Linux Command Line Mastery · レッスン

条件分岐：`if`、`else`、`case`

さまざまな条件分岐構文を使って、スクリプトに判断処理を実装します。

「条件分岐：`if`、`else`、`case`」はCoddyKit上の無料Linux Command Line Masteryレッスンです。これはレッスン2/4です。下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。これはLinux Command Line Mastery学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 Linux Command Line Masteryコースには全4レッスンが含まれています。

このレッスンの一部はまだ翻訳されておらず、英語で表示されています。

Making Decisions in Scripts

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 Basic `if` Statement

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!

Simple `if` Example

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!

#!/bin/bash

num=7

if [ "$num" -gt 5 ]; then
  echo "Number is greater than 5."
fi

num=3

if [ "$num" -gt 5 ]; then
  echo "Number is greater than 5. (This won't print)"
fi

`if-else`: Two Paths

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

Using `if-else`

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!

#!/bin/bash

num=4

if (( num % 2 == 0 )); then
  echo "$num is an even number."
else
  echo "$num is an odd number."
fi

num=9

if (( num % 2 == 0 )); then
  echo "$num is an even number. (This won't print)"
else
  echo "$num is an odd number."
fi

`elif`: Multiple Conditions

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

Chain of `elif`

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.

#!/bin/bash

score=85

if [ "$score" -ge 90 ]; then
  echo "Grade: A"
elif [ "$score" -ge 80 ]; then
  echo "Grade: B"
elif [ "$score" -ge 70 ]; then
  echo "Grade: C"
else
  echo "Grade: F"
fi

`case`: Simple Selection

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

Using `case` Statements

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.

#!/bin/bash

action="start"

case "$action" in
  "start")
    echo "Starting service..."
    ;;
  "stop")
    echo "Stopping service..."
    ;;
  "restart")
    echo "Restarting service..."
    ;;
  *) # Default case for anything else
    echo "Invalid action: $action"
    ;;
esac

action="status"

case "$action" in
  "start") echo "Starting service..." ;;
  "stop") echo "Stopping service..." ;;
  "restart") echo "Restarting service..." ;;
  *) echo "Invalid action: $action" ;;

esac

Conditional Challenge

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

#!/bin/bash

color="blue"

if [ "$color" == "red" ]; then
  echo "It's a primary color."
elif [ "$color" == "yellow" ]; then
  echo "It's another primary color."
elif [ "$color" == "blue" ]; then
  echo "Yes, blue is primary!"
else
  echo "Unknown color."
fi

Decisions Made Easy

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.

よくある質問

「条件分岐：`if`、`else`、`case`」レッスンは無料ですか？

はい。「条件分岐：`if`、`else`、`case`」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、Linux Command Line Masteryコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 Linux Command Line Masteryコースには全4レッスンが含まれています。

「条件分岐：`if`、`else`、`case`」で何を学びますか？

さまざまな条件分岐構文を使って、スクリプトに判断処理を実装します。ブラウザで直接実行するハンズオンコードでLinux Command Line Masteryを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。

Linux Command Line Masteryを始めるのに経験は必要ですか？

事前経験は必要ありません。CoddyKitのLinux Command Line Masteryは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。これはレッスン2/4です。

「条件分岐：`if`、`else`、`case`」レッスンにはどのくらい時間がかかりますか？

ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。

このLinux Command Line Masteryレッスンでコードを書いて実行できますか？

はい。すべてのLinux Command Line Masteryレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。

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