Oracle needs to have a set of context variables that allow Oracle to know where the programmes are located. Upon installation, you have to check if these variabled are set. If not, you have to set them yourself. In linux, such variables can ne set in the .bash_profile file that is stored as a file in the home directory of the oracle user. This file looks like:
export ORACLE_HOSTNAME=localhost.domain
export ORACLE_UNQNAME=ORCL
export ORACLE_BASE=/orasoft
export ORACLE_HOME=/orasoft
export ORACLE_SID=orcl
PATH=$PATH:$HOME/bin
export PATH=$ORACLE_HOME/bin:$PATH
The hostname is a name of the server that can also be stored in the hosts file. This then allows the system to refer to a certain IP number that is connected to the server.
The oracle_sid is the name that is also used when one uses the tnsping command or when someone wants to set up a connection to oracle.