Suppose I have 20 process/deqamons running in my linux system,
How different the HALT will have an effect on my process/deamons, when compared to a SHUTDOWN
Generally, one uses the shutdown command. It allows a time delay and warning message before shutdown or reboot, which is important for system administration of multiuser shell servers; it can provide the users with advance notice of the downtime.
As such, the shutdown command has to be used like this to halt/switch off the computer immediately (on Linux and FreeBSD at least):
shutdown -h now
Or to reboot it with a custom, 30 minute advance warning:
shutdown -r +30 "Planned software upgrades"
After the delay, shutdown tells init to change to runlevel 0 (halt) or 6 (reboot). (Note that omitting -h or -r will cause the system to go into single-user mode (runlevel 1), which kills most system processes but does not actually halt the system; it still allows the administrator to remain logged in as root.)
Once system processes have been killed and filesystems have been unmounted, the system halts/powers off or reboots automatically. This is done using the halt or reboot command, which syncs changes to disks and then performs the actual halt/power off or reboot.
On Linux, if halt or reboot is run when the system has not already started the shutdown process, it will invoke the shutdown command automatically rather than directly performing its intended action. However, on systems such as FreeBSD, these commands first log the action in wtmp and then will immediately perform the halt/reboot themselves, without first killing processes or unmounting filesystems.
On POSIX systems the shutdown command switches runlevels, and executes the appropriate scripts.
On FreeBSD the "halt" command is an ACPI thing...
If you have particular concerns or would like to know things the general documentation wouldn't readily address, please feel free to refine your query.
Related
In Linux, what does shutdown -p command do, will it directly power off the system without Halting? if it halts the system first then power off, then can we directly power off without halting using any command?
shutdown -P sends an ACPI signal telling the computer to completely power off. However, the shutdown command by default powers off the machine, so unless you for some reason need to specify to power off, the normal shutdown command will do it.
You don't need to use any halt commands before using shutdown to power off, either, shutdown -P will completely power off the system without any need to run any halting commands.
I need to configure linux software watchdog (enabled in kernel configuration - CONFIG_SOFT_WATCHDOG=y, which gives me a new device /dev/watchdog1) such that if enabled and if a watchdog timeout occurs, it can launch a script/binary, instead of rebooting the system. My platform uses systemd and not init and I do not see a watchdog.conf file in /etc
Could not find a solution in how to use linux software watchdog. However, one comment says that " it is very possible to restart single or multiple processes after the watchdog signals that the systems is hanging - you can even ABORT the reboot or do a SOFT-reboot, one is able to configure "test" and "repair"-scripts / binaries which do whatever you want them to do."
How/Where can I configure /dev/watchdog1 so that it launches a script/binary instead of rebooting the system?
Eventually resorting to looking at the kernel source for watchdog drivers helped clear things for me. There is no way to configure /dev/watchdog1 or a kernel watchdog driver (hardware or software(softdog)), to be precise, to launch a script/binary instead of causing a system reboot. For this purpose, if feasible, you will have to write your own watchdog driver. The "launching script/binary" configuration that I was led to chase is associated with application space "watchdog daemon" (and has nothing to do with kernel's watchdog driver's configuration/behavior) which can launch a custom script to test your system health and try to fix things before a system reboot is necessary.
I'm new to Linux and running Mint. I've seen a lot of documentation on creating commands that run when the os is started up from the computer being powered off. Is there a way to make similar commands to run when the os wakes up from hibernate or suspend? (For context, I'm running 'rfkill block bluetooth' on startup and would like to when my pc wakes up from hibernate as well).
place your commands in a script file and ensure you have sufficient owner/permissions to execute in /lib/systemd/system-sleep/ so once your OS suspends from sleep, its going to execute.
For more information
man systemd-sleep
https://askubuntu.com/questions/226278/run-script-on-wakeup
I was asked this question from my friend that
when you login our logout from a Linux based system then what all
process run and in which sequence?
Being a noob in Linux and not into real system admin kind of job, so it was difficult for me to answer. Is there any particular tutorial on such concepts or can somebody explain what exactly happens while logging in or logging out from the Linux System?
Assume it is console terminal not GUI.
In Linux, the flow of control during a boot is from BIOS, to boot loader, to kernel. The kernel then starts the scheduler (to allow multi-tasking) and runs the first userland (i.e. outside kernel space) program Init (which is mostly responsible to run startup scripts for each runlevel), at which point the kernel goes idle unless called externally.
init (short for initialization) is a program for Unix-based computer operating systems that spawns all other processes. It runs as a daemon and typically has PID 1. The boot loader starts the kernel and the kernel starts init. If one were to delete init without a replacement, the system would encounter a kernel panic on the next reboot.
When init starts, it reads a file called inittab, usually located in /etc. This file tells init which programs should be run under which conditions. Not only does init run the startup scripts that bring the rest of the system up, but init also takes care of shutting the system down. Commonly, init will start a program called "getty" to spawn a new terminal (or tty), and "login" for login prompt. This configuration is set inside inittab. In recent version of linux, inittab is replaced with /etc/init/*.conf and scripts inside /etc/init.d/
On shutdown, Init is called to close down all user space functionality in a controlled manner, again via scripted directions, following which Init terminates and the Kernel executes its own shutdown.
See Linux startup process Wiki page
Login: Getty is the process which will take care of complete login process.
Init creates the getty process
getty process initiates login command
login command try to check user credentials
getty creates user shell process
getty read shell property files
getty provides you with PS1 prompt
On logout, the shell program exits and we return to step 1.
See login process at this website 1
See login process at this website 2
We have about 40 computers running identical hardware and software. They all run Ubuntu 11.10. They all have just one user account to log in. The .profile file is set up to launch a daemon process. The code for the daemon is written in C.
Once in a few weeks, we get a report that the daemon is no longer running. This does not happen on all computers but just one or two. We cannot reproduce the problem consistently.
Looking at the code, the application quits when it receives either SIGHUP or SIGTERM.
As I understand, SIGHUP is generated when a user logs off. In our case, the user never logs off. I am wondering if it is possible that SIGHUP could have been generated for some other reason. Any other thought would be appreciated.
Well, there are a couple of things to note about SIGHUP. Firstly, its origin is from the concept of a hang-up, i.e. loss of connection to a console over something like a modem. In modern parlance this generally means it has lost its controlling tty. Unless you've taken care to detach from your tty, any program started in a given terminal will receive a SIGHUP when the terminal is closed. See here for details on how to do this in your program. Other options include:
running your program inside screen or tmux
run your program with nohup or some other daemonising framework
The other possibility is something is deliberately sending your process a SIGHUP which by "tradition" is often used to signal a process that it should re-read its configuration.
Signals can be sent using kill utility or kill syscall.
Of course, you can try and find out who is sending that signal or disconnecting your terminals or network connections, but there is simpler practical way to fix your problem.
When code is supposed to run as a daemon, but really isn't (just like yours), there is a wrapper that can turn any program into daemon. Surprise - this wrapper is called daemon! It has lots of options, probably most importantly for you, option to automatically restart your utility should it ever die for any reason.
If this command is not installed on your Ubuntu, just sudo apt-get install daemon, and man daemon to get started.