This is because you haven't set your ORACLE_HOME environment variable. This should be set to the directory in which you have installed your client libraries e.g.
#for ORACLE_HOME ORACLE_HOME=/home/oracle/OraHome1
Note If you are using the Instant Client, you will need to set LD_LIBRARY_PATH instead of ORACLE_HOME. Set LD_LIBRARY_PATH to specify the location where the Instant Client is installed. For example:
LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/lib/instantclient export LD_LIBRARY_PATH
There are other environments that you will probably also need to set:
#for Easysoft EASYSOFT_HOME=/usr/local/easysoft export EASYSOFT_HOME #for libclntsh.so LD_LIBRARY_PATH=LD_LIBRARY_PATH:$ORACLE_HOME/lib export LD_LIBRARY_PATH #for libodbcinst.so LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$EASYSOFT_HOME/unixODBC/lib export LD_LIBRARY_PATH #for isql PATH=$PATH:$EASYSOFT_HOME/unixODBC/bin export PATH
Please be aware that the command LD_LIBRARY_PATH will be different across the various flavours of Unix and also the file extensions will differ.