一、IFS引入

        IFS(Internal Field Seprator)，即内部域分隔符，完全定义是“The shell uses the value stored in IFS, which is the space, tab, and newline characters by default, to delimit words for the read and set commands, when parsing output from command substitution, and when performing variable substituioin.”。        IFS(Internal Field Seprator)，即内部域分