Everything I mentioned so far has been my Bionic desktop.  Which, 
incidentally was an upgrade from Xenial, not a clean install. But... in 
my Focal headless container, if I run:

     >$ run-mailcap myscript

it invokes emacs.  Yay!  It works!  DISPLAY, EDITOR, and VISUAL are all 
unset or empty.  There's nothing in /etc/mailcap for emacs, and what 
*IS* in /etc/mailcap is (amongst others of course):

    application/x-shellscript; vim %s; needsterminal

So I really need to spend some time reading man pages about mailcap, 
mime, etc because that's not what I would expect, and I'm really confused.

On 5/23/22 8:59 AM, Craig STCR wrote:
> I guess maybe I should have given a little better description of what 
> I tried that did NOT work?  But it's a little off-topic for this 
> mailing list.  Nevertheless, here it is...
>
> I created a ~/.mailcap file and put this in it, which I cut and pasted 
> from /etc/mailcap:
>
>     application/x-shellscript; emacs27 %s; test=test -n "$DISPLAY"
>
> But obviously that's not going to change anything, since it's already 
> in the system mailcap file, /etc/mailcap.  DOH!  And sure enough, 
> running '>$ run-mailcap myscript' invokes 'less'.  But what I wasn't 
> expecting is that running '>$ update-mime -- local' gives me: "Error: 
> '/home/user/.mailcap' is not in required format -- not updated".   Not 
> sure why I'm getting that when I cut-and-pasted from /etc/mailcap.
>
> No worries.  I'll look in more depth later.
>
>
> On 5/23/22 8:40 AM, Craig STCR wrote:
>> Thanks all for your help!
>>
>> On 5/20/22 9:44 PM, Ihor Radchenko wrote:
>>> Dear Craig, ... or provide plain/text handler in ~/.mailcap.
>>>
>> OK, I did a first-try on this and was unsuccessful, but I'm sure it's 
>> user error.  I need to refresh my knowledge on how to customize 
>> user-local mime database, and that will write-out a new ~/.mailcap, 
>> etc, I think?  I've done it before, but it was awhile ago, and I 
>> wasn't paying attention to ~/.mailcap when I did it.  I know for 
>> Gnome I can create a .desktop file.  But I know there's a way to 
>> customize user-local mime database without Gnome desktop.  I'll take 
>> a closer look when I have a little more time.
>>
>>
>> On 5/20/22 9:44 PM, Ihor Radchenko wrote:
>>> However, I am not sure what to do on Windows/Mac.
>> Maybe try a quick-and-dirty, cross-platform solution that checks 
>> non-binary files for a first-line shebang?  Could use existing Emacs 
>> hooks that determine major-mode when opening files.
>>
>> Again, thanks all for your help!
>>
>> Best,
>> -Craig
>>
>