when trying to use oracle database with different user, not oracle

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ERROR:
ORA-01034 (See ORA-01034.ora-code.com): ORACLE not available
ORA-27123 (See ORA-27123.ora-code.com): unable to attach to shared memory 
segment Linux Error: 13:
Permission denied 

This may be solved following way:

# chmod 6751 $ORACLE_HOME/bin/oracle 


when trying to use oracle database with different, not oracle user
with following errors:

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# sqlplus /nolog

ERROR:
ORA-09925: Unable to create audit trail file
Linux Error: 13: Permission denied
Additional information: 9925
ORA-01017: invalid username/password; logon denied
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Then


Add user to dba group
# gpasswd -a user dba
Then 
# cd $ORACLE_HOME/rdbms/
# chmod -R g+w audit
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That should solve an issue
with following output

SQL>  connect sys/change_on_install as sysdba
Connected to an idle instance

Now you can start or stop database :)

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 start database

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dbstart
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shutdown database

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dbshut
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How to tart listener

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# lsnrctl

LSNRCTL for Linux: Version 9.2.0.4.0 - Production on 04-MAY-2006 17:41:05

Copyright (c) 1991, 2002, Oracle Corporation.  All rights reserved.

Welcome to LSNRCTL, type "help" for information.

LSNRCTL> 
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Then type start or stop :)