[Solr Wiki] Update of "SolrConfigXml" by ErikHatcher

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The following page has been changed by ErikHatcher:
http://wiki.apache.org/solr/SolrConfigXml

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    </admin>
  }}}
  
+ == System property substitution ==
+ Solr supports system property substitution, allowing the launching JVM to specify string substitutions within either of Solr's configuration files.  The syntax {{{${property[:default value]}}}}.  Substitutions are valid in any element or attribute text.  Here's an example of allowing the runtime to dictate the data directory:
+ 
+ {{{
+    <dataDir>${solr.data.dir:./solr/data}</dataDir>
+ }}}
+ 
+ And using the example application, Solr could be launched in this manner:
+ 
+ {{{
+ java -Dsolr.data.dir=/data/dir -jar start.jar
+ }}}
+ 
+ If no default value is provided, the system property MUST be specified otherwise a Solr startup failure occurs indicating what property has no value.
+