if的基本语法if [ command ];then符合该条件执行的语句elif [ command ];then符合该条件执行的语句else符合该条件执行的语句fiOperatorDescriptionSymbol! EXPRESSIONThe EXPRESSION is false.-n STRINGThe length of STRING is greater than zero. 非空即是真-z STRINGThe lengh of STRING is zero (ie it is empty). 空即是真STRING1 = STRING2STRING1 is equal to STRING2STRING1 != STRING2