Path to dynamic libraries in ROS root system

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I don't really see any difference between the two cases you give us (both are installed system-wide by the apt package manager, they're not 'setup' by ROS nor Gazebo).

On Debian/Ubuntu, you could use something like dpkg -S <name_of_the_file> to find both the package name and the location of the file. In your case:

dpkg -S libgazebo_ros_camera.so

Alternatively, you could use a command like locate libgazebo_ros_camera.so, which should give you the location as well (but it depends on whether the db has been updated).

To find out what files were installed by apt for a specific package, you could use:

dpkg -L libgazebo6-dev

you could add | grep .so to find just the shared libraries, or | grep lib to find all occurrences of lib in the listing.

PS: note that this is not a ROS-specific problem, but rather a question about interacting with the apt package manager on a Debian/Ubuntu system.