#!/bin/bash

 #shell使用的熟练成都反映用户对Unix/Linux使用的熟练程度

 #shell 有两种执行命令的方式:交互式和批处理

 #常见的shell脚本解释器有bash,sh,csh等等

 #chmod +x ./hello.sh  #使脚本具有执行权限

 #./hello.sh  #执行脚本

 #交互输入

 echo "whats your name?"

 read person

 echo "hello,$person"

 #变量 定义不加$，引用要加$

 myname="shiqing"

 echo ${myname}

 #特殊变量

 echo "filename:$0"

 echo "first parameter:$1"

 echo "second parameter:$2"

 echo "all parameter:$@"

 echo "all parameter:$*"

 echo "total number of parameters:$#"

 #命令替换 ～下 `command`

 USERS=`who | wc -l`

 echo "Logged in user are $USERS“

　　#返回这个脚本文件放置的目录

　　cd `dirname $0`

 　　echo `pwd`

 #表达式,原生bash不支持简单的数学运算，expr 是一种表达式计算工具

 val=`expr  + `

 echo "value is $val"

 a=

 b=

 val=`expr $a \* $b`

 echo "a * b: $val"

 #字符串

 str="abcd"

 echo ${#str}

 echo ${str::}

 #数组

 arr=(a,b,c,d)

 echo "first :${arr[0]}"

 echo "all:${arr[*]}" #获取所有元素

 echo "length:${#arr[*]}" #获取数组长度

 #if...elif...else...，条件表达式要放在方括号之间，并且要有空格

 a=

 b=

 if [ $a == $b ]

 then

   echo "a is equal to b"

 elif [ $a -gt $b ]

 then

   echo "a is g reater than b"

 else

   echo "a is less than b"

 fi

 #case ...easc...

 echo 'Input a number between 1 to 4'

 echo 'Your number is:\c'

 read aNum

 case $aNum in

     )  echo 'You select 1'

     ;;

     )  echo 'You select 2'

     ;;

     )  echo 'You select 3'

     ;;

     )  echo 'You select 4'

     ;;

     *)  echo 'You do not select a number between 1 to 4'

     ;;

 esac

 #for...in...

 for loop in

 do

     echo "The value is: $loop"

 done

 #while...do...done...

 echo 'type <CTRL-D> to terminate'

 i=

 while read FILM

 do

     echo "Yeah! great film the $FILM"

     i=`expr $i + `

     b=

     if [ $i-gt$b ] #expression 和方括号([ ])之间必须有空格，否则会有语法错误。

       then

         break

     fi

 done

 #until...do...done

 a=

 until [ ! $a -lt  ]

 do

    echo $a

    a=`expr $a + `

    val=`expr $a + $b`

 done

 #函数

 funWithParam(){

     echo "The value of the first parameter is $1 !"

     echo "The value of the second parameter is $2 !"

     echo "The value of the tenth parameter is $10 !"

     echo "The value of the tenth parameter is ${10} !"

     echo "The value of the eleventh parameter is ${11} !"

     echo "The amount of the parameters is $# !"  # 参数个数

     echo "The string of the parameters is $* !"  # 传递给函数的所有参数

 }

 funWithParam           

 funWithReturn(){

     echo "The function is to get the sum of two numbers..."

     echo -n "Input first number: "

     read aNum

     echo -n "Input another number: "

     read anotherNum

     echo "The two numbers are $aNum and $anotherNum !"

     return $(($aNum+$anotherNum))

 }

 funWithReturn

 ret=$? #函数返回值在执行函数后通过$?获得

 echo "The sum of two numbers is $ret !"

 #输入输出重定向 >,>>(追加), <

 who>users

 wc -l<users