if 條件判斷式

1. 語法一：if ... then 當符合條件判斷時，就進行某項工作。

   if [ 條件判斷式 ]; then
           當條件判斷式成立時，可以進行的指令工作內容；
   fi
   

2. 範例：輸入大小寫 y 或 n，條件判斷使用 && 做 AND，|| 做 OR 的條件判斷。

   [dywang@dywmac zzz]$ vim sh06.sh
   [dywang@dywmac zzz]$ cat sh06.sh
   #!/bin/bash
   
   read -p "Please input (Y/N): " yn
   
   if [ "$yn" == "Y" ] || [ "$yn" == "y" ]; then
   	echo "OK, continue"
   	exit 0
   fi
   if [ "$yn" == "N" ] || [ "$yn" == "n" ]; then
   	echo "Oh, interrupt!"
   	exit 0
   fi
   echo "I don't know what is your choise" && exit 0
   

3. 執行結果。

   [dywang@dywmac zzz]$ sh sh06.sh 
   Please input (Y/N): y
   OK, continue
   [dywang@dywmac zzz]$ sh sh06.sh 
   Please input (Y/N): n
   Oh, interrupt!
   [dywang@dywmac zzz]$ sh sh06.sh 
   Please input (Y/N): q
   I don't know what is your choise
   

4. 語法二：if .. else ..fi

   if [ 條件判斷式 ]; then
       當條件判斷式成立時，可以進行的指令工作內容；
   else
       當條件判斷式不成立時，可以進行的指令工作內容；
   fi
   

5. 語法三：if .. elif.. else.. fi

   if [ 條件判斷式一 ]; then
       當條件判斷式一成立時，可以進行的指令工作內容；
   elif [ 條件判斷式二 ]; then
       當條件判斷式二成立時，可以進行的指令工作內容；
   else
       當條件判斷式一與二均不成立時，可以進行的指令工作內容；
   fi
   

6. 例題：改寫 sh06.sh 成 sh06-1.sh：

   [dywang@dywmac zzz]$ vim sh06-1.sh
   [dywang@dywmac zzz]$ cat sh06-1.sh
   #!/bin/bash
   
   read -p "Please input (Y/N): " yn
   
   if [ "$yn" == "Y" ] || [ "$yn" == "y" ]; then
   	echo "OK, continue"
   elif [ "$yn" == "N" ] || [ "$yn" == "n" ]; then
   	echo "Oh, interrupt!"
   else
   	echo "I don't know what is your choise"
   fi
   

7. 執行結果。

   [dywang@dywmac zzz]$ sh sh06-1.sh 
   Please input (Y/N): Y
   OK, continue
   [dywang@dywmac zzz]$ sh sh06-1.sh 
   Please input (Y/N): N
   Oh, interrupt!
   [dywang@dywmac zzz]$ sh sh06-1.sh 
   Please input (Y/N): 
   I don't know what is your choise
   

8. 例題：偵測輸入的參數是否為 hello。
   1. 如果是，顯示 "Hello, how are you?"；
   2. 如果沒有加任何參數，提示使用者必須要使用的參數下達法；
   3. 而如果加入的參數不是 hello ，就提醒使用者僅能使用 hello 為參數。

   [dywang@dywmac zzz]$ vim sh07.sh
   [dywang@dywmac zzz]$ cat sh07.sh
   #!/bin/bash
   
   if [ "$1" == "hello" ]; then
   	echo "Hello, how are you?"
   elif [ -z "$1" ]; then
   	echo "You MUST input parameters, ex> $0 someword"
   else
   	echo "The only parameter is 'hello'"
   fi
   

9. 執行結果。

   [dywang@dywmac zzz]$ sh sh07.sh
   You MUST input parameters, ex> sh07.sh someword
   [dywang@dywmac zzz]$ sh sh07.sh someword
   The only parameter is 'hello'
   [dywang@dywmac zzz]$ sh sh07.sh hello
   Hello, how are you?