mjhjmtu

remind(1) provides a function shell() documented as follows:

 shell(s_cmd [,i_maxlen])
 Executes cmd as a system command, and returns the first 511
 characters of output resulting from cmd. Any whitespace
 character in the output is converted to a space. Note that if
 RUN OFF has been executed, or the -r command-line option has
 been used, shell() will result in an error, and cmd will not be
 executed.

 …

I would like whatever s_cmd writes to stdout to be interpreted
by remind itself. E. g.:

$ echo REM Sep 13 2018 MSG test >/tmp/test.rem
$ tail -2 ~/.reminders
SET tmp shell("cat /tmp/test.rem", -1)
$tmp

Where $tmp is my unsuccessful attempt at inserting the output of the
command in the line above. When executing rem(1), it does not return an
error but it does not interpolate $tmp either:

$ rem
Reminders for Thursday, 13th September, 2018 (today):

…
$tmp 

I assume that $tmp is interpreted as an implicit REM … statement.

(The INCLUDE directive does not work in this context because I need
the output of the inclusion to be generated in situ.)

Your problem is not with the shell() function, but

a) with the way you try to interpolate expressions/variables -- you should use [tmp] instead of $tmp

b) with the fact that remind doesn't allow MSG in expressions:

$ cat /tmp/foo.rem
SET var "REM Sep 13 2018 MSG test"
[var]
$ remind /tmp/foo.rem
/tmp/foo.rem(2): Can't nest MSG, MSF, RUN, etc. in expression
No reminders.

This is what the documentation says:

 o You cannot use expression-pasting to determine the type (MSG,
 CAL, etc.) of a REM command. You can paste expressions before
 and after the MSG, etc keywords, but cannot do something like
 this:
 REM ["12 Nov 1993 AT 13:05 " + "MSG" + " BOO!"]

I'm not a remind user, but this is my first crack at fixing your problem:

SET tmp shell("cat /tmp/test.rem", -1)
REM [substr(tmp, 4, index(tmp, "MSG")-1)] MSG [substr(tmp, index(tmp, "MSG")+4)]

provided that /tmp/test.rem is of the form REM ... MSG ....

Please notice that in remind, indexes start from 1, not from 0.

Note

If your problem is actually 'how to include dynamically generated content in a remind file', you may do that by redirecting the output of the shell command to a temporary file, then INCLUDE-ing that file:

INCLUDE [shell("echo REM " + today() + " MSG hello > /tmp/foo.rem; echo /tmp/foo.rem")]

Or you could use the INCLUDE command with a fifo instead of a regular file, and have a script that writes to the fifo each time it is opened.

Before starting reminder:

$ mkfifo /tmp/remind-fifo
$ while echo 'REM Sep 18 2018 MSG test' > /tmp/remind-fifo; do sleep 1; done &

Replace the echo with whatever script you need to generate the remind commands (eg. sh my_script > /tmp/remind-fifo).

Then, in the remind file, you can simply include the fifo:

INCLUDE /tmp/remind-fifo

The fifo approach could be used with other programs that have an include mechanism (eg. the C preprocessor)