Does anything USE /dev/initctl or /run/initctl

initnutsysvinitups

I have been investigating code for a UPS for a Raspberry Pi that is using sysv init rather than systemd (personal preference and familiarity). I was looking into how a privileged script – possibly part of or run from /etc/rc.local – would tell init that there was a shortage of electrons in the power supply system. The init(8) man page tells me about writing a single capital letter ('F'ailing,'O'k or critically 'L'ow) into formerly /etc/powerstatus but now /var/run/powerstatus and then sending init a SIGPWR signal; but there is the warning that:

Usage of SIGPWR and /etc/powerstatus is discouraged. Someone wanting to interact with init should use the /run/initctl control channel – see the source code of the sysvinit package for more documentation about this.

I've looked at the source hosted by gnu.org but I've not seen any examples of anything using this new fangled interface to tell init things.

Interestingly, the hydra that is systemd does try to replicate this interface as I think somethings do not like it not being around but I have not found something that actually uses it. Is there such a thing? I'd expect some UPS related software to, e.g. nut but I don't think it does…

This question, although it mentions "Raspberry Pi" is not intended to be specific to that platform or the distributions like Raspbian that run on it.

These questions and their answers were quite enlightening on filling in some related details:

Edit: at least one of the above Q&A also indicated why some people have /dev/initctl and others /run/initctl – which was something I didn't realise (it was a Debian thing).

Best Answer

On Debian the following packages use /run/initctl: systemd, libvirt, sysvinit, refpolicy, gradm2. (from https://codesearch.debian.net/ ).

I think it is the normal way to tell init to change runlevel.

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When running to a run level does it execute previous run levels

When you switch runlevel, the only things executed are the scripts in /etc/rc.d/rc${NEW_LEVEL}.d/.

This means that you are right: Every rc*.d directory needs to be able to handle all of the process/service changes when switching from another runlevel. So every rc directory contains a full set of scripts for reaching that runlevel.

Say you're switching to runlevel 3. The /etc/rc.d/rc3.d/K* scripts will attempt to kill any processes that were running in whatever your previous runlevel was (could be any number), and /etc/rc.d/rc3.d/S* scripts will start any processes that need to be started (and weren't already started in the previous runlevel).

Clearly, managing all of these symlinks would be a real pain, so there are utilities to help manage this. On Debian and Ubuntu (at least, maybe others), you can use update-rc.d to selectively enable/disable scripts found in /etc/init.d, or to set them up to "default" or recommended settings for each script. This will create and update all the symlinks for you, to reflect any configuration changes you want to put in place. On CentOS, I understand you can use ntsysv or chkconfig to do the same thing.

Effectively, you never touch the files in /etc/rc*.d/ (or /etc/rc.d/rc*.d/) yourself; you always use the tool (eg. update-rc.d, ntsysv, chkconfig) to make any changes.

Run various shell commands when NUT reports a low UPS battery

You could try using upssched.conf(5) to run your own script on a timer after an event has fired.

A basic upssched.conf would be something like this:

CMDSCRIPT /bin/your-script.sh
AT ONBATT * EXECUTE emailonbatt
AT ONBATT * START-TIMER upsonbatt 300
AT ONLINE * EXECUTE emailonline
AT ONLINE * CANCEL-TIMER upsonbatt upsonline

With this, NUT will:

Immediately after the UPS goes on battery, run /bin/your-script.sh emailonbatt.
Five minutes after the UPS goes on battery, run /bin/your-script.sh upsonbatt.
Immediately after the UPS regains line power, run /bin/your-script.sh emailonline.
Immediately after the UPS regains line power, also cancel the upsonbatt timer if it is still active. Otherwise, run /bin/your-script.sh upsonline.

The script could be something like:

#!/bin/bash
case $1 in
        emailonbatt)
                  mail -s "UPS on battery power" your@email.com
                  ;;
        emailonline)
                  mail -s "UPS on line power" your@email.com
                  ;;
        upsonbatt)
                  ssh root@nas shutdown -h +0
                  ;;
        upsonline)
                  etherwake 01:23:45:AB:CD:EF
                  ;;
esac

The syntax for the AT statement can be found in the man page for upssched.conf(5), and the notify event types available can be found in the man page for upsmon(8).

Best Answer

Related Solutions

When running to a run level does it execute previous run levels

Run various shell commands when NUT reports a low UPS battery

Related Question