Whenever I attempt to set the EDITOR environment variable, a subsequent check reveals no value has been set.

I was attempting to edit the crontab:

sudo crontab -e -u mycooluser

which returns:

no crontab for mycooluser - using an empty one
Error opening terminal: unknown.
crontab: "/usr/bin/sensible-editor" exited with status 1

I then found this post:
Setup a cronjob from commandline which suggested I change the editor used.

So I tried (including variations):

export EDITOR="/bin/nano"
export EDITOR="usr/bin/nano"
export EDITOR=nano

and when I execute (per this link How do I find and set my $EDITOR environment variable?):

echo $EDITOR

OR

printenv | grep EDITOR

I get nothing, a quick printenv check also confirms that no EDITOR variable exists in the list. To be clear, by 'nothing' I mean that on the echo $EDITOR it prints a blank line and for printenv | grep EDITOR nothing is printed.

Any idea why the variable isn't being set or why I can't seem to change it?

System:
Ubuntu 18.04.1, Using Terminal in Webmin

You are using the so-called "popup shell". This is not a terminal. Nor is it in fact really a shell.

It's a piece of user interface fakery inside a WWW browser that executes shell commands separately from one another as individual transactions. There's no actual continually running shell to hold an environment variable. It is merely presented that way by the user interface in the WWW browser. The shell commands are run in a nonce process environment where no TERM environment variable has been set, and in a session with no controlling terminal. And even if it were set, there's no terminal emulator to handle the terminal I/O that full-screen programs like text editors use. Indeed, there is only provision for relaying output of the program back to the local machine; no input is sent in the other direction.

nano, pico, and emacs will complain about being unable to determine the terminal type and open the terminal. VIM and NeoVIM fall back to assumptions that simply do not hold in this case, and give the appearance of simply hanging. Not even line-mode editors like ex and ed will work as user input is simply not sent to the remote machine. (One possibly could drive ex non-interactively with its -c option.) Similarly, one cannot change passwords with the passwd command.

This has been a misunderstanding of the "popup shell" since at least 2016.

Further reading

• "JedMeister" (2018-03-04). Interactive Terminal. Bug #1037. authentic-theme bugs.


• Michael Hurt (2018-11-13). Error opening terminal: unknown. Bug #5214. Webmin bugs.


• "7starsone" (2016-12-08). File Manager: Command line. Bug #566. authentic-theme bugs.