Unix backup script problem

We are trying to use cron and a Unix script file to shut down Oracle every Sunday at 6:00am, do a full backup of the database, and restart the database. Everything works fine when we go through the process manually, but when cron actually runs the script we get the following:

     ORA-01034: ORACLE not available
     ORA-07429: smsgsg: shmget() failed to get segment.
     SVR4 Error: 2: No such file or directory

Oracle *is* up and running when the script gets these messages. Can anybody tell me why the script works when we enter the commands manually but croaks when we have cron submit it? I can post the script itself if it is necessary.

Thanks,

Keith

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                      Keith A. Marrocco
                      Oracle and Rdb Support
                      Computing and Information Services
                      Systems Group Database Team
                      Texas A&M University
                      (409) 845-1037