system调用虽然用了exec,但是fd, signal这些还是会保留父进程的,be careful
lthough system uses "fork" & "exec" function to replace the whole process address space, but in child process, the FDs will still be inherited from parent process. About whether signals are inheritted, I'm not sure but seems it's very possible.
So be careful with "system" function.
So be careful with "system" function.