Revised Answer (Feb 12, 2014)
the_world_is_flat=true # ...do something interesting... if [ "$the_world_is_flat" = true ] ; then echo 'Be careful not to fall off!' fi
the_world_is_flat=true # ...do something interesting... if $the_world_is_flat ; then echo 'Be careful not to fall off!' fi
From: Using boolean variables in Bash
The reason the original answer is included here is because the comments before the revision on Feb 12, 2014 pertain only to the original answer, and many of the comments are wrong when associated with the revised answer. For example, Dennis Williamson’s comment about bash builtin
true on Jun 2, 2010 only applies to the original answer, not the revised.