How can a log print to display while shutdown, reboot or startup?

Question

As we all know, when we reboot, shutdown or startup a system, some message will be printed on the screen, here is a capture:

My system is Ubuntu 16.04, I know these log messages as above come from the systemd.

As my understanding, a normal user-process can print things on the screen because the system gives it three file descriptors: 0, 1 and 2. We can find them at /proc/<PID>/fd/. Here is an example:

root@X86-Xenial-6:~# ls /proc/3467/fd
0  1  2  255

The 3467 is a hello-world program, I don't know what 225 is but I know that 0, 1 and 2 are standard input, standard output and standard error.

So, I have a question: when the system starts to shutdown, reboot or startup, user-process hasn't been created or has been destroyed, which means that /proc/ doesn't exist anymore, in this case, 0, 1 and 2 doesn't exist.

So why does the message coming from systemd can be printed on the screen? The kernel could print things because it controls the screen immediately, but I don't think systemd belongs to the kernel, so how could it print things on the screen too? What kind of function or api is used?

Answer 1

Startup and shutdown are largely conducted in user space, not by the kernel. As soon as the kernel has finished initialising, it looks for init and starts that, as a regular process, with the standard file descriptors pointing at the console. So init (in your case, the initramfs script and then systemd) can write to its standard output and whatever it writes will show up on screen (or wherever the console output is configured to show up). This continues to be the case until the kernel shuts down or reboots, which happens after user space has shut down (and after all the shut down logs have been written to the console).

As an aside, note that /proc is only a means of accessing certain pieces of information maintained by the kernel; those pieces of information exist whether /proc is mounted or not.

Note too that

As we all know, when we reboot, shutdown or startup a system, some message will be printed on the screen

isn’t necessarily true — many systems now boot and shut down without showing logs, so we can’t assume that “we all know”.

Answer 2

A short answer, to clear up some miss-understandings.

• The process is done in user space: specifically init the first user process.
• You don't need /proc for file-descriptors to exist. If the only way to access file descriptors was through a file, then how would you access a file-descriptor? (you would open the file in /proc be returned a file descriptor, try to look it up in /proc an be returned a file descriptor…). /proc is just a view, it is only needed by processes that report on other processes.
• The 255 file descriptor of ls in you example is of the directory /proc/3467/fd: ls has to open the directory, so we expect one extra file-descriptor. We have one extra, so this is what it is.
• /proc/fd/1 etc point to other devices. e.g. 1 -> /dev/pts/3. Do an ls -l /proc/self/fd
• When the kernel starts init it will connect its stdin, stdout, stderr to the screen (a tty device, that gets rendered to the screen), or to somewhere else.
• init is process 1 (On your system systemd).