Uninterruptible power supply: Difference between revisions

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      };
     };
+</syntaxhighlight>
+= The Delayed UPS Shutdown =
+As part of the system shutdown process, there needs to be an action to
+send a message to the UPS to tell it that some time later, it too will
+shut down. Note that the UPS does not shutdown at the same time as the
+system it protects.  The UPS shutdown is delayed. By default the delay
+is 20 seconds. The absence of AC power to the protected system for a
+sufficient time has the effect of resetting the BIOS options, and in
+particular the option Restore power on AC return. This BIOS option
+will be needed to restart the box.
+During the system shutdown, systemd service unit runs the command
+upsdrvctl shutdown. This tells the UPS that it is to shut down
+offdelay seconds later. The system powers down before offdelay seconds
+have passed. Wall power returns before the UPS shuts down Less than
+offdelay seconds have passed. The UPS continues it's shutdown
+process.
+After offdelay seconds the UPS shuts down, disconnecting it's
+outlets. The beeping stops.  With some UPS units, there is an audible
+clunk. An interval of ondelay-offdelay seconds later: After ondelay
+seconds the UPS turns itself on, and repowers it's outlets. The system
+BIOS option Restore Power on AC return has hopefully been selected and
+the system powers up.
+<syntaxhighlight lang="nix">
+  # copied from ConfigExamples 3.0 book, Appendix B.2.
+  systemd.services.nut-delayed-ups-shutdown = {
+    unitConfig = {
+      Description = "Initiate delayed UPS shutdown";
+      Before = "umount.target";
+      DefaultDependencies = false;
+    };
+    serviceConfig = {
+      Type = "oneshot";
+      ExecStart = ''${pkgs.bash}/bin/bash -c\
+       "${pkgs.util-linux}/bin/logger -t\
+        nut-delayed-ups-shutdown 'upsdrvctl shutting down UPS';\
+        ${pkgs.nut}/bin/upsdrvctl shutdown"'';
+    };
+    wantedBy = "final.target";
+  };
 </syntaxhighlight>