Oraenv
From Oracle FAQ
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Oraenv and coraenv are Unix/ Linux command line utilities that sets the required environment variables (ORACLE_SID, ORACLE_HOME and PATH) to allow a user to connect to a given database instance. If these environment variables are not set, commands such as SQL*Plus, imp, exp, or any other utility for that matter, will not work (or not be found).
Use coraenv when using the C Shell and oraenv when using a Bourne, Korn or Bash shell.
Note the syntax: ". oraenv" or {period} {space} oraenv.
Examples
Interactive:
$ . oraenv ORACLE_SID = [] ? orcl
Non-interactive (handy for scripting):
$ export ORACLE_SID=orcl $ export ORAENV_ASK=NO $ . oraenv