i=`echo  "a,b,c,d"
echo ${i%, *}
a,b,c
//从最后倒数到最短的匹配部分(用%)后面的逗号的地方被删除

echo ${i%%, *}
a
//从最后倒数到最长的匹配部分(用%)后面的逗号的地方都被删除

echo ${i#*，}
b,c,d
//从开头到最短的匹配部分(用#)后面的逗号的地方被删除

echo ${i##*, }
d
//从开头到最长的匹配部分(用##)后面的逗号的地方被删除

for  变量名  in  取值列表
do
	命令序列
done

for i in {1..10}         {1..10..2}

for i in $(seq 1 10)     $(seq 1 2 10)

for ( (i=1; i<=10; i++) )  ( (i=1; i<=10; i+=2) )
do
   echo  $i
done

for ( (i=1; i<=10; i++)) ;do echo $i ;done　　

#!/bin/bash
#Batch add users

for i in `cat /root/users.txt`
do
  useradd $i &> /dev/null
  echo "123456" |passwd --stdin $i &> /dev/null
done

#!/bin/bash
#Check host status based on IP address

for i in `cat /root/ipadds.txt`
do
  ping -c 3 -i 0.5 -w 2 $i &> /dev/null
  if [ $? -eq 0 ];then
     echo "$i is online"
  else
     echo "$i is offline"
  fi
done

while  [ 条件测试操作 ]
do						#do代表循环的开始
	判断式/命令序列
done 					#done代表循环的结束

#!/bin/bash
#Batch add users

a=1
while [ $a -le 20 ]
do
   useradd stu$a
   echo "123456" | passwd --stdin stu$a &> /dev/null
let a++
done
~         

#!/bin/bash
#The price guessing game

a=0
b=0
price=$(echo $[RANDOM%1000])
while [ $a -eq 0 ]
do
   read -p "请输入你猜的价格：" num
   if [ $price -eq $num ];then
       let b++
       echo "恭喜你猜对了，你一共猜了 $b 次！"
       let a++
   elif [ $num -lt $price ];then
       let b++
       echo "猜低了，再接再厉！"
   else
       let b++
       echo "猜高了，再接再厉！"
   fi
done

until  [ 条件测试操作 ]
do
	判断式/命令序列
done

#!/bin/bash
#Calculate the sum from 1 to 50

a=1
b=0
until [ $a -gt 50 ]
do
   b=$(($b+$a))

   let a++
done
   echo "得出的和为：$b"

#!/bin/bash
#Send an online message for a specified user

username=$1
if [ $# -lt 1 ];then
   echo "Usage:`basename $0` <username> [<message>]"
   exit 1
fi
#判断用户是否存在。如果存在则不进行任何操作
if grep "^$username:" /etc/passwd > /dev/null;then :
else
   echo "$username is not a user on this system"
   exit 2
fi
#判断用户是否在线，若不在线则每3秒联系一次
until who|grep "$username" > /dev/null;do
     echo "$username is not logged on"
     sleep 3
done
#shift去除第一个参数，$*为所有参数的值
shift;msg=$*
#判断$msg是否为空，为空则赋值
[[ "X"$msg == "X" ]] && msg="Are you ready? $username"
#通过write命令给用户发送消息
echo "$msg"| write $username