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How to deploy salesforce custom button using Apache ANT tool?

Package.xml file to retrieve and deploy using ANT or Workbench salesforce deployment

<?xml version=”1.0″ encoding=”utf-8″ standalone=”yes”?>
<Package xmlns=”http://soap.sforce.com/2006/04/metadata”>
<types>
<members>Case.SendEmail</members>
<name>WebLink</name>
</types>
<version>41.0</version>
</Package>

Here:

<members>Case.SendEmail</members> – Case is a Object API Name and SendEmail is a Button API Name

Additional resources:
Metadata types
How to create package.xml