cdist-backup/docs/src/cdist-preos.rst

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PreOS
=====
Description
-----------
With cdist you can install and configure new machines. You can use cdist to
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create PreOS, minimal OS whose purpose is to boot a new machine.
After PreOS is booted, the machine is ready for installing the desired OS and
afterwards it is ready for configuration.
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PreOS creation
--------------
With cdist you can create PreOS.
Currently supported PreOS-es include:
* debian
* ubuntu
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* devuan
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PreOS is created using the ``cdist preos`` command.
This command has subcommands that determine the desired PreOS.
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For example, to create an ubuntu PreOS:
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.. code-block:: sh
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$ cdist preos ubuntu /preos/preos-ubuntu -B -C \
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-k ~/.ssh/id_rsa.pub -p /preos/pxe-ubuntu
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For more info about the available options see the cdist manual page.
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This will bootstrap (``-B``) ubuntu PreOS in the ``/preos/preos-ubuntu``
directory, it will be configured (``-C``) using default the built-in initial
manifest and with specified ssh authorized key (``-k``).
After bootstrapping and configuration, the PXE boot directory will be
created (``-p``) in ``/preos/pxe-ubuntu``.
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After PreOS is created, new machines can be booted using the created PXE
(after proper dhcp and tftp settings).
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Since PreOS is configured with ssh authorized key it can be accessed through
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ssh, i.e. it can be further installed and configured with cdist.
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Implementing a new PreOS sub-command
------------------------------------
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preos command is implemented as a plugin system. This plugin system scans for
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preos subcommands in the ``cdist/preos/`` distribution directory and also in
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``~/.cdist/preos/`` directory if it exists.
preos subcommand is a module or a class that satisfies the following:
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* it has the attribute ``_cdist_preos`` set to ``True``
* it defines a function/method ``commandline``.
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For a module-based preos subcommand, the ``commandline`` function accepts a
module object as its first argument and the list of command line
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arguments (``sys.argv[2:]``).
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For a class-based preos subcommand ``commandline`` method should be
static-method and must accept a class as its first argument and the
list of command line arguments (``sys.argv[2:]``).
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If preos scanning finds a module/class that has ``_cdist_preos`` set
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to ``True`` and a function/method ``commandline`` then this module/class is
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registered to preos subcommands. The name of the command is set to ``_preos_name``
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attribute if defined in the module/class, defaulting to the module/class name in lowercase.
When a registered preos subcommand is specified, ``commandline``
will be called with the first argument set to module/class and the second
argument set to ``sys.argv[2:]``.
Example of writing new dummy preos sub-command
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Module-based preos:
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^^^^^^^^^^^^^^^^^^^
#. Create directory ``~/.cdist/preos/`` if it does not exist
#. Create ``~/.cdist/preos/netbsd.py`` with the following contents:
.. code-block:: python
_preos_name = 'netbsd'
_cdist_preos = True
def commandline(cls, args):
print("NetBSD PreOS: {}".format(args))
When you try to run this new preos you will get:
.. code-block:: sh
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$ cdist preos -L
Available PreOS-es:
- debian
- devuan
- netbsd
- ubuntu
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$ cdist preos netbsd
NetBSD PreOS: []
Class based preos:
^^^^^^^^^^^^^^^^^^
#. Create directory ``~/.cdist/preos/`` if it does not exist
#. Create ``~/.cdist/preos/freebsd.py`` with the following contents:
.. code-block:: python
class FreeBSD(object):
_cdist_preos = True
@classmethod
def commandline(cls, args):
print("FreeBSD dummy preos: {}".format(args))
When you try to run this new preos you will get:
.. code-block:: sh
$ cdist preos -h
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Available PreOS-es:
- debian
- devuan
- freebsd
- ubuntu
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$ cdist preos freebsd
FreeBSD dummy preos: []
In the ``commandline`` function/method you have all the freedom to actually create
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a PreOS.