TIBCO Spotfire Server upgrade started in interactive mode on Linux fails with error "No X11 DISPLAY variable was set, but this program performed an operation which requires it."

TIBCO Spotfire Server upgrade started in interactive mode on Linux fails with error "No X11 DISPLAY variable was set, but this program performed an operation which requires it."

When running the TIBCO Spotfire Server upgrade tool interactively on Linux OS, as per steps mentioned here : Running the Spotfire Server upgrade tool interactively, the following error may be seen in te upgrade.log when the upgrade fails:

Internal error
java.awt.HeadlessException: 
No X11 DISPLAY variable was set, but this program performed an operation which requires it.
    at java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:204) ~[?:1.8.0_212]
    at java.awt.Window.<init>(Window.java:536) ~[?:1.8.0_212]
    at java.awt.Frame.<init>(Frame.java:420) ~[?:1.8.0_212]
    at javax.swing.JFrame.<init>(JFrame.java:233) ~[?:1.8.0_212]
    at com.spotfire.server.tools.wizards.swing.AbstractInstallerWindow.<init>(AbstractInstallerWindow.java:153) ~[?:?]
    at com.spotfire.server.tools.upgrade.swing.UpgradeToolWindow.<init>(UpgradeToolWindow.java:47) ~[?:?]
    at com.spotfire.server.tools.upgrade.factory.UpgradeFactory.createSwingBasedWizard(UpgradeFactory.java:42) ~[?:?]
    at com.spotfire.server.tools.upgrade.UpgradeTool.runGUI(UpgradeTool.java:184) ~[?:?]
    at com.spotfire.server.tools.upgrade.UpgradeTool.run(UpgradeTool.java:98) [?:?]
    at com.spotfire.server.tools.upgrade.UpgradeLauncher.main(UpgradeLauncher.java:46) [spotfire-tss-tools.jar:?]

This error will be seen because when running the Spotfire Server upgrade tool interactively, users are prompted for information about the older installation and new installation. If the Linux OS does not have X11 display set to handle those user prompts then this error is thrown as it fails to launch the dialog for user information. 

For more information on this error, click here : To resolve, there are 2 options:

1) Run the tool interactively with X11 display installed: 
Ensure the Linux OS has an X11 display set prior to starting the interactive upgrade process. One of the popular tools is MobaXterm

2) Run the upgrade tool silently:
Launch the upgrade tool silently per the documentation reference Running the Spotfire Server upgrade tool silently  It will run the tool silently using the command line where the user will NOT be prompted for information. This will not require an X11 display to be set in the Linux OS.
  External: “No X11 DISPLAY variable” - what does it mean?

• https://stackoverflow.com/questions/662421/no-x11-display-variable-what-does-it-mean

Doc: Running the Spotfire Server upgrade tool silently

• https://docs.tibco.com/pub/spotfire_server/10.3.1/doc/html/TIB_sfire_server_tsas_admin_help/GUID-4C38DB12-9409-46E0-8A16-3D664C343DFD.html