# http://wiki.apache.org/solr/FAQ#How_can_I_delete_all_documents_from_my_index.3F
# http://wiki.apache.org/solr/UpdateXmlMessages#Updating_a_Data_Record_via_curl
curl "http://index.websolr.com/solr/a0b1c2d3/update?commit=true" -H "Content-Type: text/xml" --data-binary '<delete><query>*:*</query></delete>'
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The command by objectivePinta is shut off at least in Solr version 7.2.0 (in the out of the box configuration). However, the curl - solution of SainathSharmadha worked (for me) in this version.