`sudo reboot` vs `reboot` - what is the difference?

On Ubuntu 14.10 and older, sudo is required.

The introduction of Systemd in 15.04 changed the way Ubuntu handles shutdown and reboot:

1. When a single user is logged in, sudo isn't necessary. When more than one user is logged in then sudo is required.

2. Applications can inhibit shutdown and reboot. You override these inhibitions with sudo.

3. A single user logged in via ssh still requires sudo.

On my 14.04 machine, when I (as a normal user) type reboot, I get

reboot: Need to be root

That is the difference.

As Terrance pointed out in the comments, it works differently on later systems than mine. So you are probably seeing old writeups and/or users (like me) who have been habituated to typing sudo reboot!

sudo rebootis used in tutorials / how-tos for compatibility reasons

While reboot might work

• if you are root or
• if you are on a host with systemd and
• if no applications are blocking a reboot

sudo reboot will "always"* work, regardless of

• whether you are root
• whether there are other users logged in
• whether there are applications blocking
• whether init is systemd, System V, Upstart, whatever

* Well, it will certainly try - short of there being some kernel processes that are blocking/misbehaving it should work.