README.md

🔁 Control Flow in Bash Scripts

Control flow statements are the backbone of decision-making in Bash scripts. They allow scripts to respond to conditions and iterate actions, making automation powerful and efficient.

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🧠 What is Control Flow?

Control flow directs the sequence in which instructions are executed. Think of it as a roadmap guiding your script through decisions (using if/else) or repeating actions (using loops).

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🧪 Basic Conditional Example

Script: Determine if a Number is Positive, Negative, or Zero

#!/bin/bash
read -p "Enter a number: " num

# Input validation
if ! [[ "$num" =~ ^-?[0-9]+$ ]]; then
    echo "Invalid input. Please enter an integer."
    exit 1
fi

echo "You have entered the number $num"

if [ $num -gt 0 ]; then
    echo "The number is positive."
elif [ $num -lt 0 ]; then
    echo "The number is negative."
else
    echo "The number is zero."
fi

Example Output (Plaintext):

Enter a number: abc
Invalid input. Please enter an integer.

Enter a number: -5
You have entered the number -5
The number is negative.

Enter a number: 0
You have entered the number 0
The number is zero.

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🛠️ Script Setup and Execution

1. Create the Script

vim control_flow.sh

2. Make Executable

chmod +x control_flow.sh

3. Run Script

./control_flow.sh

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❗ Troubleshooting Tips

Issue	Cause	Solution
Permission denied	Missing execute permission	Run: chmod +x control_flow.sh
No output	read input not used correctly	Ensure echo $num is included for feedback
Numeric test error	Using == instead of -eq, etc.	Use -eq, -gt, -lt, -ge, -le for numbers
Script crashes with non-numeric input	Invalid input used in test	Add input validation with regex check

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🔄 Loops in Bash

🔹 For Loop: List Items

#!/bin/bash
for name in Alice Bob Charlie; do
  echo "Hello, $name!"
done

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🔹 For Loop with Numbers (Range)

for i in {1..5}; do
  echo "Iteration $i"
done

🔹 C-style For Loop

for (( i=0; i<5; i++ )); do
  echo "Number $i"
done

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🪄 Control Flow Syntax Summary

🔸 if / elif / else

if [ condition ]; then
  # commands
elif [ condition ]; then
  # commands
else
  # commands
fi

🔸 For Loop (List or Range)

for item in item1 item2; do
  # commands
done

🔸 C-style For Loop

for (( init; condition; increment )); do
  # commands
done

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🧪 Debugging Control Flow Scripts

Use set -x to trace script execution:

#!/bin/bash
set -x
# your script here
set +x

This helps you see each command as it's executed, aiding troubleshooting.

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📌 Bonus: Input Validation Tip

Add a numeric check to prevent invalid input:

if ! [[ $num =~ ^-?[0-9]+$ ]]; then
  echo "Invalid input: Not a number"
  exit 1
fi

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🏁 Conclusion

You’ve learned how to:

• Use if, elif, and else to make decisions
• Use for loops to automate repeated tasks
• Validate user input and prevent script errors
• Write readable, executable, and traceable scripts

Control flow turns a basic script into a responsive, intelligent automation tool.