Obtaining the full model code and associated scripting infrastructure
=====================================================================

CTSM is released via GitHub. You will need some familiarity with git in order
to modify the code and commit these changes. However, to simply checkout and run the
code, no git knowledge is required other than what is documented in the following steps.

To obtain the CTSM code you need to do the following:

#. Clone the repository. ::

      git clone https://github.com/escomp/ctsm.git my_ctsm_sandbox

   This will create a directory ``my_ctsm_sandbox/`` in your current working directory.

#. Run the script **manage_externals/checkout_externals**. ::

      ./manage_externals/checkout_externals

   The **checkout_externals** script is a package manager that will
   populate the ctsm directory with the relevant versions of each of the
   components along with the CIME infrastructure code.

At this point you have a working version of CTSM.

To see full details of how to set up a case, compile and run, see the CIME documentation at http://esmci.github.io/cime/ .

More details on checkout_externals
----------------------------------

The file **Externals.cfg** in your top-level CTSM directory tells
**checkout_externals** which tag/branch of each component should be
brought in to generate your sandbox. (This file serves the same purpose
as SVN_EXTERNAL_DIRECTORIES when CLM was in a subversion repository.)

NOTE: Just like svn externals, checkout_externals will always attempt
to make the working copy exactly match the externals description. If
you manually modify an external without updating Externals.cfg, e.g. switch
to a different tag, then rerunning checkout_externals will switch you
back to the external described in Externals.cfg. See below
documentation `Customizing your CTSM sandbox`_ for more details.

**You need to rerun checkout_externals whenever Externals.cfg has
changed** (unless you have already manually updated the relevant
external(s) to have the correct branch/tag checked out). Common times
when this is needed are:

* After checking out a new CTSM branch/tag

* After merging some other CTSM branch/tag into your currently
  checked-out branch

**checkout_externals** must be run from the root of the source
tree. For example, if you cloned CTSM with::

  git clone https://github.com/escomp/ctsm.git my_ctsm_sandbox

then you must run **checkout_externals** from
``/path/to/my_ctsm_sandbox``.

To see more details of **checkout_externals**, issue ::

  ./manage_externals/checkout_externals --help

Customizing your CTSM sandbox
=============================

There are several use cases to consider when you want to customize or modify your CTSM sandbox.

Switching to a different CTSM branch or tag
-------------------------------------------

If you have already checked out a branch or tag and **HAVE NOT MADE ANY
MODIFICATIONS** it is simple to change your sandbox. Say that you
checked out ctsm1.0.0 but really wanted to have ctsm1.1.0;
you would simply do the following::

  git checkout ctsm1.1.0
  ./manage_externals/checkout_externals

You should **not** use this method if you have made any source code
changes, or if you have any ongoing CTSM cases that were created from
this sandbox. In these cases, it is often easiest to do a second **git
clone**.

Pointing to a different version of a component
----------------------------------------------

Each entry in **Externals.cfg** has the following form (we use CIME as an
example below)::

  [cime]
  local_path = cime
  protocol = git
  repo_url = https://github.com/CESM-Development/cime
  tag = cime5.4.0-alpha.20
  required = True

Each entry specifies either a tag or a branch. To point to a new tag:

#. Modify the relevant entry/entries in **Externals.cfg** (e.g., changing
   ``cime5.4.0-alpha.20`` to ``cime5.4.0-alpha.21`` above)

#. Checkout the new component(s)::

     ./manage_externals/checkout_externals

Keep in mind that changing individual components from a tag may result
in an invalid model (won't compile, won't run, not scientifically
meaningful) and is unsupported.

Committing your change to Externals.cfg
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After making this change, it's a good idea to commit the change in your
local CTSM git repository. First create a branch in your local
repository, then commit it. (Unlike with subversion, branches are stored
locally unless you explicitly push them up to GitHub. Feel free to
create whatever local branches you'd like.) For example::

  git checkout -b my_ctsm_branch
  git add Externals.cfg
  git commit -m "Update CIME to cime5.4.0-alpha.20"

