backup file system

Hi :

I am very new in postgres and hope someone can give me some suggestions

I am working on a shell script to backup my postgres database. Compared
with pg_dump and pg_dumpall, I would
like to backup the file system instead of dump the database because I can
shutdown the database server in
midnight for a few hours. I also can sure I will have a stable backup. ( No
user is doing anything at the same time )

I search internet but did not find anyone mention this method. Did I miss
anything? I plan to backup all
files under data directory. Is that enough?

thanks
Fushan
DBA