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Download and install Okapi Longhorn as shown here: http://okapiframework.org/wiki/index.php?title=Longhorn#Download_and_Installation.
If you have a working and running Tomcat that has autoDeploy set to "true" in the /etc/tomcat9/server.xml file, than to deploy your okapi-longhorn.war file you can simply copy it to the webapps-folder of your Tomcat installation (on Ubuntu this is located under /var/lib/tomcat9/webapps). You may have to set the file access rights of the war file, so that Tomcat can handle the file and you may need to restart Tomcat.
Important hints for Okapi with Tomcat9
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Translate5 Configuration of the Okapi Plug-In
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