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 4 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 |
0
active
oldest
votes
Your Answer
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "106"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f496127%2fhow-does-a-log-is-printed-while-shutdown-reboot-or-startup%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
0
active
oldest
votes
0
active
oldest
votes
active
oldest
votes
active
oldest
votes
Thanks for contributing an answer to Unix & Linux Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2funix.stackexchange.com%2fquestions%2f496127%2fhow-does-a-log-is-printed-while-shutdown-reboot-or-startup%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
