在Bash中模拟OOP | Simulating OOP in Bash

Made by Mike_Zhang

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Intro

导言

众所周知，Java对面向对象编程(Object-Oriented Programming, OOP)的支持非常好。因此，我特别喜欢Java这门编程语言，配合OOP，可以十分快速和规范的对问题进行抽象和建模。

最近在学习Shell Script，这是一门结构很直接的语言，在Shell中运行。其支持十分典型的面相过程编程 (Process-Oriented Programming, POP)，并不支持OOP。但是在用Shell Script处理一些复杂问题时，直接的POP并不能很好的对问题进行抽象和建模。因此，我想尝试用Shell Script来实现模拟OOP。

受限于Shell Script的语法，本文只会模拟一下OOP的基础特性，欢迎大家在下方交流。

本文使用Bash编写Shell Script。

1. Field Abstraction

1. 属性抽象

属性(Field)作为对象的静态成员变量，用来描述对象的状态。

在Java中，一般用以下方式对属性进行抽象：

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public class Book{ 
    private String bookName; 
    private int bookAmount; 
}

但是在Shell Script中，没有类的概念，因此我们不妨用函数来模拟类，比如：

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Book() {

}

然后在函数中用变量模拟类的属性，比如：

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Book() {
    self=$1
    bookName=$2
    bookAmount=$3
}

注意：

  • self为当前对象的对象名，用来区分不同的对象，类似Python中的self；
  • bookName和bookAmount为对象的属性；

以上我们完成了对对象属性的抽象，接下来我们需要对对象的行为进行抽象。

2. Method Abstraction

2. 方法抽象

方法定义了此类实例的行为，能够对此类实例进行的操作。

2.1 Constructor

2.1 构造方法

类的构造方法(Constructor)是一种的特殊的方法, 每次用类实例化一个对象时, 构造方法会自动调用。

在Java中，构造方法的例子如下：

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public class Book{ 
    private String bookName; 
    private int bookAmount; 

    // Constructor
    public Book(String inName, int inAmount){ 
        bookName = inName; 
        bookAmount = inAmount; 
    }
}

在Java实例化对象时，我们通常用以下语句：

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Book myBook = new Book("Harry Potter", 300);

在Shell Script中，我们可以用以下语句来模拟Java中的构造方法：

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Book() {
    self=$1
    bookName=$2
    bookAmount=$3
}
  • 没错，这与上文模拟属性的代码是一样的，同时完成了构造方法的模拟；

并用以下语句来模拟Java中的实例化对象：

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# Create a new book object, object name is myBook
Book myBook "Harry Potter" 300

对比Java中：

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Book myBook = new Book("Harry Potter", 300);

对比看出，用Shell Script的函数来模拟类，可以很好的“实现”Java中的构造方法。

完成了构造方法的模拟，接下来我们当然还要模拟一下Java中经典的getter和setter方法。

2.2 Getter Method

2.2 访问器方法

访问器方法是用来访问并获取对象的属性值的方法，在Java中如：

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public class Book{ 
    private String bookName; 
    private int bookAmount; 

    // ...

    // Getter method
    // Get book name
    public String getBookName(){
        return bookName;
    }

    // Get book amount
    public int getBookAmount(){
        return bookAmount;
    }
}

并可以通过以下语句来调用：

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myBook.getBookName();
myBook.getBookAmount();

在Shell Script中，我们可以用环境变量来巧妙的模拟getter方法，如下：

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Book() {
    self=$1
    bookName=$2
    bookAmount=$3

    # ...

    # Getter method
    # Get book name
    export ${self}_getBookName="$bookName"

    # Get book amount
    export ${self}_getBookAmount="$bookAmount"
}

并可以通过以下语句来调用：

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$myBook_getBookName
$myBook_getBookAmount

对比Java：

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myBook.getBookName();
myBook.getBookAmount();
  • 形式上十分相似；
  • 但在本质上不同，Java为调用方法，而Shell Script只是访问环境变量；

2.3 Setter Method

2.3 修改器方法

修改器方法是用来修改对象的属性值的方法，在Java中如：

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public class Book{ 
    private String bookName; 
    private int bookAmount; 

    // ...

    // Setter method
    // Set book name
    public void setBookName(String inName){
        bookName = inName;
    }
}

并可以通过以下语句来调用：

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myBook.setBookName("Lord of the Rings");

在Shell Script中，我们可以用其函数的模拟setter方法，如下：

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Book() {
    self=$1
    bookName=$2
    bookAmount=$3

    # ...

    # Setter method
    # set book name
    setBookName() {
        object=$1
        name=$2
        export ${object}_getBookName="$name"
    }
}

并可以通过以下语句来调用：

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setBookName myBook "The Lord of the Rings"

对比Java：

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myBook.setBookName("Lord of the Rings");
  • 可以发现十分类似；
  • 都有对象名和参数值，都调用了函数或方法；

2.4 General Method

2.4 一般方法

我们还可以尝试一些其他的方法，如：

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public class Book{ 
    // ...

    // Increase book amount
    public void increaseBookAmount(int inAmount){
        bookAmount += inAmount;
    }

    // Decrease book amount with error checking and handling
    public void decreaseBookAmount(int inAmount){
        if (bookAmount - inAmount < 0){
            System.out.println("Error: Not enough books to decrease");
        }
        else {
            bookAmount -= inAmount;
        }
    }
}

用Shell Script来模拟：

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Book() {
    # ...

    # Increase book amount
    increaseBookAmount() {
        object=$1
        amount=$2
        cmd1=${object}_getBookAmount
        oldAmount=${!cmd1} # to execute the cmd1 variable
        newAmount=$(($oldAmount + $amount))
        export ${object}_getBookAmount="$newAmount"
    }

    # Decrease book amount with error checking and handling
    decreaseBookAmount() {
        object=$1
        amount=$2
        cmd1=${object}_getBookAmount
        oldAmount=${!cmd1} # to execute the cmd1 variable
        newAmount=$(($oldAmount - $amount))
        if [ $newAmount -lt 0 ]; then
            echo "Error: Not enough books to decrease"
        else
            export ${object}_getBookAmount="$newAmount"
        fi
    }
}

分别调用：

Java:

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myBook.increaseBookAmount(100);
myBook.decreaseBookAmount(200);
myBook.decreaseBookAmount(300);

Bash:

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increaseBookAmount myBook 100
decreaseBookAmount myBook 200
decreaseBookAmount myBook 300

3. Code Example

3. 代码示例

以下为完整代码和运行示例：

Java:

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public class Book{ 
    private String bookName; 
    private int bookAmount; 

    // Constructor
    public Book(String inName, int inAmount){ 
        bookName = inName; 
        bookAmount = inAmount; 
    }

    // Getter method
    public String getBookName(){
        return bookName;
    }

    public int getBookAmount(){
        return bookAmount;
    }

    // Setter method
    public void setBookName(String inName){
        bookName = inName;
    }

    // Increase book amount
    public void increaseBookAmount(int inAmount){
        bookAmount += inAmount;
    }

    // Decrease book amount with error checking and handling
    public void decreaseBookAmount(int inAmount){
        if (bookAmount - inAmount < 0){
            System.out.println("Error: Not enough books to decrease");
        }
        else {
            bookAmount -= inAmount;
        }
    }

    public static void main(String[] args){
        Book myBook = new Book("Harry Potter", 300);
        System.out.println(myBook.getBookName()); // Harry Potter
        System.out.println(myBook.getBookAmount()); // 300
        myBook.setBookName("Lord of the Rings");
        System.out.println(myBook.getBookName()); // Lord of the Rings
        myBook.increaseBookAmount(100);
        System.out.println(myBook.getBookAmount()); // 400
        myBook.decreaseBookAmount(200);
        System.out.println(myBook.getBookAmount()); // 200
        myBook.decreaseBookAmount(300); // Error: Not enough books to decrease
    }
}

Bash:

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#!/bin/bash

# Book myBook "The Hobbit" 300
Book() {
    # Field and constructor
    self=$1
    bookName=$2
    bookAmount=$3

    # Getter method
    export ${self}_getBookName="$bookName"
    export ${self}_getBookAmount="$bookAmount"

    # Setter method
    # e.g. setBookName myBook "The Lord of the Rings"
    setBookName() {
        object=$1
        name=$2
        export ${object}_getBookName="$name"
    }

    # Increase book amount
    # e.g. increaseBookAmount myBook 100
    increaseBookAmount() {
        object=$1
        amount=$2
        cmd1=${object}_getBookAmount
        oldAmount=${!cmd1} # to execute the cmd1 variable
        newAmount=$(($oldAmount + $amount))
        export ${object}_getBookAmount="$newAmount"
    }

    # Decrease book amount with error checking and handling
    # e.g. decreaseBookAmount myBook 100
    decreaseBookAmount() {
        object=$1
        amount=$2
        cmd1=${object}_getBookAmount
        oldAmount=${!cmd1} # to execute the cmd1 variable
        newAmount=$(($oldAmount - $amount))
        if [ $newAmount -lt 0 ]; then
            echo "Error: Not enough books to decrease"
        else
            export ${object}_getBookAmount="$newAmount"
        fi
    }
}

Book myBook "Harry Potter" 300
echo $myBook_getBookName # Harry Potter
echo $myBook_getBookAmount # 300
setBookName myBook "The Lord of the Rings"
echo $myBook_getBookName # The Lord of the Rings
increaseBookAmount myBook 100
echo $myBook_getBookAmount # 400
decreaseBookAmount myBook 200
echo $myBook_getBookAmount # 200
decreaseBookAmount myBook 300 # Error: Not enough books to decrease

Outro

尾巴

本文简单地用Shell Script模拟了Java中的OOP，但只模拟了一些基础特性。OOP的更多经典特性并未体现，如继承、多态、权限符、抽象类、接口等。或许可以尝试用Shell Script来模拟这些特性，我相信会非常有趣，并且会对面向对象编程用更深的理解，也是对Shell Script的练习。

References

参考资料

Hipersayan_x, “The Samurai Code: Object Oriented Programming in Bash,” The Samurai Code, Dec. 22, 2012. https://hipersayanx.blogspot.com/2012/12/object-oriented-programming-in-bash.html (accessed Feb. 07, 2023).

“Simulating OOP in Bash,” DEV Community. https://dev.to/leandronsp/simulating-oop-in-bash-3mop (accessed Feb. 07, 2023).

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Made by Mike_Zhang




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在Bash中模拟OOP | Simulating OOP in Bash
https://ultrafish.io/post/oop-in-bash/
Author
Mike_Zhang
Posted on
February 7, 2023
Licensed under