rosmsg show beginner_tutorials/Num can‘t work

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Maybe you haven't defined correctly your workspace. The issue is, you have to add your workspace path to the file .bashrc The command is: sudo gedit ~/.bashrc, and the end of the document you must write these lines:

1. First of all, you should write your source, I mean, for example:

source ~/my_workspace

1. Then, you add to the ROS_PACKAGE_PATH the path of your workspace, I mean, the path of the folder that you want to be your workspace, for example:

export ROS_PACKAGE_PATH=/home/user/my_workspace:$ROS_PACKAGE_PATH

1. Finally you must export your new workspace too. The command is

export ROS_WORKSPACE=/home/user/my_workspace

I hope you solve your problem!