colcon build 使用问题

[1628799315]

1. source 环境后 还是会出现 警告

foxy@ubuntu:~/robot_ws$ colcon build
[2.986s] WARNING:colcon.colcon_core.prefix_path.colcon:The path '/home/foxy/install' in the environment variable COLCON_PREFIX_PATH doesn't exist
[2.987s] WARNING:colcon.colcon_ros.prefix_path.ament:The path '/home/foxy/install/mqtt_ros_pkg' in the environment variable AMENT_PREFIX_PATH doesn't exist
[2.987s] WARNING:colcon.colcon_ros.prefix_path.catkin:The path '/home/foxy/install/mqtt_ros_pkg' in the environment variable CMAKE_PREFIX_PATH doesn't exist

2. colcon build 为什么 经常 第一次编译黄色警告编译失败，第二次 就能成功