Ubuntu which command find nothing

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In ROS 2.0 tutorial : Using ament to build a custom package: Create an overlay
It says that we can use Ubuntu command which to see the path before and after we source ~/ros2_overlay_ws/install/local_setup.bash. But it just shows nothing as I typed which talker. I also tried my ros working space, it shows nothing, too. No matter I source setup.bash/local_setup.bash or not. If I type which ls, it works fine to show me /bin/ls. Although I can still run these ros package correctly, but I'm really curious to know what's wrong in my environment setting, should I set any ros environment variable to let which command works fine?