📁 Shell Scripting for Beginners – Part 7: File Handling in Shell.

• In Middleware & DevOps automation, file handling is everywhere — reading logs, writing reports, rotating backups, or watching live output.
• Let's learn how to read, write, append, and monitor files in shell scripts with real-world Middleware examples.

---

📑 Table of Contents

• Reading Files
• Writing & Appending Files
• Monitoring Log Files
• Error Handling & Debug Tips
• Summary

---

📖 1️⃣ Reading Files

Example 1: Read File Line by Line

#!/bin/bash
filename="/opt/scripts/serverlist.txt"
while read -r line; do
  echo "Processing server: $line"
done < "$filename"

💬 Output:

Processing server: app1
Processing server: app2
Processing server: app3

Example 2: Display Tomcat Log Errors

#!/bin/bash
grep "ERROR" /opt/tomcat/logs/catalina.out

💬 Output:

ERROR [localhost-startStop-1] ... Application failed to start
ERROR [main] ... Database connection refused

💡 Tip: Use grep -i error to ignore case.

---

✍️ 2️⃣ Writing & Appending Files

Example 3: Write Output to File

#!/bin/bash
echo "Backup started at $(date)" > /opt/scripts/backup.log
echo "Compressing files..." >> /opt/scripts/backup.log
tar -czf /tmp/backup.tar.gz /opt/data && echo "Backup successful" >> /opt/scripts/backup.log

💬 Output File Content:

Backup started at Sat Nov 1 02:00:00 IST 2025
Compressing files...
Backup successful

Example 4: Append Deployment Status

#!/bin/bash
env=$1
status=$2
echo "$(date) - $env deployment $status" >> /opt/scripts/deploy_history.log

💬 Output File Content:

Sat Nov 1 02:15:00 IST 2025 - dev deployment SUCCESS
Sat Nov 1 02:17:00 IST 2025 - prod deployment FAILED

Example 5: Log WebSphere Cleanup Activity

#!/bin/bash
logfile="/opt/IBM/WebSphere/AppServer/logs/cleanup.log"
echo "$(date): Cleaning old logs..." >> $logfile
find /opt/IBM/WebSphere/AppServer/logs -type f -mtime +7 -delete && echo "✅ Logs cleaned" >> $logfile

💬 Output File:

Sat Nov 1 02:30:00 IST 2025: Cleaning old logs...
✅ Logs cleaned

---

🔍 3️⃣ Monitoring Log Files

Example 6: Live Tail of Tomcat Logs

#!/bin/bash
echo "Monitoring Tomcat logs..."
tail -f /opt/tomcat/logs/catalina.out

💬 Output:

Monitoring Tomcat logs...
[INFO] Server startup completed in 2200 ms
[WARN] Deprecated API in use
[ERROR] Connection timeout to DB

Example 7: Auto-restart Jenkins on Log Failure

#!/bin/bash
tail -Fn0 /var/log/jenkins/jenkins.log | \
while read line; do
  echo "$line" | grep "ERROR" &>/dev/null
  if [ $? = 0 ]; then
    echo "🚨 Error detected, restarting Jenkins..."
    systemctl restart jenkins
  fi
done

💬 Output:

[ERROR] Jenkins job queue stuck
🚨 Error detected, restarting Jenkins...

Example 8: Monitor NGINX Access Logs for High Load

#!/bin/bash
tail -n 100 /var/log/nginx/access.log | awk '{print $9}' | sort | uniq -c

💬 Output:

500 3
404 7
200 90
301 5

💡 Tip: Use this for quick traffic analysis on your Middleware or DevOps pipeline servers.

---

⚠️ 4️⃣ Error Handling & Debugging

• Use set -e to exit on first failure.
• Redirect errors → 2>> error.log to capture exceptions.
• Validate file existence before reading:

if [ ! -f "$filename" ]; then
  echo "❌ File not found: $filename"
  exit 1
fi

If your script suddenly exits, check for missing files or permission issues in /var/log/messages or your custom error log.

---

🏁 Summary

• Read files line by line to automate configuration or host processing.
• Write and append logs for auditing and historical tracking.
• Monitor live logs (Tomcat, Jenkins, NGINX) using tail -f.
• Add error handling to avoid silent file issues in production.

← Previous: Part 6 – Arrays & Arguments Next: Part 8 – Debugging & Logging →