`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.

For myself many times if I type reboot it won't let me due to inhibitors from Chrome when watching YouTube and other opened tabs. So I'm forced to use sudo reboot--a bigger hammer.

This in Ubuntu 16.04

Thanks for the discussion, I'm glad to understand this now!

As Terrance mentioned, in version 15.04, they switched from Upstart to Systemd for the commands. This change means that reboot no longer requires root privileges.

As for why the *nix community instructs to sudo reboot, there are a few potential reasons:

• Habit - People were so used to having to sudo reboot to reboot, they continue to do it despite the fact it is not needed

• Non-updated users - The people instructing to sudo reboot are on versions of Ubuntu lower than 15.04, or other distros that use Upstart for commands.

• Compatibility - This is what seems the most plausible to me: people are instructing to sudo reboot because it is guaranteed to reboot across all *nix systems, no matter what.

Additional note: from user535733's answer:

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.