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I got used to using the shutdown command like:

shutdown -t now

So when I started wanting to use reboot in the same way I was confused at first because to reboot now you need to use:

reboot -n

I looked more into and I noticed reboot could also be used to shutdown the system with:

reboot -p

What is the history behind these two commands? Did one come out before the other and are they both on most systems historically?

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    I think halt, reboot, and shutdown first appeared in 4BSD, and a separate shutdown was developed for USG Unix to go with the new multiple run level init. Then, as was typical in the 1980's, various vendors added various options without worrying much about compatibility with each other. – Mark Plotnick Jan 15 '17 at 23:50