rosdep is a command-line tool for installing system dependencies.
Do prefer apt over pip as it's updated automatically and faster.
sudo apt-get install python-rosdep
On non ubuntu platforms
sudo pip install -U rosdep
Needs to be called only once after installation:
sudo rosdep init
Needs to be called at least once or when updating (there might be new needed definitions your local installation of rosdep doesn't know about yet):
Do NOT run rosdep update with sudo. It is not required and will result in permission errors later on.
You can use rosdep and rosmake to build any package in the ROS repository. Say that you want to build a package called AMAZING_PACKAGE, that you hadn't built on your machine before.
rosdep install AMAZING_PACKAGE
Use source-installed rosdep
Clone the development repository. This doesn't need to be done in a directory in ROS_PACKAGE_PATH. Then at its top directory, source setup.sh. In concrete:
$ git clone https://github.com/ros-infrastructure/rosdep $ cd rosdep $ source setup.sh
Then calling rosdep should use the source just downloaded.