Bash Shell | 如何使用條件式指令 select Command

select 命令的語法如下

select name [in words …]; do commands; done

select 命令可以用來產生選單，底下是一個能夠自動產生兩個數字相加的練習程式，數字由 $RANDOM 產生，使用 $PS3 可以設定 select 命令的提示訊息，使用者選擇後 name 會是被選到之選項的值，而 $REPLY 是被選到之選項的代號，當使用者輸入超過選項的代號時，name 會是空字串。

範例：

#!/bin/sh

value1=$((RANDOM%100))
value2=$((RANDOM%100))

equation="$value1 + $value2"

# PS3 is the prompt message
PS3="What is the answer of ( $equation )? "

ans=$(($equation))

order=$((RANDOM%4))

# ans_list contains the items within the menu
for ((i=0; i<4; i++))
do
        if [ "$i" -eq "$order" ]; then
                ans_list="$ans_list $ans"
        else
                ans_list="$ans_list $((RANDOM%200))"
        fi
done

select answer in $ans_list;
do
        # answer will be empty if the selection is out of range.
        if [ -n "$answer" ]; then
                echo You selected the answer $REPLY\).
                if [ "$(($equation))" -eq "$answer" ]; then
                        echo Great, \"$answer\" is correct.
                        break;
                fi
                echo \"$answer\" is incorrect.
                echo Calculate again.
        else
                echo NOTICE: Please select between 1 to 4.
        fi
done

結果：

1) 121

2) 106

3) 17

4) 128

What is the answer of ( 7 + 10 )? 1

You selected the answer 1).

"121" is incorrect.

Calculate again.

What is the answer of ( 7 + 10 )? 3

You selected the answer 3).

Great, "17" is correct.