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[#]: subject: "Introduction to Bash Scripting"
[#]: via: "https://itsfoss.com/bash-scripting-tutorial/"
[#]: author: "Abhishek Prakash https://itsfoss.com/author/abhishek/"
[#]: collector: "lujun9972"
[#]: translator: " "
[#]: reviewer: " "
[#]: publisher: " "
[#]: url: " "
Introduction to Bash Scripting
======
Shell is the core part of Linux. It allows you to interact with the Linux kernel by using various commands like cd, ls, cat etc.
Bash is one of the many available shells for Linux. They have mostly common but not identical syntax. Bash is the most popular shell out there and is the default on most Linux distributions.
You open a terminal or SSH session and you have a shell running in it even if you cannot really visualize it.
![][1]
When you type a command, it is interpreted by the shell. If the command and syntax are correct, it will be executed otherwise you'll see an error.
### Why bash scripts when you can just run Linux commands?
You can enter the commands directly in the terminal and they will be executed.
```
abhishek@itsfoss:~$ echo "hello world"
hello world
```
And the same can be done in a script as well:
```
abhishek@itsfoss:~$ cat >> script.sh
#!/bin/bash
echo "hello world"
abhishek@itsfoss:~$ bash script.sh
hello world
```
Why do you need shell scripts then? Because you don't have to type the same command again and again. You just run the shell script.
Also, if you have complicated logic in your script, typing it all in the terminal won't be a good idea.
For example, if you enter the command below, it will work. But it is not easy to understand and typing it again and again (or even searching for it in the bash history) is a pain.
```
if [ $(whoami) = 'root' ]; then echo "root"; else echo "not root"; fi
```
Instead, you can put in a shell script so that it is easier to understand and run it effortlessly:
```
#!/bin/bash
if [ $(whoami) = 'root' ]; then
echo "You are root"
else
echo "You are not root"
fi
```
This was still simple. Imagine a complicated script with fifty or a hundred lines!
### What will you learn?
There are nine sections in this bash scripting tutorial. You'll learn to:
* Create and run your first bash shell script
* Use variables
* Pass arguments and accept user inputs in your bash scripts
* Perform mathematical calculations
* Manipulate strings
* Use conditional statements like if-else
* Use for, while and until loops
* Create functions
💡
All the sections will give you a brief example. If you wish, you can extend on the section by visiting the detailed chapter for each section. These chapters also contain practice exercises.
### Who is the target audience?
Anyone who wants to start learning bash shell scripting.
If you are a student with shell scripting as part of your course curriculum, this series is for you.
If you are a regular desktop Linux user, this series will help you understand most shell scripts you come across while exploring various software and fixes. You could also use it to automate some common, repetitive tasks.
By the end of this bash scripting tutorial, you should be able to write simple bash scripts.
🚧
It is desirable that you have basic knowledge of the Linux command line and any programming language.
If you are absolutely new to the Linux command line, I advise you to get the basics right first.
![][2]
You should understand how to go to a specific location in the command line. For that, you need to understand how path works in the Linux filesystem works.
![][3]
Next, this tutorial series gives you the basic of directory navigation and file manipulation.
![][2]
### 1\. Writing your first bash shell script
Create a new file named `hello.sh`:
```
nano hello.sh
```
This will open the nano editor in the terminal. Enter the following lines to it:
```
#!/bin/bash
echo "Hello World"
```
Save and exit the nano editor by pressing the Ctrl+X key.
Now, you can run the bash shell script in the following manner:
```
bash hello.sh
```
And you should see the following output:
```
Hello World
```
Another way is to give the script execute permission first:
```
chmod u+x hello.sh
```
And then run it in this manner:
```
./hello.sh
```
💡
You can also use a GUI based text editor to write the script. That probably be easier for writing longer scripts. However, you need to switch to the directory where the script is saved to run it.
Congratulations! You just ran your first bash script.
![][2]
### 2\. Using variables in bash scripts
Variables are declared in the following manner:
```
var=some_value
```
And then the variable is accessed like this:
```
$var
```
🚧
There must not be a space before and after = while declaring variable.
Let's modify the previous script by adding a variable.
```
#!/bin/bash
message="Hello World"
echo $message
```
The output will still remain the same if you run this script:
```
Hello World
```
![][2]
### 3\. Passing arguments to bash script
You can pass arguments to a bash script while running it in the following manner:
```
./my_script.sh arg1 arg2
```
Inside the script, you can use $1 for the 1st argument, $2 for the 2nd argument and so on. $0 is a special variable that holds the name of the script being executed.
Now, create a new shell script named `arguments.sh` and add the following lines to it:
```
#!/bin/bash
echo "Script name is: $0"
echo "First argument is: $1"
echo "Second argument is: $2"
```
Make it executable and run it like this:
```
abhishek@itsfoss:~$ ./argument.sh abhishek prakash
Script name is: ./argument.sh
First argument is: abhishek
Second argument is: prakash
```
Here's a quick look at the special variables:
Special Variable | Description
---|---
$0 | Script name
$1, $2...$9 | Script arguments
${n} | Script arguments from 10 to 255
$# | Number of arguments
$@ | All arguments together
$$ | Process id of the current shell
$! | Process id of the last executed command
$? | Exit status of last executed command
You can also make your bash script interactive by accepting user input from the keyboard.
For that, you'll have to use the read command. You can also use `read -p` command to prompt the user for the keyboard input without echo command.
```
#!/bin/bash
echo "What is your name, stranger?"
read name
read -p "What's your full name, $name? " full_name
echo "Welcome, $full_name"
```
Now if you run this script, you'll have to enter the 'arguments' when you are prompted for it.
```
abhishek@itsfoss:~$ ./argument.sh
What is your name, stranger?
abhishek
What's your full name, abhishek? abhishek prakash
Welcome, abhishek prakash
```
![][2]
### 4\. Perform arithmetic operation
The syntax for arithmetic operations in the bash shell is this:
```
$((arithmetic_operation))
```
Here's the list of the arithmetic operations you can perform in bash
Operator | Description
---|---
+ | Addition
- | Subtraction
* | Multiplication
/ | Integer division (without decimal)
% | Modulus division (only remainder)
** | Exponentiation (a to the power b)
Here's an example of performing summation and subtraction in bash script:
```
#!/bin/bash
read -p "Enter first number: " num1
read -p "Enter second number: " num2
sum=$(($num1+$num2))
sub=$(($num1-$num2))
echo "The summation of $num1 and $num2 is $sum"
echo "The substraction of $num2 from $num1 is $sub"
```
You can execute the shell script with random number of your choice.
![][4]
There is a big problem if you try the divison. Bash only works with integers. It doesn't have the concept of decimal numbers by default. And thus you'll get 3 as the result of 10/3 instead of 3.333.
For floating point operations, you'll have to use the bc command in this manner:
```
#!/bin/bash
num1=50
num2=6
result=$(echo "$num1/$num2" | bc -l)
echo "The result is $result"
```
And this time, you'll see accurate result.
```
The result is 8.33333333333333333333
```
![][2]
### 5\. Using arrays in bash scripts
Instead of using multiple variables, you can use arrays in bash to store values in the same category.
You can declare an array like this:
```
distros=(Ubuntu Fedora SUSE "Arch Linux" Nix)
```
To access an element, use:
```
${array_name[N]}
```
Like most other programming languages, the array index starts at 0.
You can display all the elements of an array like this:
```
${array[*]}
```
And get the array length like this:
```
${#array_name[@]}
```
![][2]
### 6\. Basic string operation in Bash
Bash is capable of performing a number of string operations.
You can get the string length in this manner:
```
${#string}
```
Join two strings:
```
str3=$str1$str2
```
Extract a substring by providing the starting position of the substring and its length:
```
${string:$pos:$len}
```
Here's an example:
![][5]
You can also replace a portion of the given string:
```
${string/substr1/substr2}
```
And you can also delete a substring from the given string:
```
${string/substring}
```
![][2]
### 7\. Use conditional statements in Bash
You can add conditional logic to your bash scripts by using if or if-else statements. These statements end with `fi`.
The syntax for a single if statement is:
```
if [ condition ]; then
your code
fi
```
Pay attention to the use `[ ... ];` and `then` .
The syntax for if-else statement is:
```
if [ expression ]; then
## execute this block if condition is true else go to next
elif [ expression ]; then
## execute this block if condition is true else go to next
else
## if none of the above conditions are true, execute this block
fi
```
Here's a sample bash script that uses if-else statement:
```
#!/bin/bash
read -p "Enter the number: " num
mod=$(($num%2))
if [ $mod -eq 0 ]; then
echo "Number $num is even"
else
echo "Number $num is odd"
fi
```
Run it and you should see a result like this:
![][6]
The `-eq` is called test condition or conditional operator. There are many such operators to give you different types of comparison:
Here are the test condition operators you can use for numeric comparison:
Condition | Equivalent to true when
---|---
$a -lt $b | $a < $b ($a is **l** ess **t** han $b)
$a -gt $b | $a > $b ($a is **g** reater **t** han $b)
$a -le $b | $a <= $b ($a is **less or **e** qual than $b)**
$a -ge $b | $a >= $b ($a is **g** reater or **e** qual than $b)
$a -eq $b | $a is equal to $b
$a -ne $b | $a is not equal to $b
If you are comparing strings, you can use these test conditions:
Condition | Equivalent to true when
---|---
"$a" = "$b" | $a is same as $b
"$a" == "$b" | $a is same as $b
"$a" != "$b" | $a is different from $b
-z "$a" | $a is empty
There are also conditions for file type check:
Condition | Equivalent to true when
---|---
-f $a | $a is a file
-d $a | $a is a directory
-L $a | $a is a link
🚧
Pay special attention to space. There must be space between the opening and closing brackets and the conditions. Similarly, space must be before and after the conditional operators (-le, == etc).
![][2]
### 8\. Using loops in bash scripts
Bash support three types of loops: for, while and until.
Here's an example of the **for loop** :
```
#!/bin/bash
for num in {1..10}; do
echo $num
done
```
Run it and you'll see the following output:
```
1
2
3
4
5
6
7
8
9
10
```
If you take the previous example, it can be rewritten using the **while loop** like this:
```
#!/bin/bash
num=1
while [ $num -le 10 ]; do
echo $num
num=$(($num+1))
done
```
And the same can be rewritten using the **until loop** :
```
#!/bin/bash
num=1
until [ $num -gt 10 ]; do
echo $num
num=$(($num+1))
done
```
💡
The while and until loop are pretty similar. The difference is that while loop runs as long as the condition is true and until loop runs as long as the condition is false.
![][2]
### 9\. Using functions in bash scripts
Bash shell does support the use of functions so that you don't have to write the same piece of code again and again.
Here's the generic syntax for declaring a bash function:
```
function_name() {
commands
}
```
Here's a sample bash script that uses function with arguments:
```
#!/bin/bash
sum() {
sum=$(($1+$2))
echo "The sum of $1 and $2 is: $sum"
}
echo "Let's use the sum function"
sum 1 5
```
If you run the script, you'll see the following output:
```
Let's use the sum function
The sum of 1 and 5 is: 6
```
![][2]
### Where to go from here?
This is just a glimpse. This bash scripting tutorial is just a primer. There is a lot more to bash scripting and you can explore it slowly and gradually.
The GNU bash reference is an excellent online resource to clean your bash doubts.
![][7]
###Apart from that, you can download this free book to learn more bash stuff which is not covered here:
[Download Bash Beginner Guide][8]
Once you have enough knowledge of the bash basics, you can learn advance bash scripting with this free book:
[Download Adavanced Bash Scripting Guide][9]
Both of these books are at least a decade old but you can still use them to learn bash.
💬 I hope you like this tutorial as the starting point of your bash script learning. Please provide your feedback in the comments section.
--------------------------------------------------------------------------------
via: https://itsfoss.com/bash-scripting-tutorial/
作者：[Abhishek Prakash][a]
选题：[lujun9972][b]
译者：[译者ID](https://github.com/译者ID)
校对：[校对者ID](https://github.com/校对者ID)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://itsfoss.com/author/abhishek/
[b]: https://github.com/lujun9972
[1]: https://itsfoss.com/content/images/wordpress/2021/12/linux-terminal-introduction.png
[2]: https://itsfoss.com/content/images/size/w256h256/2022/12/android-chrome-192x192.png
[3]: https://linuxhandbook.com/content/images/size/w256h256/2021/08/Linux-Handbook-New-Logo.png
[4]: https://itsfoss.com/content/images/2023/07/addition-substraction-bash-script.png
[5]: https://itsfoss.com/content/images/2023/07/extract-substring-bash.png
[6]: https://itsfoss.com/content/images/2023/07/bash-if-else-example.png
[7]: https://www.gnu.org/favicon.ico
[8]: https://tldp.org/LDP/Bash-Beginners-Guide/Bash-Beginners-Guide.pdf?ref=itsfoss.com
[9]: https://tldp.org/LDP/abs/abs-guide.pdf?ref=itsfoss.com

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[#]: subject: "Introduction to Bash Scripting"
[#]: via: "https://itsfoss.com/bash-scripting-tutorial/"
[#]: author: "Abhishek Prakash https://itsfoss.com/author/abhishek/"
[#]: collector: "lujun9972"
[#]: translator: "ChatGPT"
[#]: reviewer: "wxy"
[#]: publisher: " "
[#]: url: " "
Bash 脚本编程入门
=====
> 初学者们，让我们在这个大型的教程中来认识一下 Bash 脚本编程。
Shell 是 Linux 的核心部分，它允许你使用各种诸如 `cd`、`ls`、`cat` 等的命令与 Linux 内核进行交互。
Bash 是 Linux 上众多可用的 Shell 中的一个。这些 Shell 主要的语法基本相同，但并非完全一样。Bash 是目前最受欢迎的 Shell，并在大多数 Linux 发行版中被设为默认 Shell。
当你打开一个终端或 SSH 会话时，即使你无法真切地看到它，你其实已经在运行着一个 Shell。
![][1]
当你输入一个命令，它会被 Shell 解释。如果命令和语法是正确的，它就会被执行，否则你会看到一个错误。
### 当你可以直接运行 Linux 命令时，为什么还需要 Bash 脚本？
你可以直接在终端输入命令，它们就会被执行。
```
$ echo "hello world"
hello world
```
并且，同样的操作也可以在脚本中进行：
```
$ cat >> script.sh
#!/bin/bash
echo "hello world"
$ bash script.sh
hello world
```
那么，为什么我们需要 Shell 脚本呢？因为你不必一遍又一遍地输入同一个命令，你只需运行 Shell 脚本即可。
此外，如果你的脚本中有复杂的逻辑，把所有的命令都输入到终端中可能并不是一个好主意。
例如，如果你输入下面的命令，它会奏效，但这并不容易理解。不断地输入相同的命令（甚至要在 Bash 历史记录中搜索）会造成困扰。
```
if [ $(whoami) = 'root' ]; then echo "root"; else echo "not root"; fi
```
相反，你可以把命令放进 shell 脚本中，这样就更容易理解并且可以轻松运行了：
```
#!/bin/bash
if [ $(whoami) = 'root' ]; then
echo "You are root"
else
echo "You are not root"
fi
```
这还是比较简单的情况。尝试想象一下，一个复杂的脚本可能有五十行或一百行！
### 你将会学到什么？
在这个 Bash 脚本教程中，有九个部分。你将会学到：
* 创建并运行你的第一个 Bash Shell 脚本
* 使用变量
* 在你的 Bash 脚本中传递参数和接受用户输入
* 进行数学计算
* 操作字符串
* 使用条件语句，例如 `if-else`
* 使用 `for`、`while` 和 `until` 循环
* 创建函数
> 💡 所有的部分都会给你一个简单的例子。如果你愿意，你可以通过访问每个部分的详细章节来更深入地学习。这些章节也都包含了实践练习。
### 这个教程的目标读者是谁？
这个教程适合任何想要开始学习 Bash Shell 脚本的人。
如果你是一名学生，而你的课程里包括了 Shell 脚本，那么这个系列就是为你准备的。
如果你是一个常规的桌面 Linux 用户，这个系列将会帮助你理解在探索各种软件和修复问题时遇到的大多数 Shell 脚本。你也可以使用它来自动化一些常见的、重复的任务。
在这个 Bash 脚本教程结束时，你应该可以编写简单的 bash 脚本。
> 🚧 希望你已经拥有 Linux 命令行和编程语言的基础知识。
如果你对 Linux 命令行完全不熟悉，我建议你先掌握基础知识。
> **[19 个你应该知道的基础而重要的 Linux 终端技巧][1a]**
你应该了解如何在命令行中进入特定的位置。为了做到这一点，你需要理解 Linux 文件系统中的路径是如何工作的。
> **[Linux 中的绝对路径和相对路径有什么不同][1b]**
接下来，这个教程系列会给你介绍目录导航和文件操作的基本知识。
> **[终端基础：Linux 终端入门][1c]**
### 1、编写你的第一个 Bash Shell 脚本
创建一个名为 `hello.sh` 的新文件：
```
nano hello.sh
```
这将在终端中打开 nano 编辑器。在其中输入以下几行代码：
```
#!/bin/bash
echo "Hello World"
```
通过按 `Ctrl+X` 键可以保存并退出 nano 编辑器。
现在，你可以以以下方式运行 Bash Shell 脚本：
```
bash hello.sh
```
你应该可以看到以下的输出：
```
Hello World
```
另一种方式是首先赋予脚本执行权限：
```
chmod u+x hello.sh
```
然后这样运行它：
```
./hello.sh
```
> 💡 你也可以使用基于图形用户界面的文本编辑器来编写脚本。这可能更适合编写较长的脚本。然而，你需要切换到保存脚本的目录中才能运行它。
恭喜！你刚刚运行了你的第一个 Bash 脚本。
> [Bash 基础知识系列 #1：创建并运行你的第一个 Bash Shell 脚本][1d]
### 2、在 Bash 脚本中使用变量
变量的声明方式如下：
```
var=some_value
```
然后可以像这样访问变量：
```
$var
```
> 🚧 在声明变量时，等号（`=`）前后不能有空格。
我们通过添加一个变量来修改前面的脚本。
```
#!/bin/bash
message="Hello World"
echo $message
```
如果运行这个脚本，输出仍然会保持不变。
```
Hello World
```
> **[Bash 基础知识系列 #2：在 Bash 中使用变量][1e]**
### 3、向 Bash 脚本传递参数
你可以在运行 Bash 脚本时以以下方式传递参数：
```
./my_script.sh arg1 arg2
```
在脚本中，你可以使用 `$1` 来代表第 1 个参数，用 `$2` 来代表第 2 个参数，以此类推。`$0` 是一个特殊变量，它代表正在运行的脚本的名字。
现在，创建一个新的 shell 脚本，命名为 `arguments.sh`，并向其中添加以下几行代码：
```
#!/bin/bash
echo "Script name is: $0"
echo "First argument is: $1"
echo "Second argument is: $2"
```
使其可执行并像这样运行它：
```
$ ./argument.sh abhishek prakash
Script name is: ./argument.sh
First argument is: abhishek
Second argument is: prakash
```
让我们快速看一下特殊变量：
特殊变量 | 描述
---|---
`$0` | 脚本名称
`$1`、`$2`...`$9` | 脚本参数
`${n}` | 10 到 255 的脚本参数
`$#` | 参数数量
`$@` | 所有参数一起
`$$` | 当前 shell 的进程 id
`$!` | 最后执行命令的进程 id
`$?` | 最后执行命令的退出状态
你也可以通过接受键盘输入使你的 Bash 脚本变得交互式。
为此，你必须使用 `read` 命令。你还可以使用 `read -p` 命令提示用户进行键盘输入，而不需要 `echo` 命令。
```
#!/bin/bash
echo "What is your name, stranger?"
read name
read -p "What's your full name, $name? " full_name
echo "Welcome, $full_name"
```
现在，如果你运行这个脚本，当系统提示你输入“参数”时，你必须输入。
```
$ ./argument.sh
What is your name, stranger?
abhishek
What's your full name, abhishek? abhishek prakash
Welcome, abhishek prakash
```
> **[Bash 基础知识系列 #3：传递参数和接受用户输入][1f]**
### 4、执行算术运算
在 Bash Shell 中执行算术运算的语法是这样的：
```
$((arithmetic_operation))
```
下面是你可以在 Bash 中执行的算术运算的列表：
操作符 | 描述
---|---
`+` | 加法
`-` | 减法
`*` | 乘法
`/` | 整数除法（没有小数）
`%` | 模运算（只余）
`**` | 指数（a 的 b 次方）
以下是在 Bash 脚本中进行加法和减法的示例：
```
#!/bin/bash
read -p "Enter first number: " num1
read -p "Enter second number: " num2
sum=$(($num1+$num2))
sub=$(($num1-$num2))
echo "The summation of $num1 and $num2 is $sum"
echo "The substraction of $num2 from $num1 is $sub"
```
你可以执行 Shell 脚本，使用你选择的任意数字作为参数。
![][4]
如果你尝试除法，会出现一个大问题。Bash 只使用整数。默认情况下，它没有小数的概念。因此，你会得到 10/3 的结果为3，而不是 3.333。
对于浮点数运算，你需要这样使用 `bc` 命令：
```
#!/bin/bash
num1=50
num2=6
result=$(echo "$num1/$num2" | bc -l)
echo "The result is $result"
```
这个时候，你将看到准确的结果。
```
The result is 8.33333333333333333333
```
> **[Bash 基础知识系列 #4：算术运算][4a]**
### 5、在 Bash 脚本中使用数组
你可以使用 Bash 中的数组来存储同一类别的值，而不是使用多个变量。
你可以像这样声明一个数组：
```
distros=(Ubuntu Fedora SUSE "Arch Linux" Nix)
```
要访问一个元素，使用：
```
${array_name[N]}
```
像大多数其他的编程语言一样，数组的索引从 0 开始。
你可以像这样显示数组的所有元素：
```
${array[*]}
```
这样获取数组长度：
```
${#array_name[@]}
```
> **[Bash 基础知识系列 #5：在 Bash 中使用数组][4b]**
### 6、Bash 中的基础字符串操作
Bash 能够执行许多字符串操作。
你可以使用这种方式获取字符串长度：
```
${#string}
```
连接两个字符串：
```
str3=$str1$str2
```
提供子字符串的起始位置和长度来提取子字符串：
```
${string:$pos:$len}
```
这里有一个例子：
![][5]
你也可以替换给定字符串的一部分：
```
${string/substr1/substr2}
```
并且你也可以从给定字符串中删除一个子字符串：
```
${string/substring}
```
> **[Bash 基础知识系列 #6：处理字符串操作][4c]**
### 7、在 Bash 中使用条件语句
你可以通过使用 `if` 或 `if-else` 语句为你的 Bash 脚本添加条件逻辑。这些语句以 `fi` 结束。
单个 `if` 语句的语法是：
```
if [ condition ]; then
your code
fi
```
注意使用 `[ ... ];` 和 `then` 。
`if-else` 语句的语法是：
```
if [ expression ]; then
## execute this block if condition is true else go to next
elif [ expression ]; then
## execute this block if condition is true else go to next
else
## if none of the above conditions are true, execute this block
fi
```
这里有一个使用 `if-else` 语句的 Bash 脚本示例：
```
#!/bin/bash
read -p "Enter the number: " num
mod=$(($num%2))
if [ $mod -eq 0 ]; then
echo "Number $num is even"
else
echo "Number $num is odd"
fi
```
运行它，你应该能看到这样的结果：
![][6]
`-eq` 被称为测试条件或条件操作符。有许多这样的操作符可以给你不同类型的比较：
这些是你可以用来进行数值比较的测试条件操作符：
条件 | 当...时，等同于 true
---|---
`$a -lt $b` | `$a < $b` （`$a` 是小于 `$b`）
`$a -gt $b` | `$a > $b` （`$a` 是大于 `$b`）
`$a -le $b` | `$a <= $b` （`$a` 是小于或等于 `$b`）
`$a -ge $b` | `$a >= $b` （`$a` 是大于或等于 `$b`）
`$a -eq $b` | `$a == $b` （`$a` 等于 `$b`）
`$a -ne $b` | `$a != $b` （`$a` 不等于 `$b`）
如果你在进行字符串比较，你可以使用以下这些测试条件：
条件 | 当...时，等同于 true
---|---
`"$a" = "$b"` | `$a` 等同于 `$b`
`"$a" == "$b"` | `$a` 等同于 `$b`
`"$a" != "$b"` | `$a` 不同于 `$b`
`-z "$a"` | `$a` 是空的
还有些条件用于检查文件类型：
条件 | 当...时，等同于 true
---|---
`-f $a` | `$a` 是一个文件
`-d $a` | `$a` 是一个目录
`-L $a` | `$a` 是一个链接
> 🚧 要特别注意空格。开括号和闭括号、条件之间必须有空格。同样地，条件操作符（`-le`、`==` 等）之前和之后必须有空格。
> **[Bash 基础知识系列 #7：If Else 语句][6a]**
### 8、使用 bash 脚本中的循环
Bash 支持三种类型的循环：`for`、`while` 和 `until`。
这是 `for` 循环的一个例子：
```
#!/bin/bash
for num in {1..10}; do
echo $num
done
```
运行它，你将看到以下输出：
```
1
2
3
4
5
6
7
8
9
10
```
如果你选择使用上面的示例，可以使用 `while` 循环这样重写：
```
#!/bin/bash
num=1
while [ $num -le 10 ]; do
echo $num
num=$(($num+1))
done
```
同样，可以使用 `until` 循环来重写：
```
#!/bin/bash
num=1
until [ $num -gt 10 ]; do
echo $num
num=$(($num+1))
done
```
> 💡 `while` 循环和 `until` 循环非常相似。区别在于：`while` 循环在条件为真时运行，而 `until` 循环在条件为假时运行。
> [Bash 基础知识系列 #8：for、while 和 until 循环][6b]
### 9、在 bash 脚本中使用函数
Bash Shell 支持使用函数，这样你不必反复编写相同的代码片段。
这是声明 Bash 函数的通用语法：
```
function_name() {
commands
}
```
这是一个使用带参数的函数的 Bash 脚本样例：
```
#!/bin/bash
sum() {
sum=$(($1+$2))
echo "The sum of $1 and $2 is: $sum"
}
echo "Let's use the sum function"
sum 1 5
```
如果你运行该脚本，你将看到以下输出：
```
Let's use the sum function
The sum of 1 and 5 is: 6
```
> [Bash 基础知识系列 #9：函数][6c]
### 接下来去哪里？
这只是一个初窥。这个 Bash 脚本教程只是一篇引言。Bash 脚本的内容还有很多，你可以慢慢地、逐渐地探索。
GNU Bash 参考是一份优秀的在线资源，可以解答你的 bash 疑问。
> **[GNU Bash 参考][7]**
### 除此之外，你可以下载这本免费书籍来学习更多在此未涵盖的 bash 内容：
> **[下载 Bash 初学者指南][8]**
一旦你具有足够的 bash 基础知识，你可以通过这本免费书籍来学习高级 bash 脚本：
> **[下载高级 Bash 编程指南][9]**
这两本书至少都有十年的历史，但你仍然可以使用它们来学习 Bash。
💬 希望你喜欢这个作为学习 Bash 脚本起点的教程。请在评论区提供你的反馈。
--------------------------------------------------------------------------------
via: https://itsfoss.com/bash-scripting-tutorial/
作者：[Abhishek Prakash][a]
选题：[lujun9972][b]
译者：ChatGPT
校对：[wxy](https://github.com/wxy)
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
[a]: https://itsfoss.com/author/abhishek/
[b]: https://github.com/lujun9972
[1]: https://itsfoss.com/content/images/wordpress/2021/12/linux-terminal-introduction.png
[1a]: https://itsfoss.com/basic-terminal-tips-ubuntu/
[1b]: https://linuxhandbook.com/absolute-vs-relative-path/
[1c]: https://linux.cn/article-16104-1.html
[1d]: https://linux.cn/article-15921-1.html
[1e]: https://linux.cn/article-15991-1.html
[1f]: https://linux.cn/article-16001-1.html
[2]: https://itsfoss.com/content/images/size/w256h256/2022/12/android-chrome-192x192.png
[3]: https://linuxhandbook.com/content/images/size/w256h256/2021/08/Linux-Handbook-New-Logo.png
[4]: https://itsfoss.com/content/images/2023/07/addition-substraction-bash-script.png
[4a]: https://linux.cn/article-16006-1.html
[4b]: https://linux.cn/article-16016-1.html
[4c]: https://linux.cn/article-16047-1.html
[5]: https://itsfoss.com/content/images/2023/07/extract-substring-bash.png
[6]: https://itsfoss.com/content/images/2023/07/bash-if-else-example.png
[6a]: https://linux.cn/article-16083-1.html
[6b]: https://itsfoss.com/bash-loops/
[6c]: https://itsfoss.com/bash-function/
[7]: https://www.gnu.org/software/bash/manual/bash.html
[8]: https://tldp.org/LDP/Bash-Beginners-Guide/Bash-Beginners-Guide.pdf
[9]: https://tldp.org/LDP/abs/abs-guide.pdf