I don't work much with Linux systems these days, but I would vote for $ sudo over #. Two reasons:

1. It's easy to overlook the prompt. That part is basically "some characters before the actual command", so I don't normally pay attention to it.
2. # is also used for comments. I think it would be confusing to use the same character for two wildly different things.

I have a fairly opinionated stance on this. Except in your sudo example where you’re specifically using sudo for a reason, I document all commands as non-root, and do not instruct them to raise privs. Whether or not they have, want or need privs, and how they raise them, is their system not mine.

It’s not exactly user friendly, but I don’t like to encourage people to blindly copy & paste commands that raise privs. That should be a conscious decision where they stop and ask themselves if & why it’s necessary.

I dislike when documentations add sudo because what if I am root already or what if sudo is not installed on my machine and I cannot just copy and paste the lines because I have to avoid pasting sudo.

Also fyi ArchWiki also uses the # approach.

You should consider who your audience is, are they all CLI experts familiar with the difference in syntax? That seems unlikely.

I'd always write documentation in a way that's accessible to most users. The difference between $ and # syntax is highly esoteric.

sudo on the other hand is familiar to almost everyone. It's one of the first things mentioned in beginners guides.

I wouldn't even prefix your commands with $ as an experienced user is quite likely to include that when copying the command.

A lot of people are citing the arch wiki as a standard that uses # but isn't the entire meme around arch that its a notably complex system?

I would use $ too for the example you’ve posted - it is just assuming a different user.

As long as it is not root, I wouldn’t put it with #.

I seem to be in the minority here but I personally prefer using $ and # to denote root. I like this because not everyone uses sudo and might not even have it installed.

That being said, if you already have other commands that are using sudo -u ... to run commands as a different user then it might be best to just be consistent and prefix everything with it, but if there is only a few of those maybe a # cp foo bar && chown www-data bar is an alternative.

# is a standard shell prompt for root, and only for root. For commands executed by any other user, including sudo, use $.

In general it is a bad practice to use sudo in documentation because in many distros it is not available by default. I would use su for your example. However system users have no passwords, so you need to become root first, and only after that change user to avoid prompting a password. So I would write

# su -s /bin/bash www-data
$ cp /var/www/html/html1 /var/www/html/html2

or

# su -s /bin/sh -c 'cp /var/www/html/html1 /var/www/html/html2' www-data

But if you are sure that sudo is installed and configured on a user's machine, you may write

$ sudo -u www-data cp /var/www/html/html1 /var/www/html/html2

Neither because it makes it hard to copy paste. If you have to pick one then $ because # is for comments in bash.

Edit: looks like this is wrong lol, that's what I get for not verifying. So maybe $ does make more sense!

Original message:

I think I'd go with #.

The non-root user probably doesn't have permission to run the sudo command as www-data user, but root does.

Unless you previously set permissions for the non-root user to sudo as www-data.

Personally i prefer the $ sudo because honestly i don't always notice the symbol at the beginning but sudo is really easy to keep track of whats root and what isn't