let $success= 1;

--echo # The next two variants do not expand the content of $success
--echo #     I would call this a bug
--echo # Execute: --echo # <whatever> success: $success
--echo # <whatever> success: $success
--echo # Execute: echo # <whatever> success: $success ;
echo # <whatever> success: $success ;

--echo # The next two variants work fine and expand the content of $success
--echo # Execute: --echo $success
--echo $success
--echo # Execute: echo $success ;
echo $success ;

--echo # The workaround with "eval SELECT ... $success is uncomfortable, because
--echo # - the statement itself is not of interest
--echo #   --> temporary suppress protocolling with "--disable_query_log" 
--echo # - a running server is needed
--echo # - the statement causes annoying activities when we somebody executes the test 
--echo #   within a debugger
--echo # - the statement might cause printouts in some logs
--echo # Execute: eval SELECT "# check <whatever> success: $success" AS '';
--disable_query_log
eval SELECT "# check <whatever> success: $success" AS '';
--enable_query_log