linux bash 入门

 #!/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