@EliahKagan when I run your command, it only opens the file in nano

@Zanna Which command?

sudo SUDO_EDITOR='sed -i 1s/^/#/' visudo -f /etc/sudoers.d/keep-home

?

yes

And it's empty in nano?

no

since I had given it the same contents as yours

And it showed those contents correctly?

yes

What Ubuntu release is this on?

18.04

I ran the command on 19.10. I don't know why it doesn't work on 18.04.

I am wondering what silly thing I'm doing

Nothing?

I assume the command just doesn't work on 18.04, having to do with what works as an editor for sudo -e, sudoedit, and visudo in what versions of sudo.

That wouldn't change the behavior of sudo on 18.04, since before 19.10 it already preserves $HOME, but that doesn't address why sed -i 1s/^/#/ couldn't be used as an "editor."

Oh.

Is SUDO_EDITOR not used until recent versions of sudo?

That appears to be the case for visudo specifically.

I don't know, but it's not set

What happens if you use EDITOR or VISUAL instead of SUDO_EDITOR?

zanna@xubi:~$ sudo EDITOR='sed -i 1s/^/#/' visudo -f /etc/sudoers.d/keep-home
zanna@xubi:~$ cat $_
#Defaults	env_keep+="HOME"

works

On my 16.04 and 18.04 systems, the manpage for visudo doesn't mention SUDO_EDITOR. It appears that, until recently, SUDO_EDITOR has been used only by sudo -e and sudoedit, not by visudo. I also just got the same behavior you saw--where my default interactive editor opens--when I tried the command on a 16.04 system.

I don't understand why adding a space is a problem

@EliahKagan I guess it's another upgrade :)

@Zanna I do consider having visudo use SUDO_EDITOR to be an improvement.

@Zanna sed -i 1s/^/#/ is parsed as three arguments, like we want.

Adding a space after # causes it to be parsed as four arguments.

why is that? A quoting issue?

Not a quoting issue in the shell.

sudo -e/sudoedit and visudo are not using the value of EDITOR, VISUAL, or SUDO_EDITOR as a command name. If they were, this wouldn't work at all; there is no command called sed -i 1s/^/#/. What allows that to work is that the value is being used as a command name followed by zero or more arguments to the command. The filename of the temporary file is passed as a final argument after that.

The variable's value is split into arguments (or, if one doesn't consider the command name itself to be an argument, then split into a command name and arguments) at the spaces.

I don't know of any way to prevent it from being split on spaces.

this is very interesting!

Yeah, it's cool. :)

In particular, preceding the space with a backslash does not work.

This situation is similar to how hashbang lines are parsed, except that in this situation it can be split into many arguments.

With GNU sed, I should be able to give a hex or octal code for the space character.

I could also make a temporary script and run it.

Neither of those solutions is especially appealing, though.

None of this is in any way necessary or even helpful for my sudo / $HOME answer.

@Zanna ...but you can see why I've allowed myself to be distracted by it. :)

@EliahKagan haha yes definitely