http://www.dba-oracle.com/t_linux_profile_alias_change_oracle_database.htm

Many folks do not like to use the "oraenv" script to change Oracle environments, and some Oracle professionals write a Linux login script to perform the same task.

The script resides in the Linux users "profile" (i.e. .profile, .cshrc), and it build an alias with the same name as each $ORACLE_SID.  

Once in-place, this alternative to oraenv allows you to change database environments by entering the ORACLE_SID name:

 for DB in `cat /etc/oratab|grep -v \#|grep -v \*|cut -d":" -f1`
 do
   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;\
   export SCRIPT_HOME=$DBA/scripts;\
   export PATH=$PATH:$SCRIPT_HOME;\
   export LIB_PATH=$ORACLE_HOME/lib64:$ORACLE_HOME/lib '
 done

Now, simply enter the ORACLE_SID as a UNIX alias, and the alias will set your Oracle environment.