command  -options  arguments
   |         |         |
 command   switches   inputs

ls -la /home

ls -a          # short
ls --all       # long
ls -la         # multiple combined switches

/etc/nginx/nginx.conf    # absolute (starts with /)
./config.txt             # relative (from the current path)
../logs/                 # relative (one level up)

pwd
# /home/ali

ls                # simple listing
ls -l             # detailed listing
ls -a             # show hidden files
ls -lh            # human-readable sizes (KB, MB)
ls -la /etc       # combined options on a specific path
ls -R             # recursive

-rw-r--r-- 1 ali ali 4096 Sep 11 10:30 file.txt
│└┬┘└┬┘└┬┘ │  │   │    │        │      │
│ │  │  │  │  │   │    │        │      └─ file name
│ │  │  │  │  │   │    │        └─ date
│ │  │  │  │  │   │    └─ size
│ │  │  │  │  │   └─ group
│ │  │  │  │  └─ owner
│ │  │  │  └─ link count
│ └──┴──┴─ permissions
└─ file type (- file, d directory, l link)

cd /etc           # go to an absolute path
cd nginx          # go to a directory inside the current path
cd ..             # go one level up
cd ~              # go to home directory
cd -              # return to the previous directory
cd                # go to home directory

touch file.txt              # create an empty file
mkdir mydir                 # create a directory
mkdir -p a/b/c              # create nested directories

cp file.txt /tmp/           # copy a file
cp -r mydir /tmp/           # recursively copy a directory
cp -v file.txt backup.txt   # show details

mv file.txt /tmp/           # move
mv old.txt new.txt          # rename

rm file.txt                 # delete a file
rm -r mydir                 # delete a directory
rm -rf mydir                # force delete without confirmation (dangerous)
rm -i file.txt              # delete with confirmation

cat file.txt                # print the whole file
less file.txt               # page-by-page view (q to quit)
head file.txt               # first ten lines
head -n 20 file.txt         # first twenty lines
tail file.txt               # last ten lines
tail -f /var/log/syslog     # follow the log live (Ctrl+C to exit)

stat file.txt                     # show full metadata
stat -c '%U:%G %a %n' file.txt    # custom output: owner:group perms name
stat -c '%a' file.txt             # octal permissions only
stat -c '%U' file.txt             # owner name only
stat -c '%G' file.txt             # group name only

# Inspect multiple files with one command
stat -c '%U:%G %a %n' /srv/app/deploy.sh /srv/app/lib.sh /srv/app/audit.sh

root:root 755 /srv/app/deploy.sh
root:root 755 /srv/app/lib.sh
root:root 755 /srv/app/audit.sh

find /srv -type f -exec stat -c '%a %n' {} \; | grep -E '^77[0-7]'

find /etc -name "*.conf"              # .conf files
find / -type d -name "nginx"          # directory named nginx
find /var/log -size +10M              # files larger than 10MB
find /home -mtime -7                  # modified in the past seven days

grep "error" /var/log/syslog          # lines containing error
grep -i "error" file.txt              # case-insensitive
grep -r "localhost" /etc/             # recursive search in a directory
grep -n "root" /etc/passwd            # show line numbers
grep -v "comment" file.txt            # lines NOT matching the pattern

ps aux | grep nginx                   # nginx processes
cat /etc/passwd | wc -l               # number of users
ls -la | less                         # view the listing page by page
history | grep "apt"                  # apt commands in history

ls > files.txt                # write output (overwrite)
ls >> files.txt               # append output
cat < files.txt               # use file as input
command 2> error.log          # store errors only
command &> all.log            # store output and errors
command > /dev/null           # discard output

cd /var/log && ls -lh          # only run ls if cd succeeded
apt update && apt upgrade       # update then upgrade
command1 || echo "failed"       # print a message if the first command failed

nano /etc/hosts

man ls                # full manual
ls --help             # concise help
man -k "copy"         # search the manuals
apropos network       # commands related to networking

# 1. Go to /var/log and list its contents
cd /var/log && ls -lh

# 2. Create a practice directory
mkdir ~/practice && cd ~/practice

# 3. Create three files
touch file1.txt file2.txt file3.txt

# 4. Add content
echo "hello linux" > file1.txt
echo "server admin" >> file1.txt

# 5. View the content
cat file1.txt

# 6. Search across all files
grep "linux" *.txt

# 7. Count lines in the passwd file
wc -l /etc/passwd

# 8. View the last five users on the system
tail -n 5 /etc/passwd

# 9. Clean up
cd ~ && rm -rf practice

lsb_release -a          # show all distribution information
lsb_release -d          # description only
lsb_release -r          # release number only
lsb_release -c          # codename only
lsb_release -i          # distributor ID only
lsb_release -s -r       # release only, without label (script-friendly)

No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 22.04.3 LTS
Release:        22.04
Codename:       jammy

#!/bin/bash
VERSION=$(lsb_release -rs)
if [ "$VERSION" = "22.04" ]; then
    echo "This script targets Ubuntu 22.04"
else
    echo "Unsupported version: $VERSION"
    exit 1
fi

cat /etc/os-release         # newer standard, available on most distributions
cat /etc/issue              # simple and quick
hostnamectl                 # system and distribution info
uname -a                    # kernel info (not distribution)

ping google.com              # test connectivity to a host
ping -c 4 8.8.8.8            # send four packets and stop

ip a                         # show interfaces and addresses
ip r                         # show the routing table
ip link                      # link status

ss -tulnp                    # open ports and their processes
ss -tan                      # TCP connections

apt install tcpdump
sudo tcpdump -D                  # list capture-capable interfaces

sudo tcpdump -ni eth0                            # all traffic on eth0
sudo tcpdump -ni eth0 'host 8.8.8.8'            # traffic with a specific host
sudo tcpdump -ni eth0 'tcp port 443'            # HTTPS only
sudo tcpdump -ni eth0 'udp port 53'             # DNS only
sudo tcpdump -ni eth0 'src 192.168.1.10'        # packets originating from this IP
sudo tcpdump -ni eth0 'dst 192.168.1.10'        # packets sent to this IP
sudo tcpdump -ni eth0 'net 192.168.1.0/24'      # an entire subnet
sudo tcpdump -ni eth0 'tcp[tcpflags] & tcp-syn != 0'  # SYN packets only

sudo tcpdump -ni tun0 -nn 'host 192.168.9.101 and tcp port 443'
sudo tcpdump -ni eth0 'not port 22 and not arp'

sudo tcpdump -ni any -nn 'host 192.168.9.101 and tcp port 443'

14:23:01.123456 IP 192.168.9.50.54321 > 192.168.9.101.443: Flags [S], seq ...
14:23:01.123789 IP 192.168.9.101.443 > 192.168.9.50.54321: Flags [S.], ack ...
14:23:01.124001 IP 192.168.9.50.54321 > 192.168.9.101.443: Flags [.], ack ...

sudo tcpdump -ni eth0 -s 0 -w capture.pcap 'tcp port 80'

sudo tcpdump -ni eth0 -A 'tcp port 80 and host example.com'

sudo tcpdump -ni eth0 -vv 'port 67 or port 68'    # DHCP
sudo tcpdump -ni eth0 'udp port 53'               # DNS

sudo tcpdump -ni tun0 -nn 'host 10.8.0.1'

# Count packets sent to a specific IP
sudo tcpdump -ni eth0 -c 100 'dst 8.8.8.8' 2>/dev/null | wc -l

# Save to a compressed file for transfer
sudo tcpdump -ni eth0 -s 0 -w - 'port 443' | gzip > capture.pcap.gz

# Read from a file and search
sudo tcpdump -r capture.pcap -nn | grep '192.168.9.101'

curl https://example.com                   # fetch content
curl -I https://example.com                # headers only
curl -o file.zip https://example.com/f.zip # save to a file
curl -fsSL https://example.com/install.sh | bash   # fetch and run

wget https://example.com/file.zip          # download a file
wget -c https://example.com/file.zip       # resume a partial download

sudo apt --fix-broken install              # fix broken dependencies
sudo dpkg -i package.deb                   # install a downloaded .deb file
sudo apt install -f                        # complete installation after dpkg

rsync -avz /home/ /backup/home/
rsync -avz --delete /src/ /dst/
rsync -avz -e ssh /home/ user@server:/backup/

apt install tmux
tmux new -s work           # create a new session
tmux attach -t work        # attach to a session
tmux ls                    # list sessions

useradd ali               # create a user
useradd -m -s /bin/bash ali   # create a user with a home directory and shell
passwd ali                # set a password
usermod -aG sudo ali      # add to the sudo group
userdel -r ali            # delete the user and their home directory

groupadd developers       # create a group
groupdel developers       # delete a group
groups ali                # show the groups a user belongs to
id ali                    # if permitted: show full user information

id                        # show full information for the current user, e.g.:
                          # uid=1001(admin) gid=1001(admin) groups=1001(admin),27(sudo),100(users)

chmod 755 script.sh       # set permissions
chmod u+x script.sh       # add execute permission for the owner
chown ali:ali file.txt    # change owner and group

-rwxr-xr-x 1 ali ali 4096 Sep 11 10:30 script.sh
│└┬┘└┬┘└┬┘
│ │  │  └── permissions for other users (others)
│ │  └───── permissions for the group (group)
│ └──────── permissions for the owner (user)
└────────── file type: - file, d directory, l link

owner  → rwx → 7
group  → r-x → 5
others → r-x → 5
Result: 755

chmod 755 script.sh       # rwxr-xr-x
chmod 644 config.txt      # rw-r--r--
chmod 600 ~/.ssh/id_rsa   # rw-------
chmod -R 755 /var/www     # recursive on a directory

chmod u+x script.sh       # add execute for owner
chmod g-w file.txt        # remove write from group
chmod o-rwx secret.txt    # remove all permissions from others
chmod a+r public.txt      # add read for everyone (a = all)
chmod u=rw,g=r,o= file    # set exact permissions

apt update                # update the package list
apt upgrade               # upgrade installed packages
apt install nginx         # install a package
apt remove nginx          # remove a package
apt autoremove            # remove unused packages
apt search nginx          # search for a package

cat /etc/apt/sources.list
add-apt-repository ppa:example/ppa

systemctl start nginx     # start a service
systemctl stop nginx      # stop a service
systemctl restart nginx   # restart a service
systemctl enable nginx    # enable at boot
systemctl status nginx    # view status

journalctl -u nginx       # logs for a specific service
journalctl -f             # follow live logs
journalctl --since "1 hour ago"

nano /etc/netplan/01-netcfg.yaml

network:
  version: 2
  ethernets:
    eth0:
      dhcp4: true

netplan apply             # apply configuration

ip a                      # show network addresses
ip r                      # show routing table
ss -tulnp                 # show open ports
ping google.com           # test connectivity

ufw enable                # enable the firewall
ufw status                # view status
ufw allow 22              # allow the SSH port
ufw allow 80/tcp          # allow HTTP
ufw deny 23               # block a port
ufw delete allow 80       # remove a rule

lsblk                     # show disks and partitions
df -h                     # Disk Free - show disk usage
du -sh /var/log           # directory size

fdisk /dev/sdb            # partition a disk
mkfs.ext4 /dev/sdb1       # create a filesystem
mount /dev/sdb1 /mnt      # mount

apt install nginx
systemctl enable --now nginx

nano /etc/nginx/sites-available/example.com
ln -s /etc/nginx/sites-available/example.com /etc/nginx/sites-enabled/
nginx -t
systemctl reload nginx

apt install certbot python3-certbot-nginx
certbot --nginx -d example.com

apt install docker.io
systemctl enable --now docker

docker run -d -p 80:80 nginx
docker ps
docker stop <container_id>
docker logs <container_id>

#!/bin/bash
echo "Starting backup..."
tar -czf /backup/home-$(date +%F).tar.gz /home
echo "Backup completed."

echo $SHELL                # the user's current shell
cat /etc/shells            # allowed shells on the system
chsh -s /bin/zsh           # change the user's default shell (optional)

nano ~/.bashrc             # edit shell settings
source ~/.bashrc           # apply changes without closing the terminal

# aliases for common commands
alias ll='ls -alF'
alias update='sudo apt update && sudo apt upgrade'
alias ports='ss -tulnp'

# add a personal path to PATH
export PATH="$HOME/bin:$PATH"

# custom environment variable
export EDITOR=nano

# colored prompt with git branch
parse_git_branch() {
    git branch 2> /dev/null | sed -e '/^[^*]/d' -e 's/* \(.*\)/ (\1)/'
}
export PS1="\u@\h \w\[\033[32m\]\$(parse_git_branch)\[\033[00m\] $ "

# typical content of .bash_profile
if [ -f ~/.bashrc ]; then
    source ~/.bashrc
fi

#!/bin/bash

NAME="Ali"
AGE=30
CITY="Tehran"

echo "Name: $NAME"
echo "Age: $AGE"
echo "City: ${CITY}"

# read-only (constant) variable
readonly PI=3.14

# special variables
echo "Script: $0"           # script name
echo "First argument: 
"    # first argument
echo "Argument count: $#"    # number of arguments
echo "All arguments: $@"     # all arguments
echo "Last exit code: $?"    # exit status
echo "Script PID: $"        # process ID

A=10
B=3
echo $((A + B))     # 13
echo $((A * B))     # 30
echo $((A / B))     # 3 (integer division)
echo $((A % B))     # 1 (remainder)

#!/bin/bash

FILE="/etc/passwd"

if [ -f "$FILE" ]; then
    echo "File exists"
elif [ -d "$FILE" ]; then
    echo "It is a directory"
else
    echo "Not found"
fi

# numeric comparison
NUM=10
if [ "$NUM" -gt 5 ]; then
    echo "Greater than 5"
fi

# string comparison
NAME="Ali"
if [ "$NAME" = "Ali" ]; then
    echo "Hello Ali"
fi

#!/bin/bash

ACTION="
"

case "$ACTION" in
    start)
        echo "Starting service..."
        ;;
    stop)
        echo "Stopping service..."
        ;;
    restart)
        echo "Restarting..."
        ;;
    *)
        echo "Usage: $0 {start|stop|restart}"
        exit 1
        ;;
esac

#!/bin/bash

# over a list
for FRUIT in apple banana orange; do
    echo "Fruit: $FRUIT"
done

# over a numeric range
for i in {1..5}; do
    echo "Number: $i"
done

# over files
for FILE in /var/log/*.log; do
    echo "Processing $FILE"
done

# C-style
for ((i=0; i<10; i++)); do
    echo "i=$i"
done

#!/bin/bash

COUNT=1
while [ "$COUNT" -le 5 ]; do
    echo "Count: $COUNT"
    COUNT=$((COUNT + 1))
done

# read a file line by line
while IFS= read -r LINE; do
    echo "Line: $LINE"
done < /etc/passwd

#!/bin/bash

COUNT=1
until [ "$COUNT" -gt 5 ]; do
    echo "Count: $COUNT"
    COUNT=$((COUNT + 1))
done

#!/bin/bash

# function definition
greet() {
    local NAME="
"        # local: local variable
    echo "Hello $NAME!"
}

# function with a return value
add() {
    echo $(( 
 + 
        
     ))
}

# function with error checking
check_root() {
    if [ "$EUID" -ne 0 ]; then
        echo "This script requires root privileges"
        exit 1
    fi
}

# calling
greet "Ali"
RESULT=$(add 5 3)
echo "Result: $RESULT"
check_root

#!/bin/bash

# read input from the user
read -p "Enter your name: " USER_NAME
echo "Hello $USER_NAME"

# read a password (hidden)
read -sp "Password: " PASSWORD
echo

# read with timeout
read -t 5 -p "Answer quickly: " ANSWER

# write to a file
echo "New text" >> /tmp/log.txt

# read from a file
while read -r LINE; do
    echo "$LINE"
done < /tmp/log.txt

#!/bin/bash
set -euo pipefail

# -e : exit on error
# -u : error on undefined variable
# -o pipefail : error if any command in a pipe fails

# run a command and check for errors
if ! command -v nginx > /dev/null 2>&1; then
    echo "nginx is not installed"
    exit 1
fi

# trap: run a command on exit
cleanup() {
    echo "Cleaning up..."
    rm -f /tmp/tempfile
}
trap cleanup EXIT

#!/bin/bash
set -euo pipefail

# settings
BACKUP_DIR="/backup"
SOURCE_DIR="/home"
DATE=$(date +%F_%H-%M)
ARCHIVE="$BACKUP_DIR/home-$DATE.tar.gz"
KEEP_DAYS=7

# check root
if [ "$EUID" -ne 0 ]; then
    echo "Please run with sudo"
    exit 1
fi

# create the backup directory
mkdir -p "$BACKUP_DIR"

# create the backup
echo "Starting backup of $SOURCE_DIR ..."
tar -czf "$ARCHIVE" "$SOURCE_DIR"
echo "Backup created: $ARCHIVE"

# remove old backups
echo "Removing backups older than $KEEP_DAYS days..."
find "$BACKUP_DIR" -name "home-*.tar.gz" -mtime +$KEEP_DAYS -delete

# show disk usage
echo "Current backup usage:"
du -sh "$BACKUP_DIR"

crontab -e
# run daily at 3 AM
0 3 * * * /usr/local/bin/backup.sh

apt install unattended-upgrades
dpkg-reconfigure --priority=low unattended-upgrades

rsync -avz /home/ user@backup-server:/backup/

apt install tree                  # install the tool

tree                              # show the current path as a tree
tree /etc                         # show a specific path as a tree
tree -L 2                         # only two levels deep
tree -d                           # directories only
tree -a                           # show hidden files
tree -h                           # show file sizes
tree -I "node_modules|.git"       # ignore patterns

apt install htop
htop

apt install ncdu
ncdu /var
ncdu /home

apt install jq

curl -s https://api.example.com/data | jq .
cat data.json | jq '.items[] | .name'

node --version               # Node.js version
npm --version                # npm version

npm install                  # install project dependencies
npm install -g <package>     # install a package globally
sudo npm install -g <package> # install globally with admin privileges
npx <command>                # run a package directly without permanent install

npm install                  # install dependencies
npm run <script>             # run a script defined in package.json
npm run build                # build a production version
npm start                    # start the project

docker --version             # Docker version

docker ps                    # running containers
docker ps -a                 # all containers
docker images                # image list

docker volume ls             # volume list
docker volume prune          # remove unused volumes

docker system df             # Docker disk usage
docker system prune          # clean up unused resources

docker compose up            # start services
docker compose up -d         # start in the background
docker compose down          # stop and remove services
docker compose ps            # service status
docker compose logs          # view logs
docker compose logs -f       # follow live logs

docker exec -it <name> bash  # enter a container
docker stop <name>           # stop a container
docker start <name>          # start a container
docker restart <name>        # restart a container
docker rm <name>             # remove a container

git status                   # repository status
git branch                   # list branches
git checkout <branch>        # switch branch (legacy)
git switch <branch>          # switch branch (modern)
git add .                    # stage changes
git commit -m "message"      # commit changes
git push                     # push to remote
git pull                     # pull from remote
git log                      # commit history
git diff                     # show changes
git remote -v                # list remotes

npx wrangler                       # run Wrangler
npx wrangler --help                # general help
npx wrangler observability --help  # observability help
npx wrangler deploy                # deploy the project

python3 --version            # Python version
python3                      # enter the Python interpreter

pip --version                # pip version
pip3 install <package>       # install a package

uv --version                 # uv version
uv run <script>              # run a script with uv
uv tool install <tool>       # install a global tool with uv