How do I authorise an app (web or installed) without user intervention?

This can be done with the Oauth2 Playground at https://developers.google.com/oauthplayground

Steps:-

1. Create the Google Account (eg. [email protected]) – Or skip this step if you are using an existing account.
2. Use the API console to register the mydriveapp (https://console.developers.google.com/apis/credentials/oauthclient?project=mydriveapp or just https://console.developers.google.com/apis/)
3. Create a new set of credentials. Credentials/Create Credentials/OAuth Client Id then select Web application
4. Include https://developers.google.com/oauthplayground as a valid redirect URI
5. Note the client ID (web app) and Client Secret
6. Login as [email protected]
7. Go to Oauth2 playground
8. In Settings (gear icon), set
   • OAuth flow: Server-side
   • Access type: Offline
   • Use your own OAuth credentials: TICK
   • Client Id and Client Secret: from step 5
9. Click Step 1 and choose Drive API v3 https://www.googleapis.com/auth/drive (having said that, this technique also works for any of the Google APIs listed)
10. Click Authorize APIs. You will be prompted to choose your Google account and confirm access
11. Click Step 2 and “Exchange authorization code for tokens”
12. Copy the returned Refresh token and paste it into your app, source code or in to some form of storage from where your app can retrieve it.

Your app can now run unattended, and use the Refresh Token as described https://developers.google.com/accounts/docs/OAuth2WebServer#offline to obtain an Access Token.

NB. Be aware that the refresh token can be expired by Google which will mean that you need to repeat steps 5 onwards to get a new refresh token. The symptom of this will be a Invalid Grant returned when you try to use the refresh token.

NB2. This technique works well if you want a web app which access your own (and only your own) Drive account, without bothering to write the authorization code which would only ever be run once. Just skip step 1, and replace “my.drive.app” with your own email address in step 6. make sure you are aware of the security implications if the Refresh Token gets stolen.

See Woody’s comment below where he links to this Google video https://www.youtube.com/watch?v=hfWe1gPCnzc

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Here is a quick JavaScript routine that shows how to use the Refresh Token from the OAuth Playground to list some Drive files. You can simply copy-paste it into Chrome dev console, or run it with node. Of course provide your own credentials (the ones below are all fake).

function get_access_token_using_saved_refresh_token() {
    // from the oauth playground
    const refresh_token = "1/0PvMAoF9GaJFqbNsLZQg-f9NXEljQclmRP4Gwfdo_0";
    // from the API console
    const client_id = "559798723558-amtjh114mvtpiqis80lkl3kdo4gfm5k.apps.googleusercontent.com";
    // from the API console
    const client_secret = "WnGC6KJ91H40mg6H9r1eF9L";
    // from https://developers.google.com/identity/protocols/OAuth2WebServer#offline
    const refresh_url = "https://www.googleapis.com/oauth2/v4/token";

    const post_body = `grant_type=refresh_token&client_id=${encodeURIComponent(client_id)}&client_secret=${encodeURIComponent(client_secret)}&refresh_token=${encodeURIComponent(refresh_token)}`;

    let refresh_request = {
        body: post_body,
        method: "POST",
        headers: new Headers({
            'Content-Type': 'application/x-www-form-urlencoded'
        })
    }

    // post to the refresh endpoint, parse the json response and use the access token to call files.list
    fetch(refresh_url, refresh_request).then( response => {
            return(response.json());
        }).then( response_json =>  {
            console.log(response_json);
            files_list(response_json.access_token);
    });
}

// a quick and dirty function to list some Drive files using the newly acquired access token
function files_list (access_token) {
    const drive_url = "https://www.googleapis.com/drive/v3/files";
    let drive_request = {
        method: "GET",
        headers: new Headers({
            Authorization: "Bearer "+access_token
        })
    }
    fetch(drive_url, drive_request).then( response => {
        return(response.json());
    }).then( list =>  {
        console.log("Found a file called "+list.files[0].name);
    });
}

get_access_token_using_saved_refresh_token();