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

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"
doif [[ $f == "exit" ]]; thenecho "bye"breakelseecho "You have selected $f"fidone

（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]

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 $f"; fi; done
What is your favourite fruit?
1) apple
2) banana
3) orrange
4) peach
5) exit
#? 1
You have selected apple
#? 3
You have selected orrange
#? 5
bye
[csdn ~]$

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]
$

（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 $f"; fi; done
What is your favourite fruit?
1) apple
2) banana
3) orrange
4) peach
5) exit
#? b
You have selected
#? 9
You have selected
#? 3
You have selected orrange
#? 5
bye
[csdn ~]$

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]
$

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

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：

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
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]
$

（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]
$

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

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

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 $f"; fi; done
What is your favourite fruit?
1) apple
2) banana
3) orrange
4) peach
5) exit
#? 1
You have selected apple
#?
[csdn ~]$

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[*]}
docase $f inapple) echo 1 apple;;banana) echo 2 banana;;orrange) echo 3 orrange;;peach) echo 4 peach;;*) echo byebreak;;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]
$

（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 $f in; apple) echo 1 apple;; banana) echo 2 banana;; orrange) echo 3 orrange;;peach) echo 4 peach;; *)echo bye; break; esac; done
bash: syntax error near unexpected token `;'
[csdn ~]$

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 ~]$

（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 $f in
apple) echo 1 apple;;
banana) echo 2 banana;;
orrange) echo 3 orrange;;
peach) echo 4 peach;;
*) echo bye
break;;
esac
done
[csdn ~]$