pwd                    # current directory
cd /path/to/dir        # change directory (absolute)
cd relative/path       # change directory (relative)
cd ..                  # go up one level
cd -                   # go to previous directory
cd ~                   # go to home directory
ls -la                 # list all files with details
ls -lh                 # human-readable file sizes
ls -lt                 # sort by modification time

touch file.txt                    # create empty file
mkdir -p dir/subdir               # create directory (with parents)
cp source dest                    # copy file
cp -r source/ dest/               # copy directory recursively
mv source dest                    # move or rename
rm file                           # delete file
rm -r directory/                  # delete directory
ln -s target linkname             # create symbolic link

cat file.txt              # print entire file
less file.txt             # page through file
head -20 file.txt         # first 20 lines
tail -20 file.txt         # last 20 lines
tail -f file.txt          # follow live updates
wc -l file.txt            # count lines

grep "pattern" file                    # search in file
grep -r "pattern" directory/           # recursive search
grep -i "pattern" file                 # case insensitive
grep -v "pattern" file                 # invert (non-matching lines)
find /path -name "*.txt"               # find files by name
find /path -type f -mtime -7           # files modified in last 7 days
find /path -size +10M                  # files larger than 10MB
which command                          # find command's binary location

cut -d: -f1 /etc/passwd          # extract field 1 (colon delimiter)
sort file.txt                    # sort alphabetically
sort -n numbers.txt              # sort numerically
sort | uniq                      # remove duplicates
sort | uniq -c                   # count occurrences
sed 's/old/new/g' file           # replace all occurrences
awk '{print $1}' file            # print first field
awk -F: '{print $1}' file        # colon-delimited first field
tr 'a-z' 'A-Z'                   # convert to uppercase

chmod 755 file              # rwxr-xr-x
chmod 644 file              # rw-r--r--
chmod 600 file              # rw------- (private)
chmod +x script.sh          # add execute permission
chmod -R 755 directory/     # recursive
chown user:group file       # change owner and group
sudo chown -R alice:alice /home/alice/

whoami                      # current user
id                          # uid, gid, and groups
id alice                    # info for another user
sudo command                # run as root
sudo -u user command        # run as specific user
adduser bob                 # create user interactively
usermod -aG docker alice    # add alice to docker group
groups                      # list current user's groups
passwd                      # change your password
sudo passwd bob             # change bob's password

ps aux                          # all running processes
ps aux | grep nginx             # search for process
top                             # live process viewer
htop                            # improved live viewer
kill PID                        # SIGTERM (polite)
kill -9 PID                     # SIGKILL (force)
pkill nginx                     # kill by name
command &                       # run in background
jobs                            # list background jobs
fg                              # bring job to foreground
nohup command &                 # run, survive logout

systemctl status nginx          # check service status
systemctl start nginx           # start service
systemctl stop nginx            # stop service
systemctl restart nginx         # restart service
systemctl reload nginx          # reload config
systemctl enable nginx          # start on boot
systemctl disable nginx         # do not start on boot
systemctl enable --now nginx    # enable and start now
journalctl -u nginx             # service logs
journalctl -u nginx -f          # follow service logs
journalctl -b                   # all logs since boot

ip addr show                    # IP addresses
ip route show                   # routing table
ping -c 4 google.com            # test connectivity
traceroute google.com           # trace path to host
ss -tlnp                        # listening TCP ports
dig google.com                  # DNS lookup
curl -I https://example.com     # HTTP headers only
ssh user@host                   # connect to server
scp file user@host:/path/       # copy file to server
rsync -avz src/ user@host:/dst/ # sync directory

df -h                       # disk space by filesystem
du -sh /var/log/            # directory size
du -sh * | sort -rh         # subdirectory sizes, largest first
lsblk                       # list block devices
mount                       # show mounted filesystems
sudo fdisk -l               # list disk partitions

# Ubuntu / Debian (apt)
sudo apt update                      # update package index
sudo apt upgrade                     # upgrade all packages
sudo apt install nginx               # install package
sudo apt remove nginx                # remove package
sudo apt autoremove                  # remove unused dependencies
apt search nginx                     # search for package
apt show nginx                       # package information
RHEL / Fedora / CentOS (dnf)
sudo dnf update
sudo dnf install nginx
sudo dnf remove nginx
dnf search nginx

command > file.txt              # stdout to file (overwrite)
command >> file.txt             # stdout to file (append)
command 2> errors.txt           # stderr to file
command > all.txt 2>&1          # stdout and stderr to file
command < input.txt             # read stdin from file
cmd1 | cmd2 | cmd3              # pipe chain
command > /dev/null 2>&1        # discard all output

man command             # full manual page
command --help          # quick help
info command            # detailed GNU documentation
apropos keyword         # find commands by keyword
type command            # what kind of command it is