# Navigation
cd /path/to/directory
cd ..  # parent directory
cd ~   # home directory
pwd    # current directory

# List files
ls
ls -l   # detailed
ls -a   # hidden files
ls -lh  # human readable

# File operations
cp file1 file2
mv file1 file2
rm file
rm -r directory
mkdir directory

# View files
cat file
less file
head -n 10 file
tail -n 10 file
tail -f file  # follow

# Search content
grep "pattern" file
grep -r "pattern" directory
find . -name "*.txt"
find . -type f -mtime -7

# View permissions
ls -l file
stat file

# Change permissions
chmod 755 file
chmod u+x file
chmod -R 755 directory

# Change ownership
chown user:group file
chown -R user:group directory

# View processes
ps
ps aux
top
htop

# Process control
kill PID
kill -9 PID  # force kill
killall process_name
pkill pattern

# Background jobs
command &
nohup command &
jobs
fg %1
bg %1

# System information
uname -a
hostnamectl
lsb_release -a

# Hardware info
lscpu
free -h
df -h
du -sh directory

# Network info
ifconfig
ip addr
netstat -tulpn
ss -tulpn

# Debian/Ubuntu
apt update
apt upgrade
apt install package
apt remove package
apt search package

# Red Hat/CentOS
yum update
yum install package
yum remove package
yum search package

# Arch Linux
pacman -Syu
pacman -S package
pacman -R package
pacman -Ss package

# Network configuration
ifconfig
ip addr
ip route
nmcli

# Network testing
ping host
traceroute host
mtr host
nslookup domain
dig domain

# Network monitoring
netstat -tulpn
ss -tulpn
iftop
nethogs

# iptables
iptables -L
iptables -A INPUT -p tcp --dport 80 -j ACCEPT
iptables -D INPUT -p tcp --dport 80 -j ACCEPT

# ufw (Ubuntu)
ufw status
ufw allow 80/tcp
ufw deny 80/tcp
ufw enable
ufw disable

# firewalld (RHEL)
firewall-cmd --list-all
firewall-cmd --add-port=80/tcp
firewall-cmd --remove-port=80/tcp

# SSH connection
ssh user@host
ssh -p port user@host
ssh -i key.pem user@host

# SSH config
Host myserver
    HostName example.com
    User username
    Port 22
    IdentityFile ~/.ssh/id_rsa

# SSH key management
ssh-keygen -t rsa
ssh-copy-id user@host

# User operations
useradd username
userdel username
usermod -aG group username
passwd username

# Group operations
groupadd groupname
groupdel groupname
groupmod -n newname oldname

# User info
id username
groups username
who
w

# systemd
systemctl status service
systemctl start service
systemctl stop service
systemctl restart service
systemctl enable service

# Service files
[Unit]
Description=My Service
After=network.target

[Service]
Type=simple
User=myuser
ExecStart=/usr/bin/myprogram
Restart=always

[Install]
WantedBy=multi-user.target

# System logs
journalctl
journalctl -f
journalctl -u service
journalctl --since "1 hour ago"

# Log files
/var/log/syslog
/var/log/auth.log
/var/log/kern.log
/var/log/dmesg

# Log rotation
logrotate -d /etc/logrotate.conf

# Security measures
- Regular updates
- Firewall configuration
- SSH hardening
- User permissions
- File permissions

# Password policy
- Strong passwords
- Regular rotation
- Password complexity
- Account locking
- Failed login monitoring

# System hardening
- Disable unused services
- Remove unnecessary packages
- Configure SELinux/AppArmor
- Regular security audits
- Backup strategy

# System monitoring
- Resource usage
- Process management
- Disk usage
- Network traffic
- Log analysis

# Optimization
- Kernel parameters
- File system tuning
- Network optimization
- Service optimization
- Cache management

# Maintenance
- Regular updates
- Disk cleanup
- Log rotation
- Backup verification
- System health checks

# Common issues
- System boot problems
- Network connectivity
- Service failures
- Disk space issues
- Permission problems

# Debugging tools
- System logs
- Process monitoring
- Network tools
- Disk utilities
- Performance analysis

# Recovery
- Backup restoration
- System rescue
- Data recovery
- Service recovery
- Configuration fixes