I'm interested in how rebooting is implemented in Linux. When I press ctrl-alt-del or click "restart" in the menu bar, what happens next?
Thanks!
brings the system down in a secure way. All logged-in users are notified that the system is going down, and login(1) is blocked. It is possible to shut the system down immediately or after a specified delay. All processes are first notified that the system is going down by the signal SIGTERM.
It does its job by signalling the init process, asking it to change the runlevel. Runlevel 0 is used to halt the system, runlevel 6 is used to reboot the system, and runlevel 1 is used to put to system into a state where administrative tasks can be performed;
So basically reboot calls the "shutdown".
Quick answer is that all the scripts that are in /etc/rc6.d are executed.
scripts that start with "K" are executed with the "stop" parameter.
scripts that start with "S" are executed with the "start"parameter.
For more you can start reading about runlevels here: http://en.wikipedia.org/wiki/Runlevel
There are different init systems on Linux, and they also control what happens on restart/shutdown. See https://unix.stackexchange.com/questions/18209/detect-init-system-using-the-shell to tell which you're using.
If you're using SysVinit, then there is a runlevel associated with the overall system status. The init system will first run all the kill scripts associated with your current runlevel and then the start scripts associated with runlevel 6. If your current runlevel was 5, it would run /etc/rc5.d/K* and then /etc/rc6.d/S*. They might be in another directory, such as /etc/init.d/rc5.d/k*, depending on your Linux distribution.
If you're using systemd, then instead of having an overall "runlevel", there would be a list of defined targets and services. A list of targets is essentially a runlevel. These are defined in .service and .target files under /etc/systemd. There will likely be a "reboot.target" defined under there, and other services with a dependency on that will be run on reboot. See the systemd homepage or this stackexchange question for an example.
Some Ubuntu versions also use upstart, but I think it's been replaced by systemd in more recent versions. If you are using upstart, see this guide or this askubuntu question.
One thing to be careful of is that regardless of which init system you're using you may be using init scripts generally associated with another one. So you may be using sysVinit, but some of the rc*.d scripts may be links to things that invoke systemd scripts. Or vice-versa.
Related
Is there any way to detect in a program that it is run by systemd as a daemon?
systemd API
sd_booted()
is used to detected if the whole system is booted by systemd, but says nothing about the program itself.
Thanks
Get the parent process id and see whether that process is systemd.
Starting from systemd v232, an environment variable INVOCATION_ID is given to all processes started as (part of) a service unit. This is a nice trait from systemd and not any other service manager, so it can be used as a convenient way to detect systemd, but not necessarily reliable.
Personally I use this to disable timestamp in logging as systemd journal already does that.
You could set a magic environment variable in the daemon's service file and look for this variable.
What append in Linux when the user type "halt" in? Is there any script that force process to end as soon as possible? If yes, is there any way to schedule process shutdown?
The purpose of this scheduler will be force GPIO management to be the last one to exit.
Thanks in advance
Depending init system it can force killing services. General way to shedule stop order is sorting "K*" links in /etc/rc.d/
To understand deeper first look in /etc/inittab at lines:
::shutdown:/etc/rc.d/rcS K shutdown
::shutdown:/bin/umount -a -r
In this example shutdown sheduled by /etc/rc.d/rcS script.
After typing halt init executes inittab rules, then killing himself with still live childrens. Then kernel stops CPU.
As i understand question, one of solution is:
remove stop link for gpio-management service
add script to ensure that gpio-sensible services stopped/killed to inittab
add script to stop gpio-management service at end
I have a script to start and stop my services. My server is based on Linux. How do I automate the process such that when OS is shutdown the stop script runs and when it is starting up, the start script runs?
You should install init script for your program. The standard way is to follow Linux Standards Base section 20 subsections 2-8
The idea is to create a script that will start your application when called with argument start, stop it when called with argument stop, restart it when called with argument restart and make it reload configuration when called with argument reload. This script should be installed in /etc/init.d and linked in various /etc/rd.* directories. The standard describes a comment to put at the beginning of the script and a uitlity to handle the installation.
Please, refer to the documentation; it is to complicated to explain everything in sufficient detail here.
Now that way should be supported by all Linux distribution. But Linux community is currently searching for better init system and there are two new, improved, systems being used:
systemd is what most of the world seems to be going to
upstart is a solution Ubuntu created and sticks to so far
They provide some better options like ability to restart your application when it fails, but your script will then be specific to the chosen system.
In the Supervisord conf files you can specify to autorestart a certain program with:
autorestart=true
But is there an equivalent for [Supervisord] itself?
What is the recommended method of making sure Supervisord continues running unconditionally, especially if the Supervisord process gets killed.
Thanks!
Actually your question is a particular application of the famous "Quis custodiet ipsos custodes?" that is "Who will guard the guards?".
In a modern Linux system the central guarding point is init process (the process number 1). If init dies, the Linux kernel immediately panics, and thus you have to go to your data center (I mean go afoot) and press reset button. There're a lot of alternative init implementations, here is one of those "comparison tables" :)
The precise answer how to configure a particular init implementation depends on what init version you use in that system. For example systemd has its own machinery for configure service restart upon their deaths (directives Restart=, RestartSec=, WatchdogSec= etc in a corresponding unit-file. Other init implementations like Ubuntu Upstart also has its analogues (respawn directive in a service configuration file). Even old good SysV init has respawn option for a service line in /etc/inittab, but usually user-level services aren't started directly inittab, only virtual console managers (getty, mgetty etc)
I want to make a process alive during reboot. I am thinking that, I can backup all process related information and i will store it on some file. After reboot i will take that data back and by using that i will create a process again. Is my thinking is correct? Please clarify.
Your question is too vague. Can you provide with more information? I can give a basic outline of what is required to run a process on system startup.
Deamonize
A process must be a deamon (it shouldn't run in the context of a terminal) Read deamon process for more information.
chkconfig
Use this to add your deamon to your system startup run levels. sudo chkconfig deamon_name on