postalias
This command is used to create the alias map.

# postalias hash:/etc/postfix/aliases

You will have to create the /etc/postfix/aliases file in order to run the command. Note: the /etc/aliases file is created by default and you can use this file by running the command newaliases.

Edit your /etc/aliases file and enter an email address for root and then run the command:

newaliases

Here is an example entry in /etc/aliases:

root:             john