why catkin package is still needed at ROS runtime?

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Hi all,

This is probably a stupid question, but I can't wave it out of my head... Why we still need catkin when we run ROS? Isn't it a package building tool? Why it's still needed at runtime?

Because in my understanding, catkin is a tool we used to build packages, almost like an enhanced version of cmake. So normally cmake is not needed when we run executables right?

But when I tried to run ROSCORE in my own environment without catkin package, I got errors when importing.

$ roscore

Traceback (most recent call last): File "/sdcard/ros_runtime/runtime_env/bin/roscore", line 72, in <module> import roslaunch File "/sdcard/ros_runtime/runtime_env/lib/python2.7/dist-packages/roslaunch/__init__.py", line 57, in <module> from .launch import ROSLaunchRunner File "/sdcard/ros_runtime/runtime_env/lib/python2.7/dist-packages/roslaunch/launch.py", line 55, in <module> from roslaunch.nodeprocess import create_master_process, create_node_process File "/sdcard/ros_runtime/runtime_env/lib/python2.7/dist-packages/roslaunch/nodeprocess.py", line 50, in <module> from roslaunch.node_args import create_local_process_args File "/sdcard/ros_runtime/runtime_env/lib/python2.7/dist-packages/roslaunch/node_args.py", line 53, in <module> import roslib.packages File "/sdcard/ros_runtime/runtime_env/lib/python2.7/dist-packages/roslib/__init__.py", line 54, in <module> import roslib.stacks File "/sdcard/ros_runtime/runtime_env/lib/python2.7/dist-packages/roslib/stacks.py", line 46, in <module> import roslib.packages File "/sdcard/ros_runtime/runtime_env/lib/python2.7/dist-packages/roslib/packages.py", line 49, in <module> from catkin.find_in_workspaces import find_in_workspaces as catkin_find ImportError: No module named catkin.find_in_workspaces

Does catkin serve as other functions other than a building tool? What's the concept of "workspace" here? It is different than "build" and "devel" workspace right?

Thanks a lot.