How to declare and use boolean variables in shell script?

Question

Title.

The way I tried declaring a boolean variable is by:

variable=$false

variable=$true

Is the syntax correct? Also, if I wanted to update that variable would I just do the same format? Finally, is the following the proper syntax for using boolean variables as expressions:

if [ $variable ]

if [ !$variable ]

Thanks! Any help is appreciated!

Answer 1

From: Using boolean variables in Bash

the_world_is_flat=true
# ...do something interesting...
if $the_world_is_flat ; then
    echo 'Be careful not to fall off!'
fi