I follow this post How to declare and use boolean variables in shell script?
and developed a simple shell script
#!/bin/sh
a=false
if [[ $a ]];
then
echo "a is true"
else
echo "a is false"
fi
The output is
a is true
What is wrong?
if [[ $a ]]
is not one of the options listed in any of the answers of the question you linked.