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Linux Command Line Mastery · 课时

循环：`for`、`while`、`until`

掌握 Shell 脚本中的不同循环类型，自动执行重复性任务。

循环：`for`、`while`、`until` 是 CoddyKit 上的免费 Linux Command Line Mastery 课时。 这是第 3 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。

Why Use Loops?

Imagine you need to perform the same action many times, like saying 'Hello' to everyone in a list, or counting from 1 to 100. Doing this manually would be tedious and error-prone.

This is where loops come in! Loops allow your script to repeat a set of commands multiple times, saving you effort and making your scripts more efficient.

Looping Through Items with `for`

The for loop is perfect when you know exactly what items you want to iterate over. It goes through a list, processing each item one by one.

Its basic syntax looks like this:

  • for item in list; do
  • # commands to run for each item
  • done

The item variable will hold each value from the list in turn.

`for` Loop in Action

Let's see the for loop iterate through a list of names. Each name is assigned to the name variable, and then printed.

Try running this example:

#!/bin/bash

echo "--- Greeting Friends ---"
for name in Alice Bob Charlie;
do
  echo "Hello, $name!"
done
echo "--- All greeted! ---"

`for` Loop for Number Ranges

What if you need to loop a specific number of times, like counting from 1 to 5? You can generate sequences for the for loop:

  • Brace Expansion: {1..5} creates "1 2 3 4 5".
  • seq command: $(seq 1 5) also creates "1 2 3 4 5".

Both are handy for numerical iterations.

Counting with `for` Loop

Here's how you can use brace expansion to count up to a specific number. The loop variable i takes on each number in the sequence.

Try running this example:

#!/bin/bash

echo "Counting up!"
for i in {1..3};
do
  echo "Count: $i"
done
echo "Done counting."

Repeating with `while`

The while loop is used when you want to repeat commands as long as a certain condition remains true. It continuously checks the condition before each iteration.

Its basic structure is:

  • while condition; do
  • # commands to run
  • done

The loop stops as soon as the condition becomes false.

`while` Loop Counter

Let's use a while loop to count. We initialize a variable count, and the loop continues as long as count is less than 3.

Remember to update the variable inside the loop, or it might run forever (an 'infinite loop')!

#!/bin/bash

count=0
echo "Starting while loop..."
while [ $count -lt 3 ]; do
  echo "Current count: $count"
  count=$((count + 1))
done
echo "While loop finished."

Repeating `until` True

The until loop is similar to while, but with an inverse condition. It executes commands until a specified condition becomes true.

Its syntax is:

  • until condition; do
  • # commands to run
  • done

The loop continues as long as the condition is false.

`until` Loop in Action

Here's an until loop waiting for a variable num to reach 3. It will keep running as long as num is not greater than or equal to 3.

Try running this example:

#!/bin/bash

num=0
echo "Starting until loop..."
until [ $num -ge 3 ]; do
  echo "Current number: $num"
  num=$((num + 1))
done
echo "Until loop finished."

When to Use Which Loop?

Choosing the right loop makes your scripts clearer and more efficient:

  • for loop: Best for iterating over a known list of items (files, names, numbers in a range).
  • while loop: Ideal when you need to repeat commands as long as a condition holds true (e.g., reading lines from a file, waiting for user input, counting).
  • until loop: Useful when you want to repeat commands until a specific condition becomes true (e.g., waiting for a process to finish, retrying an operation).

Loop Challenge

Let's test your understanding of shell loops.

Loops: A Quick Recap

Well done! You've learned the fundamental looping constructs in shell scripting:

  • The for loop iterates over a fixed list of items or a range.
  • The while loop repeats commands as long as a condition is true.
  • The until loop repeats commands until a condition becomes true.

These loops are essential for automating repetitive tasks and making your scripts dynamic and powerful. Keep practicing!

常见问题解答

「循环：`for`、`while`、`until`」课时是免费的吗？

是的 — 「循环：`for`、`while`、`until`」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Linux Command Line Mastery 课程的其余内容，请升级到 CoddyKit PRO。 Linux Command Line Mastery 课程共包含 4 节课。

「循环：`for`、`while`、`until`」这节课中我会学到什么？

掌握 Shell 脚本中的不同循环类型，自动执行重复性任务。 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。

学习 Linux Command Line Mastery 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Linux Command Line Mastery 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。 这是第 3 节课，共 4 节。

「循环：`for`、`while`、`until`」课时需要多长时间？

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。

我能在这节 Linux Command Line Mastery 课中编写并运行代码吗？

能。每节 Linux Command Line Mastery 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

  1. 变量、输入与输出
  2. 条件语句：`if`、`else`、`case`
  3. 循环：`for`、`while`、`until`
  4. Shell 脚本中的函数与参数
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