一个简答的命令解释器（基于Linux开发）
Usage: mydef [FILE]...
Commands start|end: start | end clear all elements
def: def [var]... define some variables
set: set set value to name
undef: undef [var]... delete variables that given
print: print [var]... print the value of variables
add: add var1 = var1 + var2
sub: sub var1 = var1 - var2
mul: mul var1 = var1 * var2
div: div var1 = var1 / var2 Note: var2 can not be 0