Sometimes, when using a Windows box, Really Bad Things (TM) happen, forcing the system to shut down. For example, if someone exploits the system, and their exploit accidentally kills lsass.exe or services.exe, Windows is very unhappy. It pops up a dialog box expressing its discontent, telling you that it will reboot in 60 seconds.
But, suppose you don't want it to reboot that quickly? Maybe you need just a little more time to save a file, close something out, launch your retaliatory missiles, or whatever. Most of the time, you can abort a shutdown by running:
C:\> shutdown /a
Of course, without an lsass.exe or services.exe process, the box is pretty well hosed. But, this command can give you a little bit of extra time in event of dire emergencies, limping along with a machine that is only partly dead. You can then make the box reboot on your own time frame with the following command:
C:\> shutdown /r /t [N_seconds]
If you omit the /t, it'll reboot in 30 seconds. Use /t 0 to make it reboot now.
I've always hated the Unix shutdown command. I find the "write all" behavior more annoying than useful. I normally use "reboot", "halt", or "init 0" (stop and power down). That being said:
# shutdown -c # cancels scheduled shutdown
# shutdown -r +1 # shut down and reboot in 1 minute
# shutdown -r 14:30 # shut down and reboot at 2:30pm
Interestingly, you can't schedule shutdowns with finer than one-minute granularity, though I suppose you could do something like:
# sleep 30; shutdown -r now
Interesting to note that the OS X shutdown command does not have the "-c" option allowing you to halt the shutdown.