pwd                      # print working directory
ls -la                   # list all files with details
cd /path/to/dir          # change directory
cd ~                     # go to home directory
cd -                     # go to previous directory

mkdir -p path/to/dir     # create directories (with parents)
touch file.txt           # create empty file
cp -r source/ dest/      # copy (recursive)
mv old new               # move/rename
rm -r directory/         # remove (recursive)

cat file.txt             # print entire file
less file.txt            # paginated viewer (q to quit)
head -20 file.txt        # first 20 lines
tail -20 file.txt        # last 20 lines
tail -f /var/log/app.log # follow (live updates)
wc -l file.txt           # count lines

find . -name "*.py"      # find files by name
grep -r "pattern" dir/   # search file contents
which command            # find command location
locate filename          # fast search (uses index)

whoami                   # current user
id                       # user ID, groups
sudo command             # run as root
su - username            # switch user

-rwxr-xr-- 1 alice devs 4096 Jan 1 12:00 script.sh
 │││ │││ │││
 │││ │││ └── Others: read only
 │││ └──── Group: read + execute
 └────── Owner: read + write + execute

chmod 755 script.sh      # rwxr-xr-x
chmod 644 file.txt       # rw-r--r--
chown alice:devs file    # change owner:group

# Debian/Ubuntu
sudo apt update                  # refresh package index
sudo apt install nginx           # install
sudo apt remove nginx            # remove
sudo apt upgrade                 # upgrade all packages

# RHEL/Amazon Linux
sudo yum install nginx
sudo yum update

# Service control
systemctl start nginx            # start service
systemctl stop nginx             # stop service
systemctl restart nginx          # restart
systemctl status nginx           # check status
systemctl enable nginx           # start on boot
systemctl disable nginx          # don't start on boot

# View logs
journalctl -u nginx              # logs for a service
journalctl -u nginx --since today
journalctl -f                    # follow all logs

ps aux                           # all processes
ps aux | grep nginx              # find specific process
top                              # real-time process monitor
htop                             # better process monitor

kill PID                         # graceful stop (SIGTERM)
kill -9 PID                      # force kill (SIGKILL)
pkill -f "pattern"              # kill by name pattern

ip addr show                     # show IP addresses
ip route show                    # show routing table
ss -tlnp                         # listening TCP ports
curl -v http://localhost         # HTTP request
ping host                        # test connectivity
dig domain.com                   # DNS lookup

echo $PATH                       # view variable
export MY_VAR="value"            # set for current session
env                              # list all env vars

# Persistent (add to ~/.bashrc or ~/.profile)
echo 'export MY_VAR="value"' >> ~/.bashrc
source ~/.bashrc                 # reload

# Connect to remote server
ssh user@hostname
ssh -i ~/.ssh/key.pem user@host  # with private key

# Copy files
scp file.txt user@host:/path/    # local → remote
scp user@host:/path/file.txt .   # remote → local

# SSH key generation
ssh-keygen -t ed25519 -C "email@example.com"
# Public key → ~/.ssh/id_ed25519.pub (share this)
# Private key → ~/.ssh/id_ed25519 (keep secret, chmod 600)