select-case.md

• select is a Bash construct for generating interactive menus.

• Syntax looks like a for loop:

  select ITEM in [LIST]
  do
    [COMMANDS]
  done

• [LIST] can be:

  • A list of strings separated by spaces
  • A range of numbers
  • Command substitution output
  • An array

• The menu is displayed with numbered options.

• User chooses by entering a number:

  • ITEM → holds the chosen value
  • REPLY → holds the number entered

• A custom prompt is set with PS3.

Example 1 – Simple Menu

PS3="Enter a number: "
select character in Sheldon Leonard Penny Howard Raj
do
    echo "Selected character: $character"
    echo "Selected number: $REPLY"
done

• Displays:

  1) Sheldon
  2) Leonard
  3) Penny
  4) Howard
  5) Raj
  Enter a number:
  

Example 2 – Calculator (integer operations)

#!/bin/bash

PS3="Select the Operation: "

echo "Calculator"
select opr in addition substraction multiplication division quit; do 
    case $opr in 
        addition)
            read -p "Enter a first number:"  num1
            read -p "Enter a second number:" num2
            echo "$num1 + $num2 = $(( $num1 + $num2 ))"
            ;;
        substraction)
            read -p "Enter a first number: " num1
            read -p "Enter a second number: " num2
            echo "$num1 - $num2 = $(( $num1 - $num2 ))"
            ;;
        multiplication)
            read -p "Enter a first number: " num1
            read -p "Enter a second number: " num2
            echo "$num1 * $num2 = $(( $num1 * $num2 ))"
            ;;
        division)
            read -p "Enter a first number: " num1
            read -p "Enter a second number: " num2
            echo "$num1 / $num2 = $(( $num1 / $num2 ))"
            ;;
        quit)
            break
            ;;
        *)
        echo "Invalid option $REPLY"
            ;;
    esac
done

Example 3 – Advanced Calculator (supports floating numbers with bc)

calculate () {
  read -p "Enter the first number: " n1
  read -p "Enter the second number: " n2
  echo "$n1 $1 $n2 = " $(bc -l <<< "$n1$1$n2")
   echo "$n1 $1 $n2 = $(expr $n1 "$1" $n2)" 
}

PS3="Select the operation: "

select opt in add subtract multiply divide quit; do
  case $opt in
    add) calculate "+" ;;
    subtract) calculate "-" ;;
    multiply) calculate "*" ;;
    divide) calculate "/" ;;
    quit) break ;;
    *) echo "Invalid option $REPLY" ;;
  esac
done

#!/bin/bash

PS3="please select os? "   # Custom prompt for select

select os in linux windows mac
do
    case ${os} in
        linux)
            echo "you selected linux"
            echo "thanks."
            break
            ;;
        windows)
            echo "you selected windows"
            echo "thanks."
            break
            ;;
        mac)
            echo "you selected mac"
            echo "thanks."
            break
            ;;
        *)
            echo "Invalid"
            ;;
    esac
done

---

🔎 How it Works

1. PS3 → sets the prompt shown when select asks for input.

2. select os in ... → shows a numbered menu:

   1) linux
   2) windows
   3) mac
   please select os?
   

3. User enters a number (e.g., 1) → the variable $os is set (linux).

4. case statement handles each possible choice.

5. * case handles invalid/empty input.

6. break → exits the loop after a valid selection.

---

🔹 Example Runs

Case 1: Valid input

1) linux
2) windows
3) mac
please select os? 1
you selected linux
thanks.

Case 2: Invalid input

please select os? 5
Invalid

---

✅ Improvement (More Robust)

To avoid looping forever on invalid input, add an empty check:

#!/bin/bash

PS3="please select os? "

select os in linux windows mac
do
    if [[ -z "$os" ]]; then
        echo "Invalid option"
        continue
    fi

    case $os in
        linux|windows|mac)
            echo "you selected $os"
            echo "thanks."
            break
            ;;
    esac
done

Now:

• If input is invalid (5, 10, blank), it prints Invalid option and asks again.
• If input is valid, it executes and exits.

---

💡 Pro Tip: You can also use select for menus like:

• File selection
• Service management
• Script option pickers