Only supports JWT authentication currently, and only tested against Google's API https://developers.google.com/accounts/docs/OAuth2ServiceAccount
To get going quickly with service authentication by creating a JWT copy config/example.sys.config to config/sys.config and insert the appropriate values for the oauth2 service you are authenticating against.
Running make shell with pull the deps if needed, compile the project and load an Erlang shell with the proper paths set. It assumes a config/sys.config file exists, which you can then use to retrieve the necessary variables to pass to eoauth2:jwt_access_token/6 to retrieve a token.
$ make shell
Erlang R16B01 (erts-5.10.2) [source-bdf5300] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]
Eshell V5.10.2 (abort with ^G)
1> ssl:start().
ok
2> application:load(eoauth2).
ok
3> {ok, Iss} = application:get_env(eoauth2, iss).
{ok,<<"............@developer.gserviceaccount.com">>}
4> {ok, Aud} = application:get_env(eoauth2, aud).
{ok,<<"https://accounts.google.com/o/oauth2/token">>}
5> {ok, Scope} = application:get_env(eoauth2, scope).
{ok,<<"https://www.googleapis.com/auth/.........">>}
6> {ok, EncodedPrivateKey} = application:get_env(eoauth2, encoded_private_key).
{ok,<<"-----BEGIN RSA PRIVATE KEY-----\n.........">>}
7> {ok, Host} = application:get_env(eoauth2, host).
{ok,<<"https://........">>}
8> {ok, Path} = application:get_env(eoauth2, path).
{ok,<<"/......">>}
9> eoauth2:jwt_access_token(Host, Path, Iss, Scope, Aud, EncodedPrivateKey).
{ok, <<".........................">>}