为什么很多配置都的undef 和 0 呢？
speed 0 baud; rows 0; columns 0; line = 0;
intr =
eol =
susp =
flush =
-parenb -parodd cs8 -hupcl -cstopb cread -clocal -crtscts -cdtrdsr
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr -icrnl -ixon -ixoff
-iuclc -ixany -imaxbel -iutf8
-opost -olcuc -ocrnl -onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
-isig -icanon -iexten -echo -echoe -echok -echonl -noflsh -xcase -tostop -echoprt
-echoctl -echoke
man stty，里面写的很清楚。
PS. 话说OSX的man page里说的更清楚：
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/stty.1.html