Add a field

[elastic-fan]

hi,
i am getting logs from the server and in logstash i would like to add a field and value for it how can i do this

[magnusbaeck (Magnus Bäck)]

For example like this:

filter {
  mutate {
    add_field => {
      "fieldname" => "fieldvalue"
    }
  }
}

[elastic-fan]

@magnusbaeck

add_field => {
"fieldname" => "fieldvalue"
}

thanks