For whatever reason you may want to save the last command you wrote. Heres a way to do so.
trap 'previous=$this_command; this_command=$BASH_COMMAND' DEBUG
echo "Hello"
#This line removes the trap you just set.
#If you don't remove this trap,
#it will continue to override "previous"
trap - DEBUG
#now lets echo out $previous
echo $previous;
exit;
this command allows you to only capture commands that through an error
trap "command=$BASH_COMMAND" ERR