csh or tcsh:
setenv ROOTSYS pathname
set path=($path $ROOTSYS/bin)
setenv LD_LIBRARY_PATH ${LD_LIBRARY_PATH}:${ROOTSYS}/lib:.
ksh or bash:
export ROOTSYS=pathname
export PATH=$PATH:$ROOTSYS/bin
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$ROOTSYS/lib:.
FNAL: ROOTSYS=/afs/fnal.gov/files/code/cms/l/external/gcc-3.2/CMS_SOFT/lcg/external/rootcore/3.10.02/rh73_gcc32
CERN: ROOTSYS=/afs/cern.ch/cms/external/lcg/external/root/3.10.02/rh73_gcc32/root