psql - Role root does not exist

I build a bash command in a PHP script. The built command is such as : ``` su postgres -c "for tbl in `psql -qAt -c \"select tablename from pg_tables where schemaname = 'public';\" demodoo` ;do psql -c \"alter table $tbl owner to postgres\" demodoo ;done " ``` When I try to run this command in a shell, I get this error : ``` psql: FATAL: role "root" does not exist ``` Why is this occuring, whereas i execute the command under postgres user ? Thanks Cheers, EDIT I change the command to ``` sudo -u postgres for tbl in `psql -qAt -c "select tablename from pg_tables where schemaname = 'public';" demodoo` ;do psql -c "alter table $tbl owner to postgres" demodoo ;done ``` but now I get another error which I can't understand the origin : ``` -bash: syntax error near unexpected token `do' ```