系统变量：
/etc/profile:  会话时读取一次
/etc/bashrc：  每次打开终端读取
/etc/profile.d：为不同版本shell特殊用法说明

用户变量
.bash_profile:  会话时读取一次
.bashrc:    每次打开终端读取
.bash_history: 记录用户以前输入的命令
.bash_logout:  用户退出是要执行的命令

用户变量
定义：me
赋值：me=jingzhao
显示：echo $me/read me   
删除：unset me
只读：readonly me
转化成环境变量：export me

显示环境变量：env/export/set

判断语句：

第一种方式：[]/test/&&/||
[]:  进行数据判断
判断条件：
1.每个组件需要用空格来分隔；
2.变量用双引号来设置；
3.常量用单引号或者双引号来设置
例子：
[ -z "$HOME" ]
[ "$HOME" == "$MAIL" ]

test: 进行数据判断
例子：test$HOME=$MAIL 

&&:  前面的命令执行结果为正确执行后面的命令；否则就略过；
例子：ls /tmp && touch /tmp/testingagain

||:  前面的命令执行结果为错误执行后面的命令；否则就略过
例子：ls /tmp && touch /tmp/testingagain

&&和||一起用：command1&&command2||command3
例子：ls /tmp/vbirding && echo "exist"||echo "not exist"

复合使用例子：
#!/bin/bash
#Program
#    this program will show the use's choice
#History:
#2010/12/24 Tom first release
PATH=/bin;/sbin;/usr/bin;/usr/sbin;/usr/local/bin;/usr/local/sbin;~/bin
export PATH

read -p "Please input (Y/N): "yn
[ "$yn" == "Y" -o "$yn" == "y" ]&& echo "OK, continue" && exit 0
[ "$yn" == "N" -o "$yn" == "n" ]&& echo "Oh, interrupt!" && exit 0
echo "I don't know what is your choise" && exit 0

第二种方式：if...then
结构一：
if[条件判断表达式]; then
  当判断判断表达式，执行语句
fi

例子：
#!/bin/bash
#Program
#    this program will show the use's choice
#History:
#2010/12/24 Tom first release
PATH=/bin;/sbin;/usr/bin;/usr/sbin;/usr/local/bin;/usr/local/sbin;~/bin
export PATH

read -p "Please input (Y/N): "yn
if [ "$yn" == "Y" ]||[ "$yn" == "y" ]; then
  echo "OK, continue"
  exit 0
fi
if [ "$yn" == "N" ]||[ "$yn" == "n" ]; then
  echo "Oh, interrupt!"
  exit 0
fi
echo "I don't know what is your choise" && exit 0

结构二：
if[条件判断表达式1]; then
  当判断表达式1成立，执行语句
elif[条件判断表达式2]; then
  当判断表达式2成立，执行语句
else
  当判断表达式1，2均不成立，执行语句
fi

例子：
#!/bin/bash
#Program
#    this program will show the use's choice
#History:
#2010/12/24 Tom first release
PATH=/bin;/sbin;/usr/bin;/usr/sbin;/usr/local/bin;/usr/local/sbin;~/bin
export PATH

read -p "Please input (Y/N): "yn
if [ "$yn" == "Y" ]||[ "$yn" == "y" ]; then
  echo "OK, continue"
elif [ "$yn" == "N" ]||[ "$yn" == "n" ]; then
  echo "Oh, interrupt!"
else
  echo "I don't know what is your choise"
fi

第三种方式：case...esac
结构：
case $变量名称 in
   "第一个变量内容")
      符合第一个变量情况下程序段
      ;;
   "第二个变量内容")
      符合第二个变量情况下程序段
      ;;
    *)
      第一，二个变量内容均不符合情况下程序段
      exit 1
      ;;
esac

例子：
#!/bin/bash
#Program
#    this program will show the use's choice
#History:
#2010/12/24 Tom first release
PATH=/bin;/sbin;/usr/bin;/usr/sbin;/usr/local/bin;/usr/local/sbin;~/bin
export PATH

echo "This program will show the user's choice!"
#read -p "Input your choice:" choice
#case $choice in
case $1 in
  "one")
    echo "Your choice is ONE"
    ;;
  "two")
    echo "Your choice is TWO"
    ;;
  "three")
    echo "Your choice is THREE"
    ;;
  *)
    echo "Usage (one|two|three)"
    ;;
esac

循环语句：
第一种方式：while do done
结构：
while [ condition ]
do
   程序段落
done
说明：当conditions满足执行；否则终止
例子：
#!/bin/bash
#Program
#    Use loop to try find your input
#History:
#2010/12/24 Tom first release
PATH=/bin;/sbin;/usr/bin;/usr/sbin;/usr/local/bin;/usr/local/sbin;~/bin
export PATH
while [ "$yn" != "yes" ]&&[ "$yn" != "YES" ]
do
   read -p "Please input yes/YES to stop this program: " yn
done

第二种方式：until do done
结构：
until [ condition ]
do
   程序段落
done
说明：当conditions不满足执行；否则终止
例子：
#!/bin/bash
#Program
#    Use loop to try find your input
#History:
#2010/12/24 Tom first release
PATH=/bin;/sbin;/usr/bin;/usr/sbin;/usr/local/bin;/usr/local/sbin;~/bin
export PATH
until [ "$yn" == "yes" ]||[ "$yn" == "YES" ]
do
   read -p "Please input yes/YES to stop this program: " yn
done

第三种方式：for...do....done
结构：
for ((初始值；限制值；执行布长))
do
  程序段
done
说明：适用于数值运算
#!/bin/bash
#Program
#    Try do calculate 1+2+3....+100
#History:
#2010/12/24 Tom first release
PATH=/bin;/sbin;/usr/bin;/usr/sbin;/usr/local/bin;/usr/local/sbin;~/bin
export PATH
s=0
for ((i=1;i<=100;i++))
do
  s=$(($s+$i))
done
echo "The result of '1+2+3...+100' is ==> $s"