pwd                     # print working directory
ls                      # list files
ls -la                  # list all with details
ls -lh                  # human-readable sizes
cd /path/to/dir         # change directory
cd ~                    # home directory
cd ..                   # parent directory
cd -                    # previous directory

# Create
touch file.txt              # create empty file
mkdir my-folder             # create directory
mkdir -p a/b/c              # create nested dirs

# Copy
cp file.txt copy.txt        # copy file
cp -r folder/ copy/         # copy directory recursively

# Move / Rename
mv file.txt new-name.txt    # rename
mv file.txt /path/to/dir/   # move

# Delete
rm file.txt                 # delete file
rm -rf folder/              # delete directory (careful!)
rmdir folder/               # delete empty directory

# View files
cat file.txt                # print entire file
less file.txt               # paginated view (q to quit)
head -n 20 file.txt         # first 20 lines
tail -n 20 file.txt         # last 20 lines
tail -f logfile.log         # follow live (Ctrl+C to stop)

# Find files
find . -name "*.txt"                     # by name
find . -type f -name "*.js"              # files only
find . -type d -name "node_modules"      # directories
find . -mtime -7                         # modified in 7 days
find . -size +100M                       # files > 100MB

# Search inside files (grep)
grep "pattern" file.txt                  # basic search
grep -i "pattern" file.txt               # case-insensitive
grep -r "pattern" ./src/                 # recursive
grep -n "pattern" file.txt              # show line numbers
grep -l "pattern" *.js                  # show only filenames
grep -v "pattern" file.txt              # invert match
grep -E "regex+" file.txt               # extended regex

# View permissions
ls -l file.txt
# -rwxr-xr-x  1  user  group  1234  date  file.txt
#  ^--------  owner | group | others

# chmod numeric
chmod 755 file.sh    # rwxr-xr-x
chmod 644 file.txt   # rw-r--r--
chmod 600 secret.key # rw-------

# chmod symbolic
chmod +x script.sh   # add execute permission
chmod -w file.txt    # remove write permission
chmod u+x,g-w file   # owner +x, group -w

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

# View processes
ps aux                  # all running processes
ps aux | grep nginx     # filter processes
top                     # live process monitor
htop                    # better process monitor

# Kill processes
kill PID               # graceful kill (SIGTERM)
kill -9 PID            # force kill (SIGKILL)
killall nginx          # kill by name
pkill -f "python"      # kill by pattern

# Background jobs
command &              # run in background
jobs                   # list background jobs
fg %1                  # bring job 1 to foreground
nohup command &        # run even after logout

# System monitoring
df -h                  # disk usage
du -sh folder/         # folder size
free -h                # memory usage
uptime                 # system uptime

# Connectivity
ping google.com
curl https://api.example.com
wget https://file.com/download.zip

# curl with options
curl -X POST https://api.example.com/data \
  -H "Content-Type: application/json" \
  -d '{"key": "value"}'
curl -o output.html https://example.com

# Network info
ifconfig              # network interfaces
ip addr               # modern alternative
netstat -tuln         # open ports
ss -tuln              # modern alternative
nslookup google.com   # DNS lookup

# Sort and unique
sort file.txt
sort -r file.txt          # reverse
sort -n numbers.txt       # numeric sort
uniq file.txt             # remove adjacent duplicates

# Count
wc -l file.txt            # line count
wc -w file.txt            # word count
wc -c file.txt            # byte count

# sed — stream editor
sed 's/old/new/g' file.txt        # replace all
sed -i 's/old/new/g' file.txt     # in-place
sed -n '10,20p' file.txt          # print lines 10-20

# awk — text processing
awk '{print $1}' file.txt         # print first column
awk -F, '{print $2}' file.csv     # CSV second column
awk 'NR>1' file.txt               # skip first line

# tar
tar -czf archive.tar.gz folder/    # create gzip archive
tar -xzf archive.tar.gz            # extract
tar -tzf archive.tar.gz            # list contents

# zip
zip -r archive.zip folder/         # create zip
unzip archive.zip                  # extract
unzip -l archive.zip               # list contents

export MY_VAR="value"          # set env variable
echo $MY_VAR                   # print variable
unset MY_VAR                   # remove variable
env                            # all environment variables

# .bashrc / .zshrc
echo 'export PATH="$HOME/.local/bin:$PATH"' >> ~/.bashrc
source ~/.bashrc               # reload config

# Aliases
alias ll='ls -la'
alias gs='git status'

ssh user@hostname
ssh -p 2222 user@hostname      # custom port
ssh -i key.pem user@hostname   # with key file

# Copy files
scp file.txt user@host:/path/
scp user@host:/path/file.txt .
rsync -avz ./local/ user@host:/remote/

# SSH key setup
ssh-keygen -t ed25519 -C "email@example.com"
ssh-copy-id user@hostname