As we know, when we startup the system (after grub) and when we shutdown the system, many logs will be printed on screen.

I know that the kernel would write some logs of the kernel. But how about the others? Who writes them?

As I know, Ubuntu uses systemd and there are some logs from systemd. I want to know if it is possible that I could create a service of systemd, which can write some logs on screen while starting up and shutting down?

  • To me at least it is unclear what your intentions are with "creating a service of systemd. Yes, the kernel, systemd and other applications will write to the screen during the boot process. – Tommiie Jan 16 at 12:43
  • You might be looking at logs written by Plymouth, which is a boot splash manager in modern Linux distributions. – filbranden Jan 16 at 13:22
  • @Tommiie I simply want to know how to write a log message with my own service or process while startup and shutdown. – Yves Jan 17 at 1:22
  • @Yves : that is a completely different question than “who writes to the screen during startup” – Tommiie Jan 17 at 5:49

Can analyze the reboot system call source code, maybe the answer is there. PATH: linux-4.4.1/kernel/reboot.c

280 SYSCALL_DEFINE4(reboot, int, magic1, int, magic2, unsigned int, cmd,
281                 void __user *, arg)
282 {
315         switch (cmd) {
316         case LINUX_REBOOT_CMD_RESTART:
333         case LINUX_REBOOT_CMD_POWER_OFF:
334                 kernel_power_off();
335                 do_exit(0);
336                 break;

257 void kernel_power_off(void)
258 {
259         kernel_shutdown_prepare(SYSTEM_POWER_OFF);
260         if (pm_power_off_prepare)
261                 pm_power_off_prepare();
262         migrate_to_reboot_cpu();
263         syscore_shutdown();
264         pr_emerg("Power down\n");
265         kmsg_dump(KMSG_DUMP_POWEROFF);
266         machine_power_off();
267 }
  • Before kernel doing its shutdown, we can see many log have been printed on the screen. So this shouldn't have business with the kernel. – Yves Jan 17 at 8:50
  • The kernel outputs some information before the real shutdown. 265 kmsg_dump(KMSG_DUMP_POWEROFF); 266 machine_power_off(); – wzq Jan 17 at 13:35

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