คำสั่งเงื่อนไข: `if`, `else`, `case`

In shell scripting, just like in life, you often need to make decisions. Conditional statements allow your scripts to execute different commands based on whether a certain condition is true or false.

This is crucial for creating flexible and powerful scripts that can adapt to different situations or user inputs.

The simplest conditional is the if statement. It checks a condition, and if that condition is true, it executes a block of commands. If false, it skips them.

The basic syntax looks like this:

• if [ condition ]; then
• # commands to run if true
• fi

Remember fi to close the if block!

Let's see a simple if statement in action. This script checks if a variable num is greater than 5.

Try changing the value of num to see how the output changes!

What if you want to execute one set of commands if a condition is true, and a different set if it's false? That's where else comes in.

The if-else statement provides two distinct paths for your script:

• if [ condition ]; then
• # commands if condition is true
• else
• # commands if condition is false
• fi

This script uses if-else to determine if a number is even or odd. We use the arithmetic evaluation (( )) for clearer number comparisons and calculations.

The % operator gives the remainder of a division. If num % 2 is 0, it's even!

Sometimes you have more than two possible outcomes. For these situations, you can use elif (short for "else if") to check multiple conditions sequentially.

The script checks conditions one by one. The first one that's true gets its commands executed, and the rest are skipped.

• if [ cond1 ]; then ...
• elif [ cond2 ]; then ...
• else ...
• fi

Here's an example using if-elif-else to assign a letter grade based on a score. Notice how the conditions are checked from highest to lowest score.

If a score is 95, the first if condition (>= 90) is met, and it prints "A". The other elif and else blocks are then ignored.

When you have a single variable or expression that can have many possible values, and you want to perform different actions for each value, the case statement is often cleaner than a long if-elif-else chain.

It's great for menu-driven scripts or handling specific input options.

• case $variable in
• pattern1) commands ;;
• pattern2) commands ;;
• *) default commands ;;
• esac

This script demonstrates how to use case to respond to different user actions (start, stop, restart). The *) pattern acts as a default, catching any unmatched values.

Each block of commands ends with ;;, which is crucial for the case statement to work correctly.

Read the script below carefully. What will be the output when this script is run?

Great job! You've learned the fundamental ways to add decision-making logic to your shell scripts.

• if: Executes commands if a condition is true.
• if-else: Provides two paths, one for true and one for false.
• elif: Allows chaining multiple conditions for more complex logic.
• case: A clean way to handle multiple choices based on a single value.

These constructs are essential for writing scripts that can respond dynamically to different inputs and situations.