I have my ORACLE_SID set in my .bash_profile. I echo it and its there. I have until yesterday afternoon been able to log in without having to enter my ORACLE_SID so I do login: username/password now oracle needs my oracle sid. I think my sysadmin changed something. I do not know what it is. when do you need to enter a SID when you log into oracle from a unix/linux shell and when don't you?