How does a log is printed while shutdown, reboot or startup
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?
debian ubuntu systemd logs shutdown
asked 9 mins ago
YvesYves
870618
add a comment |
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?
debian ubuntu systemd logs shutdown
asked 9 mins ago
YvesYves
870618
add a comment |
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?
debian ubuntu systemd logs shutdown
asked 9 mins ago
YvesYves
870618
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?
debian ubuntu systemd logs shutdown
debian ubuntu systemd logs shutdown
asked 9 mins ago
YvesYves
870618
asked 9 mins ago
YvesYves
870618
edited 3 mins ago
Yves
asked 9 mins ago
YvesYves
870618
asked 9 mins ago
YvesYves
870618
asked 9 mins ago
YvesYves
870618
870618
add a comment |
add a comment |
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