clear源码实现：
#include <stdio.h> int clear_main(int argc, char **argv) { /* This prints the clear screen and move cursor to top-left corner control * characters for VT100 terminals. This means it will not work on * non-VT100 compliant terminals, namely Windows' cmd.exe, but should * work on anything unix-y. */ fputs("\x1b[2J\x1b[H", stdout); return 0; }fputs("\x1b[2J\x1b[H", stdout)其实是一串VT100的控制码
\x1b[2 清除整个屏幕，行属性变成单宽单高，光标位置不变
\x1b[H 光标移动到行首
