How can I only grab the field names of custom objects in the ANT Migration Tool?

Question

Is there a way to grab only the field names of custom objects using the ANT Migration Tool? I'm able to grab information about the members of Custom Objects themselves from my package.xml file, but I'm trying to filter it down to grabbing only the custom fieldnames or the <fullName> tags.

Here's my package.xml file:

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <types>
        <members>Account</members>
        <members>Opportunity</members>
        <members>Contact</members>
        <name>CustomObject</name>
    </types>
    <version>41.0</version>
</Package>

Maybe something like this?

<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
    <types>
        <members>Account</members>
        <members>Opportunity</members>
        <members>Contact</members>
        <name>CustomField</name>
        <name>CustomObject</name>
    </types>
    <version>41.0</version>
</Package>

Answer 1

No, it won't work that way, "name" can only appear once. Also, CustomField does not support wildcards. The best that you can do without making a huge manifest is to grab the entire object, via:

<types>
    <members>*</members>
    <name>CustomObject</name>
</types>

The wildcard will not retrieve standard objects, only custom ones.

If you want to just get the fields without the extras (e.g. workflow rules, list views, etc), you would build a package, and then retrieve it. Go to Setup > Create > Packages, and then add all of the custom fields. To retrieve it, use the sf:retrieve packageNames attribute:

<sf:retrieve username="${username}" password="${password}" serverurl="${serverurl}"
    retrieveTarget="src" singlePackage="true" packageNames="Fields" />

You can also do this with Salesforce DX, which I find so much easier to use:

sfdx force:mdapi:retrieve -s -r src -u sfdcfox -p Fields

Either way, the first time you do this, you'll get a package.xml inside your result folder that contains a package manifest with all the field names you can use in later scripts.