本節主要內容

while expression
do
command
command
done

（1）計數器格式

適用於迴圈次數已知或固定時

root@sparkslave02:~/ShellLearning/Chapter13# vim whileLoop.sh
.#!/bin/bash
i=1
while(($i<5))
do
echo $i
let i++
done

root@sparkslave02:~/ShellLearning/Chapter13# chmod a+x whileLoop.sh 
root@sparkslave02:~/ShellLearning/Chapter13
 
# ./whileLoop.sh 
1
2
3
4

（2）標誌符結束while迴圈

[email protected]:~/ShellLearning/Chapter13# vim flagWhileLoop.sh
#!/bin/bash
echo "Please input the num (1~~10): "
#接受使用者輸入
read num
while [[ $num != 4 ]]
do
 #if語句，後面詳細介紹，這裡判斷是否小於4
 if [ $num -lt 4 ]
 then
    echo "Too small ,Try again.."
    read num
 #判斷是否大於4
 

 elif [ $num -gt 4 ]
 then
    echo "Too big ,Try again.. "
    read num
 else
    exit 0
 fi
done
echo "Yes ,you are right !!"

[email protected]:~/ShellLearning/Chapter13# chmod a+x flagWhileLoop.sh [email protected]:~/ShellLearning/Chapter13# ./flagWhileLoop.sh 
Please input the num (1~~10): 
4
 

Yes ,you are right !!

if  條件
then
 Command
else
 Command
#if條件判斷的結束，用反拼表示
fi                           

最常用的判斷為：判斷字串、判斷數字、判斷檔案及邏輯判斷等

（1）判斷字串

1.if [ str1=str2 ];then fi  ----當兩個字串相同時返回真
2.if [ str1!=str2 ];then fi ----當兩個字串不相等時返回真
3.if [ -n str1 ];then fi    ----當字串的長度大於0時返回真 (判斷變數是否有值)
4.if [ -z str1 ];then fi    ----當字串的長度為0時返回真

root@sparkslave02:~/ShellLearning/Chapter13# vim if01.sh
str1="hello"
str2="hell"
#判斷兩字串是否相等
if [ str1=str2 ]
then
  echo "equal"
fi

#判斷兩字串是否不等
if [ str1!=str2 ]
then
  echo "not equal"
fi

#判斷字串長度是否大於0
if [ -n str1 ]
then
  echo "the length of str1 is not zero"
fi

#判斷字串長度是否等於0
if [ -z str1 ]
then
  echo "the length of str1 is not zero, it can't be executed"
fi

root@sparkslave02:~/ShellLearning/Chapter13# ./if01.sh 
not equal
the length of str1 is not zero

（2）判斷數字

1.int1 -eq int2    --相等
2.int1 -ne int2    --不相等
3.int1 -gt int2    --大於
4.int1 -ge int2    --大於等於
5.int1 -lt int2    --小於
6.int1 -le int2    --小於等於

使用示例：

root@sparkslave02:~/ShellLearning/Chapter13# vim if02.sh
i=2
j=3
if [ $i -lt $j ]
then
   echo "i is less than j"
fi

if [ $j -gt $i ]
then
   echo "j is great than i"
fi

root@sparkslave02:~/ShellLearning/Chapter13# chmod a+x if02.sh 

root@sparkslave02:~/ShellLearning/Chapter13# ./if02.sh 
i is less than j
j is great than i

（3）判斷檔案

檔案判斷常用命令如下：

1. -r file        --使用者可讀為真
2. -w file        --使用者可寫為真
3. -x file        --使用者可執行為真
4. -f file        --檔案存在且為正規檔案為真
5. -d file        --如果是存在目錄為真
6. -c file        --檔案存在且為字元裝置檔案
7. -b file        --檔案存在且為塊裝置檔案
8. -s file        --檔案大小為非0為真，可以判斷檔案是否為空
9. -e file        --如果檔案存在為真

使用示例：

root@sparkslave02:~/ShellLearning/Chapter13# vim if03.sh
root@sparkslave02:~/ShellLearning/Chapter13# chmod a+x if03.sh 
#判斷檔案是否存在
if [ -f if03.sh ]
then
   echo "if03.sh exists!!"
fi
#判斷目錄是否存在
if [ -d ../Chapter13 ]
then
   echo "directory Chapter13 exists!!"
fi

root@sparkslave02:~/ShellLearning/Chapter13# ./if03.sh 
if03.sh exists!!
directory Chapter13 exists!!

（4）邏輯判斷

邏輯判斷主要有下面三個命令

1. -a     --與
2. -o     --或
3. !      --非

使用示例：

root@sparkslave02:~/ShellLearning/Chapter13# vim if04.sh
#判斷if04.sh檔案與目錄Chapter13是否同時存在，同時存在則為真
if [ -f if04.sh -a -d ../Chapter13 ]
then
   echo "file if04.sh and directory Chapter13  both exists!!!"
fi

root@sparkslave02:~/ShellLearning/Chapter13# chmod a+x if04.sh 
root@sparkslave02:~/ShellLearning/Chapter13# ./if04.sh 
file if04.sh and directory Chapter13  both exists!!!

（5）if [] then else fi的用法

前面給出的例子都是if [] then fi的形式，這裡再給出if [] then else fi的用法

root@sparkslave02:~/ShellLearning/Chapter13# vim if05.sh
i=7
if [ $i -lt 6 ]
then
  echo "i is less than 6"
else
  echo "i is great than or equal  6"
fi

root@sparkslave02:~/ShellLearning/Chapter13# chmod a+x if05.sh 
root@sparkslave02:~/ShellLearning/Chapter13# ./if05.sh 
i is great than or equal  6

（6）if [] then elif then else fi的用法

多種判斷，看示例程式碼就能明白：

root@sparkslave02:~/ShellLearning/Chapter13# cp if05.sh if06.sh
root@sparkslave02:~/ShellLearning/Chapter13# vim if06.sh
i=7
if [ $i -le 6 ]
then
  echo "i is less than 6"
elif [ $i -eq 7 ]
then
  echo "i equal 7"
else
  echo "i is great than 7"
fi

root@sparkslave02:~/ShellLearning/Chapter13# ./if06.sh 
i equal 7

## 3. until迴圈控制結構##

語法格式：

until condition
  do
     command
 done

使用示例：

root@sparkslave02:~/ShellLearning/Chapter13# vim until01.sh
i=0
until [ $i -gt 2 ]  
 do
 let i+=1
 echo "i=$i"  
done

root@sparkslave02:~/ShellLearning/Chapter13# chmod a+x until01.sh 
root@sparkslave02:~/ShellLearning/Chapter13# ./until01.sh 
i=1
i=2
i=3