Administration in Linux

Here is a link from Oracle Documentation website about administering Oracle on Linux http://docs.oracle.com/cd/E11882_01/server.112/e10839/appc_linux.htm#UNXAR011

A question mark ? represents the value of he ORACLE_HOME environment variable.

SQL> ALTER TABLESPACE TEMP ADD DATAFILE '?/dbs/temp02.dbf' SIZE 200M

The at sign @ represents the ORACLE_SID environment variable

SQL> ALTER TABLESPACE tablespace_name ADD DATAFILE tempfile@.dbf

To display the current value of all environment variables, run the env command as follows:

$ env | more