BIOS -> MBR -> BootLoader(grub) -> kernel ->systemd->shell
启动按f2
dd if=/dev/sda of=mbr.bin bs=446 count=1
hexdump -C mbr.bin
/boot/grub2
grub2-editenv list
uname -r
top -p 1
/etc/rc.d
/etc/systemd/system
/usr/lib/systemd/system
/sbin/mk-config2