💻 Command Line Cheat Sheet

pwd                  # Print working directory
ls -la               # List all (hidden + details)
ls -lh               # Human-readable sizes
ls -lt               # Sort by modified time
cd ~/projects        # Home-relative path
cd -                 # Previous directory
mkdir -p a/b/c       # Create nested dirs
tree -L 2            # Directory tree (2 levels)
tree -I 'node_modules|.git'  # Exclude patterns

cp file.txt backup/          # Copy file
cp -r src/ dest/             # Copy directory recursively
mv old.txt new.txt           # Move / rename
rm file.txt                  # Remove file
rm -rf temp_dir/             # Remove dir recursively (CAREFUL!)
touch new_file.txt           # Create empty / update timestamp
ln -s /path/to/target link   # Symbolic link
ln target hardlink           # Hard link
stat file.txt                # Detailed file info

cat file.txt         # Print entire file
head -n 20 file.txt  # First 20 lines
tail -n 20 file.txt  # Last 20 lines
tail -f server.log   # Follow log output (live)
less large_file.txt  # Scrollable viewer (q to quit)
wc -l file.txt       # Line count
wc -w file.txt       # Word count
wc -c file.txt       # Byte count
file mystery.dat     # Detect file type
diff file1.txt file2.txt  # Compare files

find . -name '*.py'                    # Find by name
find . -name '*.log' -mtime +30        # Modified 30+ days ago
find . -type f -size +100M             # Files > 100MB
find . -name '*.tmp' -delete           # Find and delete
find . -name '*.py' -exec wc -l {} +  # Execute on results
find . -empty                          # Empty files/dirs
find . -perm 777                       # Find by permissions

# locate (faster, uses index)
locate '*.conf'
updatedb  # Refresh locate index

grep 'ERROR' app.log              # Search for pattern
grep -r 'TODO' src/               # Recursive search
grep -i 'warning' log.txt         # Case insensitive
grep -n 'def ' main.py            # Show line numbers
grep -c 'pattern' file.txt        # Count matches
grep -v 'DEBUG' app.log           # Invert (exclude)
grep -l 'import' *.py             # List files only
grep -A 3 -B 1 'ERROR' log.txt   # Context (3 after, 1 before)
grep -E '(error|warn)' log.txt    # Extended regex (egrep)
grep -P '\d{3}-\d{4}' contacts   # Perl regex

sed 's/old/new/g' file.txt         # Find & replace (stdout)
sed -i 's/http/https/g' config.yml # In-place edit
sed -n '5,10p' file.txt            # Print lines 5-10
sed '3d' file.txt                  # Delete line 3
sed '/^$/d' file.txt               # Delete empty lines
sed 's/^ *//' file.txt             # Trim leading spaces
sed -n '/START/,/END/p' file.txt   # Print between patterns

awk '{print $1, $3}' data.txt       # Print columns 1 & 3
awk -F',' '{print $2}' data.csv     # CSV: print column 2
awk '{sum += $3} END {print sum}' f  # Sum column 3
awk 'NR > 1 {print $0}' file.txt    # Skip header row
awk '$3 > 100 {print $1, $3}' f     # Filter by value
awk '{print NR": "$0}' file.txt     # Add line numbers
awk '!seen[$1]++' file.txt          # Unique by column 1

sort file.txt               # Sort lines alphabetically
sort -n file.txt             # Numeric sort
sort -k 2 -t',' data.csv    # Sort by column 2 (comma delim)
sort -u file.txt             # Sort + unique

uniq file.txt                # Remove consecutive dupes
sort file.txt | uniq -c      # Count occurrences

cut -d',' -f1,3 data.csv    # Extract columns 1 & 3
cut -c1-10 file.txt          # First 10 characters per line

tr 'a-z' 'A-Z' < file.txt   # Uppercase
tr -d '\r' < file.txt       # Remove carriage returns
tr -s ' ' < file.txt        # Squeeze spaces

cat log.txt | grep ERROR | wc -l       # Count errors
ps aux | sort -k 3 -rn | head -5      # Top 5 by CPU
history | grep 'docker'                # Search history

# Redirection
command > out.txt        # Stdout to file (overwrite)
command >> out.txt       # Stdout append
command 2> err.txt       # Stderr to file
command > out.txt 2>&1   # Both stdout+stderr
command &> all.txt       # Same (bash shorthand)
command < input.txt      # Stdin from file

# tee: stdout + file simultaneously
command | tee output.txt
command | tee -a output.txt  # Append mode

chmod 755 script.sh      # rwxr-xr-x
chmod +x script.sh       # Add execute
chmod -R 644 dir/        # Recursive

# Permission digits: r=4, w=2, x=1
# 755 = rwxr-xr-x (owner: all, group/others: read+exec)
# 644 = rw-r--r-- (owner: rw, group/others: read)
# 700 = rwx------ (owner only)

chown user:group file.txt
chown -R deploy:www /var/www/

whoami                   # Current user
id                       # User ID, groups
groups username           # User's groups
sudo useradd newuser     # Create user
sudo passwd newuser      # Set password
sudo usermod -aG docker user  # Add to group
su - username            # Switch user
sudo command             # Run as root

ps aux                    # List all processes
ps aux | grep python      # Filter processes
top                       # Real-time monitor
htop                      # Better interactive monitor

kill <PID>               # Graceful kill (SIGTERM)
kill -9 <PID>            # Force kill (SIGKILL)
killall python            # Kill all by name
pkill -f 'pattern'       # Kill by pattern

nohup script.sh &         # Background, survive logout
jobs                      # List background jobs
fg %1                    # Bring job 1 to foreground
bg %1                    # Resume in background
Ctrl+Z                    # Suspend foreground process

df -h                     # Disk space by filesystem
df -h /                   # Root partition
du -sh folder/            # Folder total size
du -sh * | sort -rh | head -10  # Top 10 largest
ncdu                      # Interactive disk usage

lsblk                     # List block devices
mount | column -t         # Mounted filesystems

curl -s https://api.example.com | jq '.'
curl -X POST -H 'Content-Type: application/json' \
  -d '{"key":"val"}' https://api.example.com

wget https://example.com/file.zip
wget -O output.zip https://example.com/file.zip

ping -c 4 google.com      # Connectivity check
nslookup domain.com        # DNS lookup
netstat -tlnp              # Open ports
ss -tulnp                  # Modern netstat
lsof -i :8080             # What's on port 8080

ssh user@host.com          # Connect
ssh -i key.pem user@host   # Connect with key
ssh -p 2222 user@host      # Custom port

# Copy files over SSH
scp file.txt user@host:/path/
scp -r folder/ user@host:/path/
scp user@host:/remote/file.txt ./local/

# SSH tunnel (port forwarding)
ssh -L 8080:localhost:80 user@host  # Local forward
ssh -N -L 5432:db-host:5432 bastion-host  # DB tunnel

# SSH config (~/.ssh/config)
# Host myserver
#   HostName 1.2.3.4
#   User deploy
#   IdentityFile ~/.ssh/my_key

# tar (archive + compress)
tar -czf archive.tar.gz folder/     # Create gzip
tar -xzf archive.tar.gz             # Extract gzip
tar -cjf archive.tar.bz2 folder/    # Create bzip2
tar -xjf archive.tar.bz2            # Extract bzip2
tar -tf archive.tar.gz              # List contents

# zip
zip -r archive.zip folder/
unzip archive.zip
unzip -l archive.zip  # List contents

# gzip single file
gzip file.txt          # Compresses to file.txt.gz
gunzip file.txt.gz     # Decompress

echo $PATH                # Print variable
export MY_VAR='value'     # Set for session
unset MY_VAR              # Remove

# Persist in ~/.bashrc or ~/.zshrc
echo 'export API_KEY=abc' >> ~/.bashrc
source ~/.bashrc          # Reload

env                        # All env vars
printenv HOME              # Specific var
which python               # Find executable
type -a python             # All locations

crontab -e                 # Edit cron jobs
crontab -l                 # List cron jobs

# Format: min hour day month weekday command
0 * * * *    /scripts/hourly.sh    # Every hour
30 2 * * *   /scripts/nightly.sh   # Daily 2:30 AM
0 0 * * 0    /scripts/weekly.sh    # Weekly Sunday
*/5 * * * *  /scripts/check.sh     # Every 5 minutes
0 9 1 * *    /scripts/monthly.sh   # 1st of month 9 AM

# Redirect output
0 * * * * /scripts/job.sh >> /var/log/job.log 2>&1

docker run -d --name myapp -p 8080:80 nginx
docker run -it ubuntu bash       # Interactive shell
docker ps                        # Running containers
docker ps -a                     # All containers
docker stop myapp
docker start myapp
docker rm myapp
docker logs -f myapp             # Follow logs
docker exec -it myapp bash       # Shell into running

docker images                    # List images
docker pull python:3.11
docker build -t myapp:v1 .
docker rmi image_name            # Remove image

# Volumes (persistent data)
docker volume create mydata
docker run -v mydata:/app/data myapp
docker run -v $(pwd):/app myapp  # Bind mount

# Docker Compose
docker compose up -d
docker compose down
docker compose logs -f

# Run command for each input line
find . -name '*.log' | xargs rm
find . -name '*.py' | xargs grep 'TODO'
cat urls.txt | xargs -I {} curl -s {}
echo '1 2 3' | xargs -n 1 echo  # One arg per run
find . -name '*.jpg' | xargs -P 4 -I {} convert {} {}.png  # Parallel

alias ll='ls -la'
alias gs='git status'
alias dc='docker compose'
alias k='kubectl'

# Persist in ~/.bashrc or ~/.zshrc
echo "alias ll='ls -la'" >> ~/.bashrc

history                    # Command history
!!                         # Repeat last command
!grep                      # Last command starting with grep
Ctrl+R                    # Reverse search history