·語法：for 變量名 in 條件; do …; done

案例1

計算1到100數字的和

#!/bin/bash
sum=0
for i in `seq 1 100`
do
    sum=$[$sum+$i]                ##核心語句
    echo $i
done
echo $sum

案例2

列出/etc/下的目錄或子目錄

#!/bin/bash
cd /etc/
for a in `ls /etc/`
do
    if [ -d $a ]
    then
       ls -d $a
    fi
done

ls 是用空格或回車作為分隔符，for循環以他對象，那麽可能會出錯

20.11/20.12 while循環

·語法 while 條件; do … ; done

案例1

#!/bin/bash
while :                                ## : 表示死循環 或者寫成 while true
do
 load=`w|head -1|awk -F 'load average: ' '{print $2}'|cut -d. -f1`
 if [ $load -gt 10 ]
 then
   top|mail -s "load is high: $load" [email protected]
##或/usr/local/sbin/mail.py [email protected] "load high" "$load"
 fi
 sleep 30
done

案例2

#!/bin/bash
while :
do
  read -p "Please input a number: " n
  if [ -z "$n" ]
  then
    echo "you need input sth."
    continue                            ## continue 指繼續重新再來一遍循環
  fi
  n1=`echo $n|sed 's/[0-9]//g'`
  if [ -n "$n1" ]
  then
    echo "you just only input numbers."
    continue                            ## continue 從頭繼續重新再來一遍循環
  fi
  break                                ##退出 while 循環
done
echo $n

20.13 break跳出循環

#!/bin/bash
for i in `seq 1 5`
do
 echo $i
 if [ $i == 3 ]                ##正常等於號應該寫為 -eq，字符串等於為 ==
 then
   break
 fi
 echo $i
done
echo aaaaaaa

[root@localhost shell]# sh -x break.sh
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1
1
+ '[' 1 == 3 ']'
+ echo 1
1
+ for i in '`seq 1 5`'
+ echo 2
2
+ '[' 2 == 3 ']'
+ echo 2
2
+ for i in '`seq 1 5`'
+ echo 3
3
+ '[' 3 == 3 ']'
+ break
+ echo aaaaaaa
aaaaaaa

原本在 i 為 3 之前，腳本一直執行到done，然後在從頭循環，而 為 3 時，之間break跳出循環，

echo aaaaaaa

·break 用在循環語句裏，for 也行，while 也行

20.14 continue結束本次循環

·忽略continue之下的代碼，直接進行下一次循環

#!/bin/bash
for i in `seq 1 5`
do
 echo $i
 if [ $i == 3 ]
 then
   continue
 fi
 echo $i
done
echo $i

[root@localhost shell]# sh -x continue.sh
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1
1
+ '[' 1 == 3 ']'
+ echo 1
1
+ for i in '`seq 1 5`'
+ echo 2
2
+ '[' 2 == 3 ']'
+ echo 2
2
+ for i in '`seq 1 5`'
+ echo 3
3
+ '[' 3 == 3 ']'                ## 等於3時，直接結束了本次循環，從頭開始
+ continue
+ for i in '`seq 1 5`'
+ echo 4
4
+ '[' 4 == 3 ']'
+ echo 4
4
+ for i in '`seq 1 5`'
+ echo 5
5
+ '[' 5 == 3 ']'
+ echo 5
5
+ echo aaaaaaa
aaaaaaa

·break會跳出循環，不再執行循環，而continue會結束本次循環，從頭開始執行循環

20.15 exit退出整個腳本

exit直接退出腳本：

#!/bin/bash
for i in `seq 1 5`
do
  echo $i
  if [ $i == 3 ]
  then
    exit
  fi
  echo $i
done
echo aaaaa

[root@localhost shell]# sh -x exit.sh
++ seq 1 5
+ for i in '`seq 1 5`'
+ echo 1
1
+ '[' 1 == 3 ']'
+ echo 1
1
+ for i in '`seq 1 5`'
+ echo 2
2
+ '[' 2 == 3 ']'
+ echo 2
2
+ for i in '`seq 1 5`'
+ echo 3
3
+ '[' 3 == 3 ']'
+ exit

2018-4-19 17周2次課 for循環、while循環、break、continue、exit