Linux system administration essentials for DevOps

                  #!/bin/bash
# Linux system administration commands

# === Process Management ===
ps aux                                # All processes
ps aux | grep nginx                   # Find specific process
top -bn1 | head -20                   # Snapshot of top processes
htop                                  # Interactive process viewer

kill -SIGTERM 1234                    # Graceful stop
kill -SIGKILL 1234                    # Force kill
pkill -f "node server.js"             # Kill by name pattern

# === Systemd Services ===
systemctl status nginx                # 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                # Enable on boot
systemctl disable nginx               # Disable on boot
systemctl list-units --type=service   # List all services

# === Logs (journalctl) ===
journalctl -u nginx                   # Service logs
journalctl -u nginx --since "1 hour ago"
journalctl -u nginx -f                # Follow logs
journalctl -p err                     # Error priority only
journalctl --disk-usage               # Log disk usage
journalctl --vacuum-size=500M         # Trim logs to 500MB

# === Disk & Storage ===
df -h                                 # Disk space usage
du -sh /var/log/*                     # Directory sizes
du -sh /var/log/* | sort -rh | head   # Largest directories
lsblk                                 # List block devices
mount                                 # List mounts
findmnt -t ext4                       # Find specific mounts

# Find large files
find / -type f -size +100M -exec ls -lh {} ;
# Find files modified in last 24h
find /var/log -mtime -1 -type f

# === Network ===
ss -tlnp                              # Listening TCP ports
ss -tunap                             # All connections
curl -v https://example.com           # HTTP debug
dig example.com                       # DNS lookup
dig +short example.com A              # Quick DNS
traceroute example.com                # Network path
mtr example.com                       # Better traceroute

# Check if port is open
nc -zv hostname 443
timeout 3 bash -c 'cat < /dev/null > /dev/tcp/hostname/443' && echo "Open"

# === Users & Permissions ===
useradd -m -s /bin/bash deploy        # Create user
usermod -aG sudo deploy               # Add to sudo group
passwd deploy                         # Set password

chmod 755 /opt/app                    # rwxr-xr-x
chmod 600 /etc/secrets.conf           # rw-------
chown -R deploy:deploy /opt/app       # Change ownership
setfacl -m u:deploy:rx /var/log/app   # ACL permissions

# === Performance Analysis ===
vmstat 1 5                            # Virtual memory stats
iostat -x 1 5                         # Disk I/O stats
sar -u 1 5                            # CPU history
sar -r 1 5                            # Memory history
free -h                               # Memory usage
uptime                                # Load averages
nproc                                 # CPU count

# === Cron Jobs ===
crontab -l                            # List cron jobs
crontab -e                            # Edit cron jobs

# Cron format: minute hour day month weekday command
# 0 2 * * * /opt/scripts/backup.sh    # Daily at 2 AM
# */5 * * * * /opt/scripts/health.sh   # Every 5 minutes
# 0 0 * * 0 /opt/scripts/weekly.sh     # Weekly on Sunday

# === Firewall (UFW) ===
ufw status verbose                    # Show rules
ufw allow 22/tcp                      # Allow SSH
ufw allow from 10.0.0.0/8 to any port 5432  # Allow DB from VPN
ufw deny 23                           # Block telnet
ufw enable                            # Enable firewall

# === Kernel Tuning ===
sysctl -a | grep net.core             # View settings
# /etc/sysctl.d/99-custom.conf
# net.core.somaxconn = 65535
# net.ipv4.tcp_max_syn_backlog = 65535
# vm.swappiness = 10
# fs.file-max = 2097152
sysctl -p /etc/sysctl.d/99-custom.conf  # Apply

# === Quick Diagnostics ===
# Check why a service failed
systemctl status myapp
journalctl -u myapp --no-pager -n 50

# Check resource usage
free -h && echo "---" && df -h / && echo "---" && uptime

# Check open file limits
ulimit -n                             # Current limit
cat /proc/sys/fs/file-nr              # System-wide file usage