一、具体问题
电脑硬盘不够用了，故加装硬盘重装系统和ROS，在执行
sudo rosdep init rosdep update
这两步时都很顺利，下面需要安装一些ROS依赖，却遇到了如下报错：
ERROR: the following packages/stacks could not have their rosdep keys resolved to system dependencies: simple_models_description: Cannot locate rosdep definition for [urdf_test] pmb2_rgbd_sensors: Cannot locate rosdep definition for [openni2_launch] omni_base_description: Cannot locate rosdep definition for [urdf_test] pmb2_description: Cannot locate rosdep definition for [urdf_test] pal_navigation_cfg_pmb2: Cannot locate rosdep definition for [spatio_temporal_voxel_layer] pal_navigation_cfg_pmb3: Cannot locate rosdep definition for [spatio_temporal_voxel_layer] pal_navigation_cfg_ari: Cannot locate rosdep definition for [slam_toolbox] pal_gripper_description: Cannot locate rosdep definition for [urdf_test] tiago_description: Cannot locate rosdep definition for [urdf_test] omni_base_laser_sensors: Cannot locate rosdep definition for [ira_laser_tools] social_navigation_layers: Cannot locate rosdep definition for [people_msgs] pal_navigation_cfg_tiago: Cannot locate rosdep definition for [spatio_temporal_voxel_layer] pmb2_laser_sensors: Cannot locate rosdep definition for [depthimage_to_laserscan] tiago_bringup: Cannot locate rosdep definition for [usb_cam] tiago_laser_sensors: Cannot locate rosdep definition for [ira_laser_tools] tiago_rgbd_sensors: Cannot locate rosdep definition for [openni2_launch] omni_base_bringup: Cannot locate rosdep definition for [twist_mux] pmb2_bringup: Cannot locate rosdep definition for [twist_mux] pal_robotiq_controller_configuration_gazebo: Cannot locate rosdep definition for [roboticsgroup_upatras_gazebo_plugins] pal_navigation_cfg_tiago_dual: Cannot locate rosdep definition for [spatio_temporal_voxel_layer] four_wheel_steering_controller: Cannot locate rosdep definition for [urdf_geometry_parser]
起初怀疑过是不是网络问题、rosdep的问题、conda环境的问题，但都没有解决，甚至卸载重装了一遍rosdep，最终找到解决方案
二、具体环境
Ubuntu 18.04 + ROS melodic
三、解决方法
上述问题其实和之前提到的想法没有关系，是在安装ROS的过程中遗漏了很多包没有安装，我意识到是因为缺少了报错中【】中的内容
因此拿第一行报错举例
simple_models_description: Cannot locate rosdep definition for [urdf_test]
解决这行的问题，就使用
sudo apt-get install ros-melodic-urdf-test
来进行安装，如何以此类推，将报错中所有的框框里的包，用
sudo apt-get install ros-你的ROS版本-包名
的方式进行单独安装，记住，报错中包名里面的下划线“_”，在安装时需要改成短横线“-”
在全部安装完成后，重新执行安装依赖的指令
最后出现#All required rosdeps installed successfully就没问题了
