first run, on a fresh database WITHOUT any nbackups
Following is the Console displaying:

2007-09-29-17-58-14.
Resource id #2
SELECT RDB$BACKUP_LEVEL FROM RDB$BACKUP_HISTORY

now begin to do the nbackups

the current directory:/usr/local/apache2/htdocs
change directory to /opt/firebird/bin
the current directory:/opt/firebird/bin
./nbackup -U sysdba -P masterkey -B 0 /home/myname/employee.fdb
/tmp/bak/employee_0-2007-09-29-17-58-14.nbk
nbackup result:

now change back to starting directory
the current directory:/usr/local/apache2/htdocs

system re-check if all nbackup files in position
file name: .
file name: employee_0-2007-09-29-17-58-14.nbk
file name: ..

nbackup file successful: /tmp/bak/employee_0-2007-09-29-17-58-14.nbk

confirm if nbackup is successful by re-connect the database
SELECT RDB$FILE_NAME FROM RDB$BACKUP_HISTORY
Resource id #2
/tmp/bak/employee_0-2007-09-29-17-58-14.nbk

disconnect from database OK !