output results of many queries to one log file

Wayne_1

I am trying to output results of many queries to one log file in vsql. Here is all commands.
\set t_pwd 'pwd'
\set t_date 'date "+%Yy%mm%dd%Hh%Mm%Ss"'
\set log_file  '\'':t_pwd'/log/incr_etl_load_':t_date'.log\''
\echo :log_file
\o :log_file

error message:
'/data/vertica/clinical/covenant/log/incr_etl_load_2014y04m15d10h32m28s.log': No such file or directory

Anybody knows what the problem is? If I hardcode lie this way. it works okay.
\set log_file '/data/vertica/clinical/covenant/log/incr_etl_load_2014y04m15d10h32m28s.log'
\o :log_file

Anybody can help? thanks.