Linux shell编程学习笔记21：用select in循环语句打造菜单

朴灿烈づ我的快乐病毒、 2024-02-19 07:46 56阅读 0赞

## 一、select in循环语句的功能 ##

Linux shell脚本编程提供了select in语句，这是 Shell 独有的一种循环语句，非常适合终端（Terminal）这样的交互场景，它可以根据用户的设置显示出带编号的菜单，用户通过输入不同的编号就可以选择不同的菜单，并执行与菜单对应的功能，这是C、C++、Java、Python 等编程语言中是没有的。

## 二、select in循环语句的语法格式 ##

> select 变量名 in 值1 \[值2 ……值n\]  
> do  
> 语句或命令1  
> \[……\]  
> \[语句或命令n\]  
> done

在do 和 done之间，我们可以用if或case语句根据变量名的值执行相应语句或命令，实现不同的功能。

## 三、实例1 ##

我们会询问你喜欢吃哪种水果，并显示4个水果选项：apple、banana、orrange、peach，以及一个退出循环的选项exit

如果你的选择4个水果中的一个，将会显示你选择的水果名称

如果你选择了是exit，将会看到提示bye，并结束循环。

代码如下：

echo "What is your favourite fruit?"
    select f in "apple" "banana"  "orrange" "peach" "exit"
    do
        if [[ $f == "exit" ]]; then
            echo "bye"
    	break
        else
            echo "You have selected $f"
        fi
     done

### （一）在zsh中 ###

> \# csdn @ edu in ~ \[22:32:22\]  
> $ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do; if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple 2) banana 3) orrange 4) peach 5) exit  
> ?\# 1  
> You have selected apple  
> ?\# 4  
> You have selected peach  
> ?\# 5  
> bye
> 
> \# csdn @ edu in ~ \[22:32:52\]

![ff09fdd5294a41398856d34d868cbfae.png][]

我们可以看到zsh 根据select in循环语句提供的值显示出了五个菜单项：

> 1) apple 2) banana 3) orrange 4) peach 5) exit

每个菜单项前有1个数字。

然后显示提示符 ?\# 让我们输入菜单项前的数字来选择相应的菜单项。

我们先输入了1，所以显示You have selected apple

接着我们输入4，所以显示You have selected peach

最后我们输入5，这是退出循环的选项，于是我们看到了bye，并退回到了命令行状态。

### （二）在bash中 ###

> \# csdn @ edu in ~ \[22:46:42\]  
> $ bash  
> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do; if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> bash: syntax error near unexpected token \`;'  
> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple  
> 2) banana  
> 3) orrange  
> 4) peach  
> 5) exit  
> \#? 1  
> You have selected apple  
> \#? 3  
> You have selected orrange  
> \#? 5  
> bye  
> \[csdn ~\]$

![3d534d97ceee411e8d6f47be93b69040.png][]

我们可以看到bash 和zsh 对脚本的支持不太一样。

刚才在zsh中执行正常的脚本，在bash中执行时出错：

> bash: syntax error near unexpected token \`;'

我们把 do 语句后面的; 去掉后，脚本可以执行了。

根据select in循环语句提供的值显示出了五个菜单项：

我们可以看到bash 像zsh 一样，根据select in循环语句提供的值显示出了五个菜单项：

> 1) apple  
> 2) banana  
> 3) orrange  
> 4) peach  
> 5) exit

每个菜单项前有1个数字。

但与zsh的**横向显示**方式不同，bash采用的是**纵向显示**方式。

然后显示提示符**\#?**  让我们输入菜单项前的数字来选择相应的菜单项。

bash的这个提示符**\#?**  跟zsh显示的提示符字符顺序是相反的，zsh显示的提示符字符是**?\#**。

我们先输入了1，所以显示You have selected apple

接着我们输入3，所以显示You have selected orrange

最后我们输入5，这是退出循环的选项，于是我们看到了bye，并退回到了命令行状态。

## 四、容错机制测试 ##

在上面的实例1中，如果我们输入的字符不属于菜单项前的数字，代码会怎么执行呢？

### （一）在zsh中 ###

> \# csdn @ edu in ~ \[23:03:43\]  
> $ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do; if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple 2) banana 3) orrange 4) peach 5) exit  
> ?\# a  
> You have selected  
> ?\# 6  
> You have selected  
> ?\# 3  
> You have selected orrange  
> ?\# 5  
> bye
> 
> \# csdn @ edu in ~ \[23:04:12\]  
> $

![40c9e4a85b4343bf8fdf9ca180c9d57b.png][]

### （二）在bash中 ###

> \# csdn @ edu in ~ \[23:04:12\]  
> $ exec bash  
> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do; if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> bash: syntax error near unexpected token \`;'  
> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple  
> 2) banana  
> 3) orrange  
> 4) peach  
> 5) exit  
> \#? b  
> You have selected  
> \#? 9  
> You have selected  
> \#? 3  
> You have selected orrange  
> \#? 5  
> bye  
> \[csdn ~\]$

![f46502848ec648b1ace6f28c864b3c57.png][] 可以看到，不管zsh还是bash，如果我们输入的字符不属于菜单项前的数字，shell并不会进行干预，代码仍然会执行，并由if语句根据$f的值进行处理。

所以为了实现预期的效果，我们需要自己对用户输入的字符的有效性进行检查和处理。

## 五、如果 传递给 select in 的值超过9个会怎么显示？（实例2） ##

我们通过下面的代码，给select in 传递了36个值来测试。

### （一） 在 zsh中 ###

> \# csdn @ edu in ~ \[23:14:29\]  
> $ echo "What is your favourite fruit?"; select f in 1 2 3 4 5 6 7 8 9 a b c d e f g h i j k l m n o p q r s t u v w x y z exit; do; if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "  
> You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) 1 3) 3 5) 5 7) 7 9) 9 11) b 13) d 15) f 17) h 19) j 21) l 23) n 25) p 27) r 29) t 31) v 33) x 35) z  
> 2) 2 4) 4 6) 6 8) 8 10) a 12) c 14) e 16) g 18) i 20) k 22) m 24) o 26) q 28) s 30) u 32) w 34) y 36) exit  
> ?\# 1  
> You have selected 1  
> ?\# 18  
> You have selected i  
> ?\# m  
> You have selected  
> ?\# 36  
> bye
> 
> \# csdn @ edu in ~ \[23:15:12\]  
> $

![fde8166635364f70b87b4a9028a1c4f4.png][]

这次在zsh也是采用纵向多列的方式来显示菜单。

### （二）在bash中 ###

> \# csdn @ edu in ~ \[23:15:12\]  
> $ exec bash  
> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in 1 2 3 4 5 6 7 8 9 a b c d e f g h i j k l m n o p q r s t u v w x y z exit; do if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) 1 3) 3 5) 5 7) 7 9) 9 11) b 13) d 15) f 17) h 19) j 21) l 23) n 25) p 27) r 29) t 31) v 33) x 35) z  
> 2) 2 4) 4 6) 6 8) 8 10) a 12) c 14) e 16) g 18) i 20) k 22) m 24) o 26) q 28) s 30) u 32) w 34) y 36) exit  
> \#? 9  
> You have selected 9  
> \#? 20  
> You have selected k  
> \#? k  
> You have selected  
> \#? 36  
> bye  
> \[csdn ~\]$

![3a83cf5ffa194fcb840af09c5897c4a4.png][]

这次bash显示菜单的方式与zsh一致。

可以看到，当传递给 select in 的值超过9个时，不管zsh还是bash会以10、11……这种方式给值赋予菜单值。

## 六、利用数组构建容错机制 ##

### （一）实例3 ###

我们可以先将要传递给select in 的值定义并存储到一个数组a中，

再把数组a传递给 select in，

当用户输入数值后，对用户输入的数值进行判断：

1.  如果用户输入的值 > 数组a的长度 或者 用户输入的值 < 1，那么就显示 bye并退出循环
2.  否则显示用户选择的fruit。

> \# csdn @ edu in ~ \[23:48:42\]  
> $ a=(apple banana orrange peach);echo "What is your favourite fruit?"; select f in $a; do; if \[\[ $f -gt $\{\#a\[\*\]\} \]\] || \[\[ $f < 1 \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple 2) banana 3) orrange 4) peach  
> ?\# 1  
> You have selected apple  
> ?\# 4  
> You have selected peach  
> ?\# 0  
> bye
> 
> \# csdn @ edu in ~ \[23:48:52\]  
> $

![5fe0b7ae9f53421097c23b6b7782210c.png][]

### （二）注意 ###

对于 if \[\[ $f -gt $\{\#a\[\*\]\} \]\] || \[\[ $f < 1 \]\]

#### 1.如果 将 $f -gt $\{\#a\[\*\]\} 写成 $f > $\{\#a\[\*\]\}，代码执行就会出现问题 ####

> $f > $\{\#a\[\*\]\} 
> 
> $ a=(apple banana orrange peach);echo "What is your favourite fruit?"; select f in $a; do; if \[\[ $f > $\{\#a\[\*\]\} \]\] || \[\[ $f < 1 \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple 2) banana 3) orrange 4) peach  
> ?\# 1  
> bye
> 
> \# csdn @ edu in ~ \[23:13:38\]  
> $

![0e6522cdc9074e20a8151c34c7ec4f27.png][]

即使我们输入1，代码也会显示bye并跳出循环返回命令行。

这是因为$f 和 $\{\#a\[\*\]\} 在这里进行的是字符串比较。

#### 2.如果将 $f < 1 写为 $f -lt 1，代码执行也会出现问题 ####

> \# csdn @ edu in ~ \[23:48:31\]  
> $ a=(apple banana orrange peach);echo "What is your favourite fruit?"; select f in $a; do; if \[\[ $f -gt $\{\#a\[\*\]\} \]\] || \[\[ $f -lt 1 \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done   
> What is your favourite fruit?  
> 1) apple 2) banana 3) orrange 4) peach  
> ?\# 1  
> bye

![499a557542fb436b8116023d8abb4971.png][]

同样，我们输入1，代码也会显示bye并跳出循环返回命令行。

这是因为$f 和1在进行的是数值比较。

## 七、如果没有设置退出值（菜单项），如何退出循环？ ##

在上面的实例1和实例2中，我们都设置了exit值给用户选择退出循环，如果我们在脚本中没有作这方面的设置，用户该如何退出循环呢？

有两种方法：

> Ctrl+C

或

> Ctrl+D

### （一）以bash中用Ctrl+C ###

> \# csdn @ edu in ~ \[23:26:14\]  
> $ exec bash  
> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do; if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> bash: syntax error near unexpected token \`;'  
> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple  
> 2) banana  
> 3) orrange  
> 4) peach  
> 5) exit  
> \#? 3  
> You have selected orrange  
> \#? ^C

![256ff7a3adea454e9bcc59863bc2b0b7.png][]

按Ctrl+C退出，屏幕上会显示^C。

### （二）以bash中用Ctrl+D ###

> \[csdn ~\]$ echo "What is your favourite fruit?"; select f in "apple" "banana" "orrange" "peach" "exit"; do if \[\[ $f == "exit" \]\]; then echo "bye"; break; else echo "You have selected $f"; fi; done  
> What is your favourite fruit?  
> 1) apple  
> 2) banana  
> 3) orrange  
> 4) peach  
> 5) exit  
> \#? 1  
> You have selected apple  
> \#?  
> \[csdn ~\]$

![bf5949cc439b47af8edf4c56b5480e92.png][]

按Ctrl+D退出，屏幕上不会显示出来，更美观一些。

## 八、与 case...esac语句配合使用 ##

当我们需要对多个数值分别做不同的处置时，用if语句太麻烦了，我们可以使用case...esca语句配合使用。

我们将实例1修改如下：

a=(apple banana orrange peach)
    echo "What is your favourite fruit?"
    select f in ${a[*]}
    do
       case $f in
         apple) echo 1 apple;;
         banana) echo 2 banana;;
         orrange) echo 3 orrange;;
         peach) echo 4 peach;;
         *) echo bye
            break;;
       esac
    done

使用case...esca语句来对用户选择的fruit进行分别处理，如果用户输入的值不在数组a中，将显示bye并退出循环。

### （一）在zsh中顺利执行 ###

> \# csdn @ edu in ~ \[22:37:41\]  
> $ a=(apple banana orrange peach);echo "What is your favourite fruit?"; select f in $a; do; case $f in; apple) echo 1 apple;; banana) echo 2 banana;; orrange)  
> echo 3 orrange;;peach) echo 4 peach;; \*)echo bye; break; esac; done  
> What is your favourite fruit?  
> 1) apple 2) banana 3) orrange 4) peach  
> ?\# 1  
> 1 apple  
> ?\# 4  
> 4 peach  
> ?\# 5  
> bye
> 
> \# csdn @ edu in ~ \[22:37:53\]  
> $

![1bb25585b14f4181aea234d352f91380.png][]

### （二）在bash中 ###

#### 1.命令行执行出错 ####

> \[csdn ~\]$ a=(apple banana orrange peach);echo "What is your favourite fruit?"; select f in $a; do; case $f in; apple) echo 1 apple;; banana) echo 2 banana;; orrange) echo 3 orrange;;peach) echo 4 peach;; \*)echo bye; break; esac; done  
> bash: syntax error near unexpected token \`;'  
> \[csdn ~\]$ a=(apple banana orrange peach);echo "What is your favourite fruit?"; select f in $a; do case $f in; apple) echo 1 apple;; banana) echo 2 banana;; orrange) echo 3 orrange;;peach) echo 4 peach;; \*)echo bye; break; esac; done  
> bash: syntax error near unexpected token \`;'  
> \[csdn ~\]$

![c55d566a3e9f47a0af2475e7dfab9575.png][]

bash对;的处理

#### 2.创建脚本文件来测试 ####

##### （1）创建脚本文件a.sh #####

> \[csdn ~\]$ echo 'a=(apple banana orrange peach)' > a.sh  
> \[csdn ~\]$ echo 'echo "What is your favourite fruit?"' >> a.sh  
> \[csdn ~\]$ echo 'select f in $\{a\[\*\]\}' >> a.sh  
> \[csdn ~\]$ echo do >> a.sh  
> \[csdn ~\]$ echo 'case $f in' >> a.sh  
> \[csdn ~\]$ echo 'apple) echo 1 apple;;' >> a.sh  
> \[csdn ~\]$ echo 'banana) echo 2 banana;;' >> a.sh  
> \[csdn ~\]$ echo 'orrange) echo 3 orrange;;' >> a.sh  
> \[csdn ~\]$ echo 'peach) echo 4 peach;;' >> a.sh  
> \[csdn ~\]$ echo '\*) echo bye' >> a.sh  
> \[csdn ~\]$ echo 'break;;' >> a.sh  
> \[csdn ~\]$ echo esac >> a.sh  
> \[csdn ~\]$ echo done >> a.sh  
> \[csdn ~\]$

\\![8856fb5297c24369b4fdbb21248724da.png][]

##### （2）查看脚本文件a.sh的内容 #####

> \[csdn ~\]$ cat a.sh  
> a=(apple banana orrange peach)  
> echo "What is your favourite fruit?"  
> select f in $\{a\[\*\]\}  
> do  
> case $f in  
> apple) echo 1 apple;;  
> banana) echo 2 banana;;  
> orrange) echo 3 orrange;;  
> peach) echo 4 peach;;  
> \*) echo bye  
> break;;  
> esac  
> done  
> \[csdn ~\]$

![e3407eff0e1f46b59dfc47d2778db7bf.png][]

##### (3)执行脚本文件a.sh #####

> \[csdn ~\]$ . a.sh  
> What is your favourite fruit?  
> 1) apple  
> 2) banana  
> 3) orrange  
> 4) peach  
> \#? 1  
> 1 apple  
> \#? 3  
> 3 orrange  
> \#? 5  
> bye  
> \[csdn ~\]$

![48fa1c5d72be4b6cb643f6ed736575ef.png][]

> **注意：**
> 
> 对于数组a=(apple banana orrange peach)'
> 
>  *  在zsh中，只需要使用$a 就可以获得所有元素的值
>  *  在bash中，使用$a 只能获得第1个元素的值，要获得所有元素的值，需要使用$\{a\[\*\]\}或$\{a\[@\]\}

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