Essential Linux Command Line Reference

File Operations

Text Processing

# View file contents
cat file.txt          # Entire file
head -20 file.txt     # First 20 lines
tail -f /var/log/syslog  # Follow file (live)
less file.txt         # Scrollable viewer

# Search in files
grep "error" /var/log/*.log
grep -r "TODO" /var/www/    # Recursive
grep -i "warning" file.txt  # Case-insensitive
grep -c "error" access.log  # Count matches

# Text manipulation
sed 's/old/new/g' file.txt         # Replace text
awk '{print $1, $4}' access.log   # Extract columns
sort file.txt | uniq               # Sort and deduplicate
wc -l file.txt                     # Count lines

System Information

uname -a          # Kernel version
hostname          # Server name
uptime            # Uptime and load
whoami            # Current user
id                # User ID and groups
lsb_release -a    # OS version
nproc             # CPU count
free -h           # Memory usage
df -h             # Disk usage
du -sh /var/*     # Directory sizes

Process Management

ps aux                    # All processes
top / htop               # Interactive process viewer
kill PID                 # Graceful stop
kill -9 PID              # Force kill
pkill -f "process name"  # Kill by name
jobs                     # Background jobs
bg / fg                  # Background/foreground
nohup command &          # Run after logout

Networking

ip addr                  # IP addresses
ss -tlnp                 # Listening ports
curl -I example.com      # HTTP headers
wget file-url            # Download file
ping host                # Connectivity test
dig domain               # DNS lookup
traceroute host          # Network path
nc -zv host port         # Port check

Piping and Redirection

# Pipe output to next command
ps aux | grep nginx | wc -l

# Redirect output
command > file.txt        # Overwrite
command >> file.txt       # Append
command 2> errors.txt     # Stderr only
command &> all.txt        # Both stdout and stderr
command 2>/dev/null       # Discard errors

Useful Shortcuts

Tip Use history | grep "keyword" or Ctrl+R to search through your command history. It's faster than retyping commands.