Linux Command Line Mastery · 课时

条件语句：`if`、`else`、`case`

学习使用各种条件结构在脚本中实现决策逻辑。

条件语句：`if`、`else`、`case` 是 CoddyKit 上的免费 Linux Command Line Mastery 课时。这是第 2 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Linux Command Line Mastery 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Linux Command Line Mastery 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

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.

常见问题解答

「条件语句：`if`、`else`、`case`」课时是免费的吗？

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「条件语句：`if`、`else`、`case`」这节课中我会学到什么？

学习使用各种条件结构在脚本中实现决策逻辑。你通过在浏览器中直接运行的动手代码来练习 Linux Command Line Mastery，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

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大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

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