translate or delete characters

  echo "hello world" | tr 'a-z' 'A-Z'
                                   # Convert to uppercase
  cat file.txt | tr -d '\r'       # Remove carriage returns (Windows line endings)
  

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• tar — Tape archive utility (most common archiver)

  tar -cvf archive.tar files...    # Create uncompressed archive (-c create, -v verbose, -f file)
  tar -xvf archive.tar             # Extract all files (-x extract)
  tar -tzf archive.tar.gz          # List contents of compressed archive (-t list)
  
  # Common compression flags:
  tar -czvf backup.tar.gz dir/     # Compress with gzip
  tar -cjvf backup.tar.bz2 dir/    # Compress with bzip2 (slower but better ratio)
  tar -cJvf archive.xz files       # XZ compression (best ratio, slowest)
  
  # Extract specific file:
  tar -xzf archive.tar.gz path/to/file.txt
  

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• gzip / gunzip — Compress single files

  gzip largefile.log               # Creates largefile.log.gz (original removed by default)
  gunzip largefile.log.gz          # Decompress back to original name
  zcat file.log.gz                 # View compressed content without extracting
  
  # Keep original:
  gzip -k file.txt                 # Compress but keep source file
  

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• bzip2 / xz — Alternative compression formats (better ratio, slower)

  bzip2 data.txt                   # Creates .bz2 compressed version
  xz archive.tar                   # XZ compression (.xz extension)
  
  # Decompress:
  bunzip2 file.bz2                 # Unbzip2
  unxz file.xz                     # Unxz (or xz -d)
  

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• zip / unzip — Cross-platform archive format

  zip archive.zip folder/*         # Create ZIP with all contents of folder/
  unzip archive.zip                # Extract to current directory
  
  # Advanced:
  zip -r backup.zip project/       # Recursive compression (directories)
  unzip -l archive.zip             # List contents without extracting
  

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• diff / patch — Compare and apply differences between files

  diff file1.txt file2.txt         # Show line-by-line differences
  diff -u old new > changes.patch  # Unified format patch file (-u)
  
  # Apply patches:
  patch < changes.patch            # Interactive application
  patch -p0 < changes.patch       # Apply without path stripping
  

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• cmp — Binary comparison of two files

  cmp file1.bin file2.bin          # Find first differing byte
  cmp -l file1 file2               # List all differences (byte by byte)
  

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• export / env — Manage environment variables

  export MY_VAR="hello world"      # Set variable for current shell session
  echo $MY_VAR                     # View value (note: no quotes needed)
  env                              # Show all exported variables
  printenv                         # Alternative to show environment
  
  # Persistent across sessions:
  echo 'export PATH=$PATH:/custom/bin' >> ~/.bashrc   # Add to bash config
  source ~/.bashrc                 # Reload configuration without restart
  

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• alias / unalias — Create command shortcuts

  alias ll='ls -la'                # Shorten ls -l with all details
  alias gs='git status'
  
  # View aliases:
  alias                          # List current aliases
  unalias my_alias                 # Remove temporary alias (current session only)
  
  # Make permanent: add to ~/.bashrc or ~/.zshrc
  

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• history — Command history management

  history                          # Show command history with line numbers
  !123                             # Re-execute command number 123 from history
  !!                               # Repeat last command
  !$                               # Last argument of previous command (shortcut)
  
  # Search and manage:
  Ctrl+R                           # Reverse search through history interactively
  history -c                       # Clear current session's history
  

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• time — Measure execution duration

  time sleep 5                     # Time how long a command takes
  
  # Output format:
  real    elapsed wall-clock time
  user    CPU time spent in user mode
  sys     CPU time spent in kernel mode
  

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• watch — Execute commands repeatedly with output refresh

  watch -n 5 df -h                 # Run every 5 seconds (default is 2s)
  watch --color 'free -m'          # Colorized output for memory monitoring
  

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• > / >> — Redirect stdout to file (overwrite vs append)

  echo "hello" > test.txt          # Overwrite existing content
  echo "world" >> test.txt         # Append to end of file
  command > output.log             # Save program output to log file
  
  # Multiple redirections:
  ls -la > files.txt 2> errors.txt   # stdout and stderr separately
  

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• | — Pipe (connect stdout of one command to stdin of next)

  ps aux | grep nginx              # Filter process list for 'nginx'
  cat logfile.log | tail -50       # Show last 50 lines via pipe
  echo "data" | tr '[a-z]' '[A-Z]' # Transform text through pipeline
  
  # Common pipelines:
  ls -lh | grep .pdf               # List PDF files with human-readable sizes
  free -h | awk '/^Mem:/ {print $3}'   # Extract used memory only (awk filters)
  

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• & — Run command in background, wait for all jobs to complete

  sleep 10 &                       # Background job
  echo "done"                      # Continue immediately while sleep runs
  
  # Wait for specific process:
  wait PID                         # Block until specified process finishes
  

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• && / || — Conditional execution (AND/OR)

  cd /var/log && ls                # Run 'ls' only if 'cd' succeeds
  make clean || echo "clean failed"   # Show error message on failure
  
  # Complex chains:
  git pull && npm install && npm run build    # Chain multiple commands (all must succeed)
  

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• ; — Run next command regardless of previous result

  cd /tmp ; ls                     # Always execute 'ls' even if directory change fails
  echo "start" ; sleep 2 ; echo "end"
  

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• reboot / shutdown — Restart or power off system

  reboot                           # Immediate restart (requires sudo)
  shutdown now                     # Power off immediately
  
  // Scheduled operations:
  shutdown -h +10                  // Halt in 10 minutes
  shutdown -r 23:59                // Reboot at specific time (HH:MM format)
  

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• systemctl — Systemd service manager (modern Linux systems)

  systemctl status nginx           # Check service state and logs
  sudo systemctl start httpd       // Start a service
  sudo systemctl stop mysql        // Stop a service
  sudo systemctl restart ssh       // Restart a service
  
  // Enable/disable at boot:
  sudo systemctl enable docker     // Auto-start on boot
  sudo systemctl disable cron      // Disable auto-start
  
  // View all services:
  systemctl list-units --type=service --state=running
  

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• journalctl — Systemd journal log viewer

  journalctl -xe                   // Recent errors with context
  journalctl -u nginx              // Logs for specific service (unit)
  
  // Real-time follow:
  journalctl -f                    // Follow live logs (like tail -f)
  journalctl --since "10 min ago"  // Filter by time range
  

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• dmesg — Kernel ring buffer messages

  dmesg                            // Show kernel boot and hardware messages
  dmesg | grep USB                 // Search for specific events (e.g., USB devices)
  
  // Clear old logs:
  sudo dmesg -C                    // Clear console log buffer
  

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• who / last — User session tracking

  who                              // Currently logged-in users with terminals
  last                             // Historical login records (from wtmp)
  last reboot                      // Show system reboots from history
  
  // Recent activity:
  last -10                         // Last 10 logins only
  

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• date / cal — Date and calendar utilities

  date                             // Current date/time (default format)
  date +%Y-%m-%d                   // Custom format: YYYY-MM-DD
  
  // Calendar:
  cal                              // Show current month's calendar
  cal -3                           // Display three months at once
  cal 2025                         // Full year calendar for specific year
  

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• bc — Basic calculator (arithmetic in shell)

  echo "10 + 5" | bc               // Simple arithmetic: result = 15
  
  // Advanced math:
  echo "scale=2; 3.14 * 7" | bc   // Floating point with precision control
  
  // Interactive mode:
  bc                               // Enter calculator shell (Ctrl+D to exit)
  

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• dd — Low-level data conversion and copying

  dd if=/dev/sdX of=backup.img bs=4M    // Create disk image backup
  
  // Write ISO to USB:
  sudo dd if=fedora.iso of=/dev/sdb bs=4M status=progress
  
  // Verify data integrity (compare source and destination):
  diff <(dd if=file1.img bs=512 count=10) <(dd if=file2.img bs=512 count=10)
  

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• locale / envsubst — Localization and variable substitution

  locale                             // Show current locale settings (language, encoding)
  
  // Set temporary environment:
  LANG=en_US.UTF-8 command           // Run with specific language setting only for this command
  

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• Only use sudo when necessary (not for every command)
• Check what a command does before running with elevated privileges:

cat /etc/passwd              // Read-only operation, often doesn't need sudo
rm -rf /var/cache/apt        // Dangerous without review!
sudo rm -rf /var/cache/apt   // Only after confirming contents are safe to remove

Add these to ~/.bashrc or ~/.zshrc:

alias ll='ls -la'
alias h='history | grep '
alias gs='git status'
alias gdiff='git diff --color-words'  // Git-friendly diff for code review

Then reload: source ~/.bashrc

# Instead of writing a script, chain commands:
cat access.log | grep "404" | awk '{print $1}' | sort -u > broken_links.txt

This approach is faster for one-off tasks and easier to debug.

tar czf /backup/config_backup.tar.gz /etc/nginx/
sudo cp /etc/passwd /etc/passwd.backup_$(date +%Y%m%d)

Always create backups before editing critical system files or running destructive commands.

• Shell Scripting: Learn bash scripting for automation (variables, loops, conditionals)
• Systemd Deep Dive: Master service management with systemctl, timers, and units
• Advanced Networking: Explore iptables, firewalld, and network namespaces
• Performance Tuning: Study strace, perf, and kernel parameters (sysctl)
• Containerization: Learn Docker/Podman for isolated environments
• Version Control: Master Git workflows alongside Linux CLI skills

This guide covers the most essential commands used daily by system administrators, developers, and power users. Practice regularly to build muscle memory and efficiency. 🚀