Do you notice there is a "&" before M_SOURCE_FULLPATH.
&M_SOURCE_FULLPATH. is a macro variables. It is also called a symbolic variable.

Its value will be resovled when it gets compiled.

You may define with %let statement.

For example,

%let M_SOURCE_FULLPATH=c:\mysas\project;

thank you for your answer!!
但是，"PIPE"在这有什么作用呀？

PIPE起管道作用，指将dir 的结果输送给FLIST文件，/B 只输出文件名，不输出文件位置，生成时间等信息，这样便于抓取同一目录下的所有文件名。

知道了，非常感谢

把sas和dos连起来的管道，大概这个意思吧