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.
As of version 0.14.0 rosdep update will only fetch ROS package names for non-EOL ROS distributions. If you are still using an EOL ROS distribution (which you probably shouldn't) you can pass the argument --include-eol-distros to also fetch the ROS package names of those.
Install dependency of a particular package
You can use rosdep and catkin 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
Install dependency of all packages in the workspace
This usecase shows even more powerful feature of rosdep.
Go to the top directory of your catkin workspace where the source code of the ROS packages you'd like to use are. Then run:
rosdep install --from-paths src --ignore-src -r -y
This command magically installs all the packages that the packages in your catkin workspace depend upon but are missing on your computer.
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.