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Control Structures in Shell Scripting

Control structures allow a script to make decisions and execute code conditionally or repeatedly. There are three main types:

  1. Conditional Statements (if, case)

  2. Loops (for, while, until)

  3. Branching (break, continue)

1. Conditional Statements

a) If-Else Statement

Used for decision-making based on conditions.

Syntax:

bash
if [ condition ]; then
# Code to execute if condition is true
elif [ another_condition ]; then
# Code for another condition
else
# Code if no condition matches
fi

Example:

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

if [ $num -gt 10 ]; then
echo "Number is greater than 10"
elif [ $num -eq 10 ]; then
echo "Number is exactly 10"
else
echo "Number is less than 10"
fi

📌 Common Comparison Operators:

Operator Description
-eq Equal to
-ne Not equal to
-gt Greater than
-lt Less than
-ge Greater than or equal to
-le Less than or equal to

b) Case Statement

Used when multiple conditions need to be checked, similar to switch in other languages.

Syntax:

bash
case $variable in
pattern1)
# Code to execute
;;
pattern2)
# Code to execute
;;
*)
# Default case
;;
esac

Example:

bash
#!/bin/bash
echo "Enter a fruit name: "
read fruit

case $fruit in
apple)
echo "You chose Apple";;
banana)
echo "You chose Banana";;
orange)
echo "You chose Orange";;
*)
echo "Unknown fruit";;
esac

2. Loops

Loops are used to execute a block of code multiple times.

a) For Loop

Repeats execution a set number of times.

Syntax:

bash
for variable in list; do
# Code to execute
done

Example:

bash
#!/bin/bash
for i in 1 2 3 4 5; do
echo "Iteration: $i"
done

Using a Range:

bash
for i in {1..5}; do
echo "Number: $i"
done

Using C-style Syntax:

bash
for ((i=1; i<=5; i++)); do
echo "Loop iteration: $i"
done

b) While Loop

Executes while a condition is true.

Syntax:

bash
while [ condition ]; do
# Code to execute
done

Example:

bash
#!/bin/bash
count=1
while [ $count -le 5 ]; do
echo "Count: $count"
((count++))
done

c) Until Loop

Similar to while, but runs until the condition becomes true.

Syntax:

bash
until [ condition ]; do
# Code to execute
done

Example:

bash
#!/bin/bash
num=1
until [ $num -gt 5 ]; do
echo "Number: $num"
((num++))
done

3. Loop Control Statements

a) Break Statement

Exits a loop immediately.

bash
#!/bin/bash
for i in {1..10}; do
if [ $i -eq 5 ]; then
echo "Breaking at $i"
break
fi
echo "Iteration: $i"
done

b) Continue Statement

Skips the rest of the loop’s body and moves to the next iteration.

bash
#!/bin/bash
for i in {1..5}; do
if [ $i -eq 3 ]; then
echo "Skipping $i"
continue
fi
echo "Processing $i"
done

🚀 Putting It All Together: A Complete Example

bash
#!/bin/bash

echo "Enter a number (1-5):"
read num

if [ $num -ge 1 ] && [ $num -le 5 ]; then
echo "Valid number, looping..."
for i in $(seq 1 $num); do
if [ $i -eq 3 ]; then
echo "Skipping 3"
continue
fi
echo "Iteration: $i"
done
else
echo "Invalid input!"
fi

🔥 Key Takeaways

✅ If-Else: Used for decision-making.
✅ Case Statement: Simplifies multiple conditions.
✅ Loops: for, while, and until automate repetitive tasks.
✅ Break & Continue: Control loop execution flow.

1️⃣ If-Else Statements (More examples, nested conditions)
2️⃣ Case Statements (Pattern matching, advanced use cases)
3️⃣ For Loop (Iterating over arrays, files, numbers)
4️⃣ While/Until Loops (Real-world automation examples)
5️⃣ Break & Continue (Use in nested loops)
6️⃣ A Complete Script Combining All Concepts