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Don Burleson Blog 







Change Oracle environment in UNIX/Linux

Oracle Database Tips by Donald BurlesonApril 28, 2015

Question:  How do I change my Oracle environment in UNI/Linux?  I want to change from one instance to another instance with a Linux command.

Answer:  A typical problem on any UNIX server is quickly setting the many Oracle environment variables necessary to change the Oracle environment to a different database.
Oracle provides a command script called oraenv to reset the Oracle environment, but it often does not work properly, and most experienced DBAs know that the following commands must be issued to change from one ORACLE_SID to another:

       export ORAENV_ASK=NO;\
       export ORACLE_SID='$DB';\
       export ORACLE_HOME;\
       export ORACLE_BASE=\
         `echo ORACLE_HOME | sed -e 's:/product/.*::g'`;\
       export DBA=$ORACLE_BASE/admin;\
       export SCRIPT_HOME=$DBA/scripts;\
       export PATH=$PATH:$SCRIPT_HOME;\
       export LIB_PATH=$ORACLE_HOME/lib:$ORACLE_HOME/lib:/usr/lib '

Most DBAs create a UNIX alias with the same name as the ORACLE_SID.  When the ORACLE_SID is entered at the command prompt, all of the required commands are executed by means of a shell script.  Below is the login profile code to perform this function.
   # For every Oracle_SID in /var/opt/oracle/oratab,
   # create an alias using the SID name.
   # Now, entering the ORACLE_SID at the UNIX prompt will completely set the
   # UNIX environment for that SID

   for DB in `cat /var/opt/oracle/oratab| \
   grep -v \#|grep -v \*|cut -d":" -f1`
      alias $DB='export ORAENV_ASK=NO; export ORACLE_SID='$DB'; .
TEMPHOME/bin/oraenv; export ORACLE_HOME; export ORACLE_BASE=`echo
ORACLE_HOME | sed -e 's:/product/.*::g'`; export DBA=$ORACLE_BASE/admin;
xport SCRIPT_HOME=$DBA/scripts; export PATH=$PATH:$SCRIPT_HOME; export
IB_PATH=$ORACLE_HOME/lib:$ORACLE_HOME/lib:/usr/lib '

Next, examine how this works. First there is a FOR loop in UNIX.  Decompose this command and see what it is doing:
for DB in `cat /var/opt/oracle/oratab|\
grep -v \#|grep -v \*|cut -d":" -f1`

The for DB in command means that the script will loop once for each value of $DB.  The argument to the for DB in command is enclosed in graves (pronounced "gra-vees"), which is the back-tick character (directly above the tab key on a PC keyboard).  Arguments enclosed in graves tell UNIX to execute the command enclosed in the graves and return the result set to UNIX. In this case, the command in the graves does the following:

cat the var/opt/oracle/oratab file (/etc/oratab in AIX).  This lists all databases defined on the UNIX server:

root>cat /var/opt/oracle/oratab


Next, notice the grep –v \# and the grep –v \* commands.  These ignores any lines in the oratab file that are commented-out:

root>cat /var/opt/oracle/oratab|grep –v \#|grep –v \*

Then issue the cut -d":" -f1 command.  This extracts the first column in the oratab file using the colon ":" as the column delimiter:

root>cat /var/opt/oracle/oratab|grep –v \#|grep –v \*|cut –d":" –f1

There is now a list of valid $ORACLE_SID values.  Inside the for loop, create an alias with the value of $DB (The $ORACLE_SID name), and perform all of the required changes to reset the UNIX environment for that database.
Now turn to the processes of dissecting complex UNIX commands.  An Oracle DBA must be able to interpret complex UNIX commands.

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