If Condition Types in Bash

Multi Line:

if [[ $INSTALLED != "yes" ]]; then
	echo "Error message."
fi

if [[ $INSTALLED != "yes" ]]; then
	echo "Error message."
else
	echo "Message."
fi

One Line:

if [[ $INSTALLED != "yes" ]];then echo "Error message."; fi

if [[ $INSTALLED != "yes" ]];then echo "Error message."; else echo "Message."; fi

Another One Line Method:

[[ $INSTALLED != "yes" ]] && echo "Error message." || sleep 0

[[ $INSTALLED != "yes" ]] && echo "Error message." || echo "Message."