These instructions assume you are in ROS noetic and ubuntu 20.04.
The default installation seems to have problems with single camera setups, see the appropriate section.
To have this working you need to first of all install the ROS dependencies.
rosdep install --from-paths src --ignore-src -r -y
Then, you will need to install the udev rules for the realsense cameras. You can do this by installing the udev rule package from the non-ROS apt repo.
sudo apt-key adv --keyserver keys.gnupg.net --recv-key F6E65AC044F831AC80A06380C8B3A55A6F3EFCDE || sudo apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-key F6E65AC044F831AC80A06380C8B3A55A6F3EFCDE
sudo add-apt-repository "deb https://librealsense.intel.com/Debian/apt-repo focal main" -u
sudo apt update
sudo apt install librealsense2-udev-rules
You should now reload the udev rules (or reboot):
sudo udevadm control --reload-rules
sudo udevadm trigger
Now it should be working.
If you are using multiple cameras, it may be better to install the non-ROS version of librealsense, which uses a different usb backend.
To do so you will need to set the appropriate PPA and install librealsense from apt. Then you will need to get realsense-ros from source, cloning it in your catkin workspace. Remember to remove the dependency in librealsense from realsense-ros package.xml, if you don't rosdep will install also the ros version of librealsense, breaking your installation.
The ROS librealsense installation does not include the tools (realsense-viewer, rs-enumerate-devices...)
To have them you can clone the librealsense repo and build it, WITHOUT INSTALLING IT.
So:
https://github.com/IntelRealSense/librealsense.git
cd librealsense
mkdir build
cd build
cmake ../
make
You can now find the binaries for the tools in the 'tools' folder