After installing Radiance on a Linux system, you must modify your environment a bit. You need to add the path to the Radiance executables to your PATH environment var, and you must add an environment variable called RAYPATH, which needs to point to the Radiance library directory.

Backing up, just how did you install Radiance? For Linux, the options are building from source, getting an archive of pre-built binaries from NREL (but these are built on Ubuntu) and putting them in the right place(s), or installing an older version from rpm.

Assuming you have everything where it needs to be (and we'll assume for this example the executables are in /usr/local/radiance/bin and libs are in /usr/local/radiance/lib), you'd need to add the following to your config (recommend ~/.bash_profile):

export PATH=$PATH:.:/usr/local/radiance/bin
export RAYPATH=.:/usr/local/radiance/lib

This will allow your system to run Radiance. As Mostapha has pointed out, even after all this, typing radiance at the command prompt will not give the desired result. Hopefully you were trying to execute an actual Radiance command, or a script or something. If not, I highly recommend you check out one of the Radiance tutorials out there.