An example on how to do idempotent PATH manipulation.

idempotent_path_add.sh

#!/usr/local/bin/bash
#How to manipulate the path in an intelligent fashion with bash. Note: this doesn't work with Bourne shell.

function idempotent_path_add {
	DIR="$1"
	PREPEND=$2
	if [[ ! "$PATH" =~ (^|:)"$DIR"(:|$) ]]
	then
		if [ $PREPEND ]
		then
			PATH="$DIR:$PATH"
		else
			PATH="$PATH:$DIR"
		fi
	fi
}

#examples:
#Add /usr/local/bin to the end of the path
# idempotent_path_add "/usr/local/bin"
#Add /sbin to the beginning of the path
# idempotent_path_add "/sbin" 1