linux, fb1.5.1 classic, login = fred

I have 2 databases I want to backup

this works:
-rw-rw-rw- 1 fred fred 2473984 Aug 17 09:25 yyy.fdb
gbak -v -user sysdba -pass masterkey -backup yyy.fdb /tmp/yyy.fbk

this doesn't:
-rwxr--r-- 1 firebird firebird 10092544 Aug 8 13:59 xxx.fdb
gbak -v -user sysdba -pass masterkey -backup xxx.fdb /tmp/xxx.fbk
with:
gbak: ERROR: lock conflict on no wait transaction
gbak: ERROR: no permission for read-write access to database xxx.fdb
gbak: Exiting before completion due to errors
gbak: ERROR: cannot open backup file /tmp/xxx.fbk
gbak: Exiting before completion due to errors

It's a file permission problem as changing the second case to -rw-rw-rw-
allows it to work
but I don't see why it should happen. I'm not accessing the file directly
but via the firebird server,
running as firebird, which does have full permission


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