• 语法：for 变量名 in 条件; do …; done
• 案列1，算出1到10的数字相加等于多少并打印出过程

  [root@akuilinux01 shell]# cat for1.sh 
  #!/bin/bash
  sum=0
  for i in `seq 1 10`
  do
  sum=$[$sum+$i]
  echo "$sum + $i"
  done
  echo $sum
  [root@akuilinux01 shell]# sh -x for1.sh 
  + sum=0
  ++ seq 1 10
  + for i in ‘`seq 1 10`‘
  + sum=1
  + echo ‘1 + 1‘
  1 + 1
  + for i in ‘`seq 1 10`‘
  + sum=3
  + echo ‘3 + 2‘
  3 + 2
  + for i in ‘`seq 1 10`‘
  + sum=6
  + echo ‘6 + 3‘
  6 + 3
  + for i in ‘`seq 1 10`‘
  + sum=10
  + echo ‘10 + 4‘
  10 + 4
  + for i in ‘`seq 1 10`‘
  + sum=15
  + echo ‘15 + 5‘
  15 + 5
  + for i in ‘`seq 1 10`‘
  + sum=21
  + echo ‘21 + 6‘
  21 + 6
  + for i in ‘`seq 1 10`‘
  + sum=28
  + echo ‘28 + 7‘
  28 + 7
  + for i in ‘`seq 1 10`‘
  + sum=36
  + echo ‘36 + 8‘
  36 + 8
  + for i in ‘`seq 1 10`‘
  + sum=45
  + echo ‘45 + 9‘
  45 + 9
  + for i in ‘`seq 1 10`‘
  + sum=55
  + echo ‘55 + 10‘
  55 + 10
  + echo 55
  55

• 文件列表循环，列出/root/shell/目录下子目录中的所有文件

  [root@akuilinux01 shell]# cat for2.sh 
  #!/bin/bash
  cd /root/shell/
  for a in `ls /root/shell`
  do
  if [ -d $a ]
  then 
    ls $a
  fi
  done
  [root@akuilinux01 shell]# sh -x for2.sh 
  + cd /root/shell/
  ++ ls /root/shell
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d 1 ‘]‘
  + ls 1
  1.txt  2.txt  3.txt
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d 1.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d 2 ‘]‘
  + ls 2
  a.txt  b.txt  c.txt
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d 3 ‘]‘
  + ls 3
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d 4 ‘]‘
  + ls 4
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d case.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d file1.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d file2.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d for1.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d for2.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d if1.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d if2.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d if3.sh ‘]‘
  + for a in ‘`ls /root/shell`‘
  + ‘[‘ -d if4.sh ‘]‘
  [root@akuilinux01 shell]# sh for2.sh 
  1.txt  2.txt  3.txt
  a.txt  b.txt  c.txt

• for循环是以空格和回车作为分割符，所以不支持列出带有空格的文件名的文件

  while循环

• 语法 while 条件; do … ; done
• 案列：当系统的负载大于10的时候就发一封邮件

  [root@akuilinux01 shell]# cat while1.sh 
  #!/bin/bash
  #死循环,:是一个死循环，也可以写成1或者true
  while : 
  do
  #截取负载的值并赋给变量load
  load=`w|head -1|awk -F ‘load average: ‘ ‘{print $2}‘|cut -d. -f1`
  #如果负载大于10就报警并发送邮件，设置30秒查一次  
  if [ $load -gt 10 ]
  then
   /usr/lib/zabbix/alertscripts/mail.py xxxxx@qq.com "load is high:$load"
  fi
  sleep 30
  done
  [root@akuilinux01 shell]# sh -x while1.sh 
  + :
  ++ head -1
  ++ cut -d. -f1
  ++ w
  ++ awk -F ‘load average: ‘ ‘{print $2}‘
  + load=0
  + ‘[‘ 0 -gt 10 ‘]‘
  + sleep 30
  ^C

• 截取负载的值

  [root@akuilinux01 shell]# uptime|awk -F ‘load average: ‘ ‘{print $2}‘
  0.01, 0.03, 0.05
  [root@akuilinux01 shell]# uptime|awk -F ‘load average: ‘ ‘{print $2}‘|cut -d. -f1
  0
  #w|head -1也可以写成uptime
  [root@akuilinux01 shell]# w |head -1
  01:02:36 up 58 min,  1 user,  load average: 0.07, 0.04, 0.05
  [root@akuilinux01 shell]# uptime
  01:02:46 up 59 min,  1 user,  load average: 0.06, 0.04, 0.05

• 案列2：数字输入交互

  [root@akuilinux01 shell]# cat while2.sh 
  #!/bin/bash
  while :
  do
  read -p "Please input a number: " n
  #如果你什么也没输入
  if [ -z "$n" ]
  then
      echo "你需要输入一个数字"
  #从头开始循环
      continue
  fi
  #过滤数字并赋予n1
  n1=`echo $n|sed ‘s/[0-9]//g‘`
  #如果n1不为空，提示
  if [ -n "$n1" ]
  then
      echo "你只能输入一个纯数字"
      continue
  fi
  #退出循环
  break
  done
  echo $n
  [root@akuilinux01 shell]# sh while2.sh 
  Please input a number: 
  你需要输入一个数字
  Please input a number: a
  你只能输入一个纯数字
  Please input a number: 2
  2

  break跳出循环

• 案列：当i等于3时，跳出循环并打印aaaa

  [root@akuilinux01 shell]# cat break.sh 
  #!/bin/bash
  for i in `seq 1 5`
  do
  echo $i
  if [ $i -eq 3 ]
   then
       break
  fi
  echo $i
  done
  echo aaaa
  [root@akuilinux01 shell]# sh -x break.sh 
  ++ seq 1 5
  + for i in ‘`seq 1 5`‘
  + echo 1
  1
  + ‘[‘ 1 -eq 3 ‘]‘
  + echo 1
  1
  + for i in ‘`seq 1 5`‘
  + echo 2
  2
  + ‘[‘ 2 -eq 3 ‘]‘
  + echo 2
  2
  + for i in ‘`seq 1 5`‘
  + echo 3
  3
  + ‘[‘ 3 -eq 3 ‘]‘
  + break
  + echo aaaa
  aaaa

  continue结束本次循环

• 忽略continue之下的代码，直接进行下一次循环
• 案列：当i不等于3的时候打印出i，当i等于3的时候退出本次循环不打印

  [root@akuilinux01 shell]# cat continue.sh 
  #!/bin/bash
  for i in `seq 1 5`
  do
  if [ $i -eq 3 ]
   then
       continue
  fi
  echo $i
  done
  echo aaaa
  [root@akuilinux01 shell]# sh -x continue.sh 
  ++ seq 1 5
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 1 -eq 3 ‘]‘
  + echo 1
  1
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 2 -eq 3 ‘]‘
  + echo 2
  2
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 3 -eq 3 ‘]‘
  + continue
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 4 -eq 3 ‘]‘
  + echo 4
  4
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 5 -eq 3 ‘]‘
  + echo 5
  5
  + echo aaaa
  aaaa

  exit直接退出脚本

• 案列：当i不等于3的时候打印出i，当i等于3的时候直接退出脚本

  [root@akuilinux01 shell]# cat exit.sh 
  #!/bin/bash
  for i in `seq 1 5`
  do
  if [ $i -eq 3 ]
   then
       exit
  fi
  echo $i
  done
  echo aaaa
  [root@akuilinux01 shell]# sh -x exit.sh 
  ++ seq 1 5
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 1 -eq 3 ‘]‘
  + echo 1
  1
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 2 -eq 3 ‘]‘
  + echo 2
  2
  + for i in ‘`seq 1 5`‘
  + ‘[‘ 3 -eq 3 ‘]‘
  + exit
  [root@akuilinux01 shell]# sh exit.sh 
  1
  2

  扩展

• select用法

for循环，while循环，break跳出循环，continue结束本次循环，exit直接退出脚本

原文：http://blog.51cto.com/akui2521/2141965