🐧 LINUX COMMANDS

pwd                        # print current directory
ls                         # list files
ls -la                     # list all files with details (including hidden)
ls -lh                     # human-readable file sizes
ls -lt                     # sort by modification time
cd /var/log               # change to absolute path
cd ..                      # go up one directory
cd ~                       # go to home directory
cd -                       # go to previous directory
tree                       # display directory tree
tree -L 2                  # limit tree depth to 2 levels

touch file.txt             # create empty file
mkdir myfolder             # create directory
mkdir -p a/b/c             # create nested directories
cp file.txt backup.txt    # copy file
cp -r folder/ dest/       # copy directory recursively
mv file.txt /tmp/         # move file
mv old.txt new.txt        # rename file
rm file.txt               # delete file
rm -rf folder/           # force delete directory (be careful!)
ln -s /etc/nginx nginx    # create symbolic link

cat file.txt              # print file contents
less file.txt             # scroll through file (q to quit)
head -n 20 file.txt       # first 20 lines
tail -n 20 file.txt       # last 20 lines
tail -f /var/log/syslog   # follow file live (great for logs)
find / -name "*.conf"     # find files by name
find /etc -type f -size +1M # find files over 1MB
locate nginx.conf         # fast file search (uses index)
which python3             # find binary location
file mystery.bin          # detect file type
stat file.txt             # detailed file metadata
du -sh folder/           # folder size, human readable
df -h                      # disk usage per filesystem

ls -l file.txt            # view permissions
chmod 755 script.sh       # set permissions (numeric)
chmod +x script.sh        # add execute for all
chmod u+x,g-w script.sh  # symbolic: user +exec, group -write
chmod -R 644 folder/      # recursive permission change
chown user:group file    # change owner and group
chown -R www-data /var/www # recursive ownership change
umask                      # view default permission mask
umask 022                 # set default mask

# View processes
ps aux                     # all running processes
ps aux | grep nginx      # find specific process
top                        # live process monitor
htop                       # better live monitor (install if missing)
pgrep nginx               # get PID by name
pidof nginx               # same as pgrep

# Control processes
kill 1234                  # send SIGTERM (graceful stop)
kill -9 1234              # send SIGKILL (force stop)
killall nginx             # kill all processes by name
pkill nginx               # kill by name pattern

# Background jobs
command &                  # run in background
jobs                       # list background jobs
fg %1                      # bring job 1 to foreground
bg %1                      # send job 1 to background
nohup script.sh &        # run immune to hangups
screen -S mysession      # start named screen session
tmux                       # terminal multiplexer

# grep — search text
grep "error" syslog        # search for pattern
grep -i "error" syslog    # case insensitive
grep -r "error" /var/log/ # recursive search
grep -n "error" syslog    # show line numbers
grep -v "debug" syslog    # exclude matching lines
grep -E "error|warn" log  # extended regex (OR)

# awk — column-based processing
awk '{print $1}' file      # print first column
awk -F: '{print $1}' /etc/passwd  # use : as delimiter
awk '$3 > 1000' /etc/passwd     # filter rows by column value

# sed — stream editor
sed 's/old/new/g' file    # replace all occurrences
sed -i 's/old/new/g' file # replace in-place
sed '/pattern/d' file     # delete matching lines
sed -n '10,20p' file      # print lines 10 to 20

# Other useful tools
sort file.txt             # sort lines alphabetically
sort -n file.txt          # sort numerically
uniq file.txt             # remove duplicate lines
wc -l file.txt           # count lines
cut -d: -f1 /etc/passwd  # cut first field using : delimiter
tr 'a-z' 'A-Z'           # translate characters (lowercase to upper)
echo "hello" | tee out.txt # write to file AND stdout

# Interfaces & IP
ip a                       # show all interfaces and IPs
ip r                       # show routing table
ip link set eth0 up        # bring interface up
ifconfig                   # legacy interface info

# Connectivity
ping -c 4 8.8.8.8         # ping 4 times
traceroute google.com     # trace network path
mtr google.com            # live traceroute (better)
curl https://example.com  # fetch URL
curl -I https://example.com # fetch headers only
wget https://example.com/file.zip # download file

# Ports & Sockets
ss -tulnp                 # show listening ports + process
netstat -tulnp            # legacy version of ss
lsof -i :80              # what's using port 80

# DNS
nslookup google.com       # basic DNS lookup
dig google.com            # detailed DNS query
dig google.com MX         # query MX records
host google.com           # simple DNS lookup

# SSH
ssh user@192.168.1.1      # connect to remote host
ssh -p 2222 user@host    # custom port
scp file.txt user@host:/tmp/ # copy file to remote
rsync -avz folder/ user@host:/dest/ # sync folder to remote

uname -a                  # kernel version + architecture
hostnamectl               # hostname, OS, kernel info
uptime                     # how long system has been running
whoami                     # current user
id                         # user ID, group ID, groups
w                          # who is logged in + what they're doing
last                       # login history
free -h                   # RAM usage, human readable
vmstat                     # memory, swap, CPU stats
lscpu                      # CPU architecture info
lsblk                      # list block devices / disks
lsusb                      # list USB devices
lspci                      # list PCI devices
dmesg | tail             # kernel ring buffer (hardware events)
journalctl -xe           # systemd logs
systemctl status nginx    # service status
systemctl start nginx     # start service
systemctl enable nginx    # enable on boot
systemctl restart nginx   # restart service

apt update                 # refresh package list
apt upgrade                # upgrade all packages
apt install nginx         # install package
apt remove nginx          # remove package
apt purge nginx           # remove + delete config files
apt autoremove             # remove unused dependencies
apt search nginx          # search for package
apt show nginx            # show package details
dpkg -l                    # list all installed packages