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I want to have a Mac Mini automatically boot when power is applied, as I cannot conveniently reach its power button, because it resides inside an art installation, where the power will be shut down during the night. I thought it might be possible to use the "Start up automatically after a power failure" in the energy system settings for this, along with the -u parameter to the shutdown command, in order to safely bring the machine in a state where I don't have to fear data loss. According to the shutdown man page, the -u parameter seems to be what I need:

-u: The system is halted up until the point of removing system power, but waits before removing power for 5 minutes so that an external UPS (uninterruptible power supply) can forcibly remove power. This simulates a dirty shutdown to permit a later automatic power on. OS X uses this mode automatically with supported UPSs in emergency shutdowns.

But shutdown -hu now does not work. On both Macs I tested it with (Mac Mini with 10.10 and a Mac Pro on 10.11), the machines still power off immediately, they do not wait for five minutes in the halt state, so that I don't have the time to power off the mains. Therefore, the autostart on power failure setting doesn't trigger, and I have to manually push the power button to make them boot again.

  • Do you have UPS physically connected while you are testing? I would expect things to fail if there isn't a USB connection to a working power supply. – bmike Jan 13 '16 at 19:28
  • I'm having the same problem. I was using the dirty shutdown command on my Mac mini, but after updating to El Capitan it stopped working (as you described). I was using the iHome Smart Plug to kill power after the dirty shutdown and to provide power again later for automatic boot. Since OS 10.11 came out, I have to manually press the power button on the Mac, which negates the reason for using the iHome Smart Plug. – user170079 Feb 13 '16 at 2:40
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The NVRAM setting to power on when power returns has nothing to do with the shutdown command, so I wouldn't worry about the shutdown options and instead focus on the NVRAM settings for what to do when the machine senses a return of power.

If you don't have an actual UPS - I would get an internet enabled power switch like WeMo or other or just train staff to power cycle the hardware.

The setting to power on automatically is quite reliable in my experience, so you should only need that to recover from 95% or more of the power failure events you have. Also, kiosk machines generally just run apps and open files, so you don't have work that needs to be saved to disk. I would just let the power cut from the Mac and make sure the filesystem is journaled so that the restart is quick.

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If you have an actual UPS - use the energy saver control panel to tell the OS to shut down N minutes before the power runs out and you will be well protected from data loss.

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Here I have a server with storage attached so I'm having the shut down scheduled 10 minutes before the battery expires. For a kiosk, I would disable all three sliders/check boxes for UPS or get a simple UPS like APS BGE70 that lacks USB port entirely and just gives you some time when people accidentally trip the power or a substation feeder shifts and your outage is a minute or so in length.

  • Am I right that the "NVRAM setting" you're talking about is the "Start up automatically after a power failure" I was using? – Johannes Maibaum Jan 13 '16 at 19:35
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    yup - both my Mac Pro and Mac Mini seem to power off with -u so if you want to file a bug with Apple - that might be worthwhile - but the end result is still fine for my needs. They power off when the battery runs low, they restart if and only if power is actually cut and then returns. You roll the dice on UPS shutdowns if the power comes back between the time you issue the shutdown and the power stays connected. For kiosk use, I just let it crash hard to ensure it powers back on. Journaling keeps the boot short and data doesn't really change on a kiosk so data loss is a rare event. – bmike Jan 13 '16 at 19:39
  • OK, so there is no other NVRAM setting that better fits my problem? And if I can't get apple to fix the probable bug with -u, the only other (still partial) solution is to buy a UPS and hope that it works with your suggestion? If so, I will probably go with the power off hard crash and hope that there will be no data loss... Thanks @bmike for the quick explanation! – Johannes Maibaum Jan 13 '16 at 19:46
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Found a workaround for "auto boot after power loss" on any OS, after few H/w tests, found that "BIOS"(or whatever they named it in APPLE) controlled by OSX OS - basically changing this setting under Power Saving - "Start up automatically after a power failure" to enabled and then wipe the local HDD and install anything you want that won't affect this setting(meaning it will remain and will power the box back on after power failure), and vise versa - removing this setting via OSX will cause Mac stop "Auto start after power failure" running no matter what OS.

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