How do I install a package on OSX without recompiling everything?

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The recommended workflow is to build another workspace on top of the one in ~/ros_catkin_ws. Also for catkin, you should use rosinstall_generator and wstool. So you would do something like this:

mkdir ~/my_ws
cd ~/my_ws
wstool init src
source ~/ros_catkin_ws/install_isolated/setup.bash
rosinstall_generator PACKAGE --deps --exclude RPP | wstool merge -t src -
wstool update -t src
rosdep install --from-paths src --ignore-src
catkin_make_isolated --install
source ~/my_ws/install_isolated/setup.bash

wstool is the ROS agnostic replacement for rosws, and has most of the same functions, but you normally have to specify the target folder to act on, hence the -t src in the merge command. The wstool update -t src command will cause the packages you just added to be fetched.

rosinstall_generator is pretty new, and currently can only operate on released packages, but should be able to get packages from source in the future. The --deps --exclude RPP options to rosinstall_generator means get me that PACKAGE and its dependencies, recursively, excluding dependencies in my ROS_PACKAGE_PATH. This is the best way to create an "extension" on top of your existing workspace of ROS packages.

The --from-paths src and --ignore-src arguments to rosdep indicate that it should install all of the dependencies of packages found in the folder src and that it should not try to install packages found of the ROS_PACKAGE_PATH respectively.

Hopefully that helps, we are working on improving these tools and getting tutorials setup to give them more visibility to them.