log-analysislisted

# Log Analysis ## When to use this skill - **Error debugging**: analyze the root cause of application errors - **Performance analysis**: analyze response times and throughput - **Security audit**: detect anomalous access patterns - **Incident response**: investigate the root cause during an outage ## Instructions ### Step 1: Locate Log Files ```bash # Common log locations /var/log/ # System logs /var/log/nginx/ # Nginx logs /var/log/apache2/ # Apache logs ./logs/ # Application logs ``` ### Step 2: Search for Error Patterns **Common error search**: ```bash # Search ERROR-level logs grep -i "error\|exception\|fail" application.log # Recent errors (last 100 lines) tail -100 application.log | grep -i error # Errors with timestamps grep -E "^\[.*ERROR" application.log ``` **HTTP error codes**: ```bash # 5xx server errors grep -E "HTTP/[0-9.]+ 5[0-9]{2}" access.log # 4xx client errors grep -E "HTTP/[0-9.]+ 4[0-9]{2}" access.log # Specific error code grep "HTTP/1.1\" 500" access.log ``` ### Step 3: Pattern Analysis **Time-based analysis**: ```bash # Error count by time window grep -i error application.log | cut -d' ' -f1,2 | sort | uniq -c | sort -rn # Logs for a specific time window grep "2025-01-05 14:" application.log ``` **IP-based analysis**: ```bash # Request count by IP awk '{print $1}' access.log | sort | uniq -c | sort -rn | head -20 # Activity for a specific IP grep "192.168.1.100" access.log