Reworded added info about shells.

        <p>

            ISIS 3 needs to know where all its pieces are located. We use an

            environment variable called "ISISROOT" to do this. Unix commands must

            be put into your startup shell scripts.  Assuming you installed ISIS in

            the directory /work1/isis3 add the following commands to your startup

            script:

            be put into your startup shell scripts. There are two different

            ways to achieve this depending on what shell you are using. If you are

            unsure what kind of shell you are using, type the following command

            in the terminal:

            <pre>

                echo $SHELL

            </pre>

            It is important to note that tsch is a C shell and bash is a Bourne shell.

            Assuming you installed ISIS in the directory /work1/isis3 add the following

            commands to your startup script:

        </p>

        <p>

    <change name="Adam Paquette" date="2016-06-24">Updated links to the support center.</change>

    <change name="Ian Humphrey" date="2017-01-25">Updated instructions for new supported systems.</change>

    <change name="Summer Stapleton" date="2017-12-29">Updated SPICE Web Service mission information to include newer missions.</change>

    <change name="Kaitlyn Lee" date="2018-04-04">Added information about tsch being a C shell and bash being a Bourne shell

      and how to tell what shell a user is using to the UNIX Enviroment Setup section. Fixes #5372.</change>

  </history>

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