str='this is a string'单引号字符串的限制：
your_name='qinjx' str="Hello, I know your are \"$your_name\"! \n"双引号的优点：
your_name="qinjx"
greeting="hello, "$your_name" !"
greeting_1="hello, ${your_name} !"
echo $greeting $greeting_1获取字符串长度string="abcd"
echo ${#string} #输出 4提取子字符串string="alibaba is a great company"
echo ${string:1:4} #输出liba查找子字符串string="alibaba is a great company" echo `expr index "$string" is`
原文地址：http://blog.csdn.net/wu20093346/article/details/47188209
