linux

## What I do - Manage Linux systems using command-line tools - Configure system services and daemons - Monitor system performance and resources - Manage users, groups, and permissions - Configure networking and firewall rules - Automate tasks with shell scripts - Manage storage and filesystems - Configure logging and auditing - Implement security hardening - Debug system issues ## When to use me When administering Linux servers, troubleshooting system issues, writing automation scripts, or configuring system services. ## Core Concepts - File system hierarchy and management - Process management and systemd - User/group administration and PAM - Network configuration (ip, nmcli, netplan) - Package management (apt, yum, dnf) - Firewall configuration (iptables, nftables) - SELinux/AppArmor mandatory access control - Log management (journald, rsyslog) - Systemd service units - Shell scripting (bash, zsh) ## Code Examples ### System Administration ```bash #!/bin/bash # System information gathering get_system_info() { echo "=== System Information ===" echo "Hostname: $(hostname)" echo "Kernel: $(uname -r)" echo "Uptime: $(uptime -p 2>/dev/null || uptime)" echo "Load Average: $(cat /proc/loadavg | awk '{print $1, $2, $3}')" } # Memory and CPU analysis analyze_resources() { echo "=== Resource Analysis ===" echo "Memory:" free -h | awk 'NR==2{printf " Used: %s/%s (%.1f%%)\n", $3, $2, $3/$2*100}' echo "CPU:" awk '/^cpu /{printf " Usage: %.1f%%\n", 100-($5+$6)*100/($2+$4+$5+$6)}' /proc/stat echo "Top Memory Processes:" ps aux --sort=-%mem | head -6 | awk '{printf " %s: %.1f%%\n", $11, $4}' } # Disk usage analysis analyze_disk() { echo "=== Disk Usage ===" df -h | awk 'NR>1 && $1!="tmpfs" && $1!="overlay" { printf " %s: %s/%s (%.0f%%)\n", $6, $3, $2, $5 }' echo "Largest directories:" du -h --max-depth=2 /var 2>/dev/null | sort -hr | head -5 } # Service management manage_service() { local service="$1" local action="${2:-status}" case "$action" in start|stop|restart|reload) sudo systemctl "$action" "$service" ;; status) systemctl is-active "$service" && echo "$service: active" || echo "$service: inactive" systemctl is-enabled "$service" && echo "enabled" || echo "disabled" ;; logs) sudo journalctl -u "$service" -n 50 --no-pager ;; *) echo "Unknown action: $action" ;; esac } # Network diagnostic network_diagnostics() { echo "=== Network Status ===" echo "Interface statistics:" ip -s link | grep -A1 "^[0-9]" | head -20 echo "Routing table:" ip route show echo "DNS resolution test:" host google.com 2>/dev/null || echo "DNS lookup failed" echo "Active connections:" ss -tunapl | head -10 } ``` ### User and Permission Management ```bash #!/bin/bash # Create user with specified groups create_application_user() { local username="$1" local groups="${2:-app}" if id "$username" &>/dev/null; then echo "User $username already exists" return 1 fi sudo useradd -r -s /sbin/nologin -M "$username" for group in $groups; do sudo usermod -aG "$group" "$username" 2>/dev/null done echo "Created user: $username" } # Audit file permissions audit_permissions() { local path="${1:-.}" local issues=0 echo "=== Permission Audit for $path ===" # World-writable files world_writable=$(find "$path" -perm -002 -type f 2>/dev/null) if [ -n "$world_writable" ]; then echo "WORLD WRITABLE FILES:" echo "$world_writable" issues=$((issues + $(echo "$world_writable" | wc -l))) fi # Files without group write sgid_files=$(find "$path" -perm -2000 -type f 2>/dev/null) if [ -n "$sgid_files" ]; then echo "SGID FILES:" echo "$sgid_files" fi # SUID files suid_files=$(find "$path" -perm -4000 -type f 2>/dev/null) if [ -n "$suid_files" ]; then echo "SUID FILES:" echo "$suid_files" fi echo "Total issues found: $issues" } # Set permissions recursively secure_directory() { local path="$1" local owner="${2:-root}" local group="${3:-root}" # Set directory ownership sudo chown -R "$owner:$group" "$path" # Set directory permissions (755 for dirs, 644 for files) find "$path" -type d -exec chmod 755 {} \; find "$path" -type f -exec chmod 644 {} \; # Make sensitive files more restrictive find "$path" -name "*.key" -o -name "*.pem" -o -name "*password*" | \ xargs -I{} chmod 600 {} 2>/dev/null echo "Secured: $path" } ``` ### Firewall Configuration ```bash #!/bin/bash # Configure firewalld with common rules configure_firewall() { local interface="${1:-eth0}" # Set default policies sudo firewall-cmd --set-default-zone=drop sudo firewall-cmd --zone=drop --add-interface="$interface" sudo firewall-cmd --zone=trusted --add-interface=lo # Allow SSH (change port if needed) sudo firewall-cmd --zone=public --add-service=ssh sudo firewall-cmd --zone=public --add-port=2222/tcp --permanent # Allow HTTP/HTTPS sudo firewall-cmd --zone=public --add-service=http sudo firewall-cmd --zone=public --add-service=https # Allow from internal network sudo firewall-cmd --zone=trusted --add-source=10.0.0.0/8 sudo firewall-cmd --zone=trusted --add-source=192.168.0.0/16 # Reload and save sudo firewall-cmd --reload sudo firewall-cmd --runtime-to-permanent echo "Firewall configured" } # Configure nftables configure_nftables() { sudo nft -f - <<'EOF' table inet filter { chain input { type filter hook input priority 0; ct state established,related accept iif lo accept ip protocol icmp accept tcp dport ssh accept tcp dport {80, 443} accept drop } chain forward { type filter hook forward priority 0; drop } chain output { type filter hook output priority 0; accept } } EOF sudo nft add table inet filter echo "nftables configured" } ``` ### Log Analysis ```bash #!/bin/bash # Analyze system logs analyze_logs() { local timeframe="${1:-24h}" local logfile="${2:-/var/log/syslog}" echo "=== Log Analysis ($timeframe) ===" # Failed login attempts echo "Failed SSH attempts:" journalctl -u sshd --since "$timeframe" | grep "Failed" | \ awk '{print $11}' | sort | uniq -c | sort -rn | head -10 # Most frequent error messages echo "Common errors:" journalctl -p err --since "$timeframe" --no-pager | \ awk '{print $5}' | sort | uniq -c | sort -rn | head -10 # System restarts echo "Recent restarts:" last reboot | head -5 } # Configure logrotate configure_logrotate() { cat > /etc/logrotate.d/app-logs <<'EOF' /var/log/app/*.log { daily rotate 30 compress delaycompress missingok notifempty create 0640 www-data adm sharedscripts postrotate systemctl reload app > /dev/null 2>&1 || true endscript } EOF echo "Log rotation configured for /var/log/app/" } ``` ## Best Practices - Use SSH keys instead of passwords for authentication - Keep systems updated with regular security patches - Configure automated security updates for production systems - Use fail2ban or similar tools to prevent brute force attacks - Implement proper backup strategies and test restores regularly - Use configuration management (Ansible, Puppet, Chef) - Monitor system metrics with Prometheus, Grafana, or similar - Enable and review audit logs for security events - Use sudo instead of root login for day-to-day administration - Document all system configurations and changes