postgres on the comman line

Hello,

I like to automate a daily check and like to run select statement via
cron but this seems to be more tricky than I thought....

I tried the following:
su postgres -c "select count(*) from TABLE where xxxxxx;"

I have the probelm that I am allways asked for the password - I did not
find a way how to pass the password. 

Also I like to direct the result to a file, I assume I can do this via >
but not haveing passed the first problem I did not check this. 

I am happy for every hint

Thanks a lot 
Michael