Linux Command Line

# Where am I?
pwd                    # Print working directory

# List files
ls                     # Basic listing
ls -la                 # Long format, show hidden files
ls -lah                # Human-readable sizes

# Change directory
cd /home               # Go to /home
cd ~                   # Go to home directory
cd ..                  # Go up one level
cd -                   # Go to previous directory

# Create and remove
mkdir new_folder       # Create directory
mkdir -p a/b/c         # Create nested directories
touch file.txt         # Create empty file
rm file.txt            # Remove file
rm -rf folder/         # Remove directory (careful!)

# Copy and move
cp file.txt backup.txt # Copy file
cp -r dir1/ dir2/      # Copy directory
mv old.txt new.txt     # Rename/move file

# View file contents
cat file.txt           # Print entire file
head -n 20 file.txt    # First 20 lines
tail -n 20 file.txt    # Last 20 lines
less file.txt          # Scrollable viewer (q to quit)

# Search inside files
grep "flag" file.txt           # Find lines containing "flag"
grep -r "CTF{" ./              # Search recursively
grep -i "password" file.txt    # Case insensitive
grep -n "error" log.txt        # Show line numbers
grep -E "flag|ctf" file.txt    # Regex (OR)

# Find files
find . -name "*.txt"           # Find by name
find . -name "flag*"           # Find files starting with "flag"
find / -name "*.conf" 2>/dev/null  # Find config files
find . -type f -size +1M       # Files larger than 1MB

# Powerful combination
find . -type f -exec grep -l "password" {} \;

# Sort and unique
sort file.txt              # Sort lines alphabetically
sort -n numbers.txt        # Sort numerically
sort file.txt | uniq       # Remove duplicates
sort file.txt | uniq -c    # Count occurrences

# Cut and extract
cut -d':' -f1 /etc/passwd  # Extract first field (usernames)
cut -c1-10 file.txt        # First 10 characters of each line

# Stream editor (sed)
sed 's/old/new/g' file.txt # Replace all occurrences
sed -n '5,10p' file.txt    # Print lines 5-10

# Awk - powerful text processing
awk '{print $1}' file.txt      # Print first column
awk -F: '{print $1}' /etc/passwd  # Custom delimiter
awk '{sum+=$1} END {print sum}' numbers.txt

# Character translation
echo "hello" | tr 'a-z' 'A-Z'  # HELLO
echo "hello" | tr -d 'l'       # heo (delete)

# Word count
wc -l file.txt     # Count lines
wc -w file.txt     # Count words
wc -c file.txt     # Count bytes

# Pipes - output of one command becomes input of next
cat file.txt | grep "flag" | sort | uniq

# Output redirection
echo "hello" > file.txt    # Overwrite file
echo "world" >> file.txt   # Append to file
command 2> errors.txt      # Redirect stderr
command > out.txt 2>&1     # Redirect both

# Input redirection
sort < unsorted.txt

# Real CTF examples:
# Extract all URLs from a file
grep -oE 'https?://[^"]+' file.html

# Decode base64 and search for flag
cat encoded.txt | base64 -d | grep -i flag

# Find all unique email addresses
grep -oE '[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+' file.txt | sort -u

# Count occurrences of each word
cat file.txt | tr ' ' '\n' | sort | uniq -c | sort -rn

# View permissions
ls -la
# -rwxr-xr-x 1 user group 1234 Jan 1 00:00 file.txt
#  ^^^        Owner permissions (read, write, execute)
#     ^^^     Group permissions
#        ^^^  Others permissions

# Change permissions
chmod 755 script.sh    # rwxr-xr-x
chmod +x script.sh     # Add execute permission
chmod u+w file.txt     # Add write for user

# Change owner
chown user:group file.txt
chown -R user folder/   # Recursive

# Special permissions (important for CTF)
find / -perm -4000 2>/dev/null  # Find SUID binaries
find / -perm -2000 2>/dev/null  # Find SGID binaries

# Current user info
whoami                 # Current username
id                     # User and group IDs
groups                 # Group memberships
sudo -l                # What can I run as sudo?