SQL & PL/SQL

As part of a bigger set of Windows/DOS batch scripts I am running I want to be able to create a .txt file containing just one line and this line should be the SID of the database.

I was thinking that a SQL*Plus script using spool would enable this, however I cannot work out how to get simply the SID written into the resultant .txt file created by the spool.

Any suggestions on how I might achieve this much appreciated.

Thanks,
Andy