⚙️ Shell Scripting for Beginners – Part 13: Subshells, Process Control & Parallel Execution.

• Now let's level up and make your shell scripts multitask like a Middleware and DevOps pro.
• In this part, you'll learn how to run processes in the background, manage subshells, and execute multiple tasks in parallel for faster automation.

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📑 Table of Contents

• 1️⃣ What is a Subshell?
• 2️⃣ Running Commands in the Background
• 3️⃣ Managing Jobs (fg, bg, kill)
• 4️⃣ Parallel Execution in DevOps
• 5️⃣ nohup & disown for Long Running Tasks
• 6️⃣ Real-World Examples (Tomcat, Jenkins, Docker)
• 🧠 Pro Tips & Troubleshooting
• 🏁 Summary

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1️⃣ What is a Subshell?

A subshell is a new child shell launched from your current shell. It allows running commands in isolation — any variable or environment change inside doesn’t affect the parent shell.

#!/bin/bash
VAR="ParentShell"
( VAR="SubShell"; echo "Inside subshell: $VAR" )
echo "Outside subshell: $VAR"

💬 Output:

Inside subshell: SubShell
Outside subshell: ParentShell

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2️⃣ Running Commands in the Background

Add & at the end of a command to run it in the background. This allows your script to keep running while other tasks continue behind the scenes.

#!/bin/bash
echo "Starting Jenkins backup..."
tar -czf /opt/backups/jenkins_backup.tar.gz /var/lib/jenkins & 
echo "Backup started in background (PID: $!)"

💬 Output:

Starting Jenkins backup...
Backup started in background (PID: 2356)

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3️⃣ Managing Jobs

When you run background processes, you can manage them using built-in commands:

• jobs → List current background jobs
• fg %1 → Bring job 1 to foreground
• bg %1 → Resume job 1 in background
• kill %1 → Kill job 1

#!/bin/bash
long_task() {
  sleep 30 &
}
long_task
jobs

💬 Output:

[1]+ Running sleep 30 &

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4️⃣ Parallel Execution in DevOps

Let’s say you want to restart multiple Tomcat servers or back up several apps simultaneously. We can use background jobs with a simple for loop.

#!/bin/bash
servers=("devbox" "testbox" "prodbox")
for srv in "${servers[@]}"; do
  echo "Restarting Tomcat on $srv..."
  ssh $srv "systemctl restart tomcat" &
done

wait  # Wait for all background jobs to finish
echo "✅ All Tomcat servers restarted in parallel!"

💬 Output:

Restarting Tomcat on devbox...
Restarting Tomcat on testbox...
Restarting Tomcat on prodbox...
✅ All Tomcat servers restarted in parallel!

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5️⃣ nohup & disown

Sometimes, you need your scripts to survive terminal disconnections — for instance, a long-running Jenkins restore or DB migration.

#!/bin/bash
nohup ./db_migration.sh > /var/log/db_migration.log 2>&1 &
disown
echo "🧠 DB migration running safely in background, even if you close terminal."

💬 Output:

nohup: ignoring input and appending output to 'nohup.out'
🧠 DB migration running safely in background, even if you close terminal.

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6️⃣ Real-World DevOps Examples

🧩 Example 1: Parallel Log Collection

#!/bin/bash
servers=("dev" "test" "prod")
for s in "${servers[@]}"; do
  scp $s:/var/log/tomcat/catalina.out /opt/logs/$s.log &
done
wait
echo "✅ Logs collected from all servers."

🧰 Example 2: Monitor Jenkins & Docker in Background

#!/bin/bash
( while true; do pgrep -f jenkins >/dev/null || systemctl restart jenkins; sleep 10; done ) &
( while true; do docker ps | grep -q "webapp" || docker start webapp; sleep 15; done ) &
echo "🩺 Jenkins and Docker monitored in background."

📦 Example 3: Run Multiple CI Jobs Simultaneously

#!/bin/bash
for job in build-test build-stage build-prod; do
  curl -s -X POST "http://jenkins.local/job/$job/build" &
done
wait
echo "✅ All Jenkins build jobs triggered in parallel!"

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🧠 Pro Tips & Troubleshooting

🧩 Process Control Best Practices

• Use wait to synchronize parallel tasks before continuing.
• Log PIDs ($!) to monitor background tasks easily.
• Redirect background job output to separate log files using > file 2>&1 &.
• For persistent background services, prefer systemd over manual nohup scripts.

⚙️ Debugging Issues

• Run ps -ef | grep scriptname to confirm background processes.
• Use set -x or bash -x script.sh for detailed command tracing.
• Use trap to clean up child processes on script exit.
• If parallel SSH commands hang, add -o ConnectTimeout=5 in SSH for safety.

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🏁 Summary

• ✅ Learned to create and manage subshells.
• ⚙️ Mastered process control using &, jobs, fg, bg, and kill.
• 🚀 Ran real-world DevOps tasks in parallel — Jenkins builds, Tomcat restarts, Docker checks.
• 💡 Used nohup and disown for persistent automation tasks.

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