Two things: If /opt/ros/hydro/share:/opt/ros/hydro/stacks is already in ROS_PACKAGE_PATH, you don't need to put it in your export line. This command overwrites the variable, and by putting :$ROS_PACKAGE_PATH at the end of your path, you're say add everything that is already in the variable. Second, I don't know if it matters, but I would try exactly what you have but without the space between the equals sign and the beginning of your path, like this: export ROS_PACKAGE_PATH=/home/user/<name_of-directory_i_want_to_add>:/opt/ros/hydro/share:/opt/ros/hydro/stacks:$ROS_PACKAGE_PATH
Two things: If /opt/ros/hydro/share:/opt/ros/hydro/stacks is already in ROS_PACKAGE_PATH, you don't need to put it in your export line. This command overwrites the variable, and by putting :$ROS_PACKAGE_PATH at the end of your path, you're say add everything that is already in the variable. Second, I don't know if it matters, but I would try exactly what you have but without the space between the equals sign and the beginning of your path, like this: export ROS_PACKAGE_PATH=/home/user/<name_of-directory_i_want_to_add>:/opt/ros/hydro/share:/opt/ros/hydro/stacks:$ROS_PACKAGE_PATH