🔁 Linux Shell Script Lab – While Loops, Status Checks & Menu-Driven Programs

In this lab, I learned how to use while loops, conditionals, and interactive menu-driven programs in Bash. The tasks focused on waiting for rocket states, fixing loop syntax errors, and building fully functional scripts that interact with the user in real time.

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📋 Lab Overview

Goal:

• Use while loops to wait for changing system states
• Fix scripts with missing loop syntax
• Understand conditional branching inside loops
• Build a menu-driven CLI calculator using infinite loops + user input

Learning Outcomes:

• Use while [ condition ] loops effectively
• Understand when to use sleep, state checks, and repeated polling
• Debug syntactical errors inside loop structures
• Apply interactive read statements inside loops
• Build reusable automation scripts that loop until user exit conditions

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🛠 Step-by-Step Journey

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Step 1 — Add a While Loop to Wait for Rocket Launch State

Script path:

/home/bob/create-and-launch-rocket

Goal: Add a loop that waits while the rocket is in the "launching" state before running the failure debug tool.

Final Script Snippet:

mission_name=$1

mkdir $mission_name
rocket-add $mission_name
rocket-start-power $mission_name
rocket-internal-power $mission_name
rocket-start-sequence $mission_name
rocket-start-engine $mission_name
rocket-lift-off $mission_name

rocket_status=$(rocket-status $mission_name)
echo "The status of launch is $rocket_status"

while [ $rocket_status = "launching" ]
do
  sleep 2
  rocket_status=$(rocket-status $mission_name)
done

if [ $rocket_status = "failed" ]
then
  rocket-debug
fi

✅ Script now waits until the rocket stops launching ✅ If FAILED → runs rocket-debug ✅ If SUCCESS → finishes normally

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Step 2 — Fix Syntax Error in print-numbers.sh

Script path:

/home/bob/print-numbers.sh

Original (broken):

to_number=$1
number=0
while [ $number -lt $to_number ]
  echo $(( number++ ))
done

Issue: Missing the do keyword.

Fixed:

to_number=$1
number=0

while [ $number -lt $to_number ]
do
  echo $(( number++ ))
done

After running:

bash print-numbers.sh 10

Output:

0
1
2
3
4
5
6
7
8
9

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Step 3 — Identify What the Script Does

Answer: ✔ It prints numbers from 0 up to (but not including) the input value.

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Step 4 — Build a Menu-Driven Calculator

Script path:

/home/bob/calculator.sh

Requirements: ✔ Show a menu: Add, Subtract, Multiply, Divide, Quit ✔ Accept two numbers when needed ✔ Loop until user chooses option 5 ✔ Print results in the format: Answer=6

Final Script:

while true
do
  echo "1. Add"
  echo "2. Subtract"
  echo "3. Multiply"
  echo "4. Divide"
  echo "5. Quit"

  read -p "Enter your choice: " choice

  if [ $choice -eq 1 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( number1 + number2 ))

  elif [ $choice -eq 2 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( number1 - number2 ))

  elif [ $choice -eq 3 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( number1 * number2 ))

  elif [ $choice -eq 4 ]
  then
        read -p "Enter Number1: " number1
        read -p "Enter Number2: " number2
        echo Answer=$(( number1 / number2 ))

  elif [ $choice -eq 5 ]
  then
        break
  fi

done

Make executable:

chmod +x calculator.sh

✔ Menu repeats indefinitely ✔ Input-driven ✔ Fully interactive ✔ Ends only when user selects option 5

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🧠 Key Concepts Reinforced

Concept	Description
While loop	Repeats while a condition remains true
Polling status	Used for waiting on processes (e.g., rocket-state)
Menu-driven programs	Built using infinite loops (while true)
User input	read accepts numbers/choices interactively
Conditional logic	if / elif / else to handle branching paths
Debugging syntax errors	Ensuring every while has a matching do

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🧩 Troubleshooting Patterns Learned

Issue	Fix
Missing do in while loop	Add do before loop body
Incorrect variable comparison	Use [ $var = "value" ]
Infinite loop without exit	Add break on quit condition
Wrong command order	Ensure checks come after updated state

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💡 Notes / Tips

• Loops are the backbone of automation — polling and planning tasks rely heavily on them.

• Always remember the structure:

  while [ condition ]
  do
     commands
  done
  

• When building interactive scripts, always loop until the user chooses to exit.

• Use sleep inside loops to avoid high CPU usage.

• Keep menus clean and user-friendly — they form the UI of shell scripts.

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✅ Summary Commands

Task	Command
Edit script	vi scriptname.sh
Save & exit	:wq
Make executable	chmod +x scriptname.sh
Run script	bash scriptname.sh
Test interactive script	Run and follow prompts
Loop through states	while [ condition ]; do ... done

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🏁 End of Lab

Successfully completed multiple loop-based scripting tasks: ✅ Added a rocket state polling loop ✅ Fixed syntactical loop errors ✅ Built an interactive menu-driven calculator ✅ Strengthened while-loop, state checking, and conditional branching skills

This lab deepened understanding of loops, interactive scripts, and continuous automation logic — foundational for real DevOps scripting.