Note that you would need to add this user to the list of sudoers first which the documentation hadn't mentioned. I ran into the same while building the ambari service. See https://issues.apache.org/jira/browse/NIFI-930
By Default NiFi will run as what ever user starts NiFi.
NiFi can however be configured to start as a specific user by setting the run.as= property inside the bootstrap.conf file.
*** Note that the user configured in this property will need to have sudo privileges to the java executable on linux based deployments. This could interfere with some processors that are dependent on the java process being owned by a particular user since it will be owned by root.
*** Setting the run.as user allows you to setup NiFi as a service that can be configured to start as part of the OS starting.