pg_upgrade_server.log file location

Hi,

I am upgrading Postgres 9.4 to 9.6 on Ubuntu 16.04. I am getting below error while running pg_upgrade command. Does anyone know "pg_upgrade_server.log" file location? I am not able to find log file.

Command :# sudo -H -u postgres /usr/lib/postgresql/9.6/bin/pg_upgrade -b /usr/lib/postgresql/9.4/bin -B /usr/lib/postgresql/9.6/bin -d /var/lib/postgresql/9.4/main -D /var/lib/postgresql/9.6/main

Error:
*failure*
Consult the last few lines of "pg_upgrade_server.log" for
the probable cause of the failure.

There seems to be a postmaster servicing the old cluster.
Please shutdown that postmaster and try again.
Failure, exiting

Thanks,
Ashish Chauhan