While adding an alias to a host is not really hard, what's usually more interesting is to ensure a host entry has an alias, i.e. make the operation idempotent.
Here is how you can do that with Augeas:
set /files/etc/hosts/*[ipaddr="127.0.0.1"]/alias[.="mycouchdb"] "mycouchdb"
which will only add the alias if it doesn't exist yet.
alias[.="mycouchdb"] refers to the alias with value
. refers to the current node).
When there is no alias yet with value
alias[.="mycouchdb"] will not match anything and Augeas will create a node with label
alias and value
mycouchdb. The rule when the node doesn't exist is to use the path label without filters, in this case
alias, so it creates a new label node and assigns it the value
When there is an alias already, the expression will match and the set command will replace the value with
mycouchdb, which will do nothing.