"sudo -u redash psql -d redash" results in "unknown user: redash"

I want to back up the data. But, I can’t even access the database. “sudo -u redash psql -d redash” does’t work. The result is that the message “sudo: unknown user: redash” comes out. What should I do. Could anyone help me?

The error is from your host, not from Redash. It seems that your UNIX host does not have a user named redash. How did you install Redash?

Thanks for your reply. I installed Redash using AWS EC2 AMI. I have never created username which is used that EC2 instance on my own. So, I thought username was autmatically created and named redash. The reason why I thought it was named redash is that there are many blogs that uses “sudo -u redash psql redash” when backing up. How can I know username which is used in my EC2 instance?