Once upon a time, there was a powerful tool called Autoyast. Using single Autoyast XML profile, one could easily install one hundred identically configured computers, exact clones of the first one. All that in (almost) unattended mode, so the sysadmin could go to the pub and have a beer (or two) in the meantime.
If you want to know more about Autoyast and how the cloning works, you'd better ask Uwe or read the official documentation maintained by him. This fairytale, liebe kinder, is about something slightly different.

In some cases, installing the whole system from scratch using Autoyast is not exactly what you have to (and want to) do. You might want to re-use only some well-defined bits and pieces of configuration that are proven to work on an old machine with a new one. And, of course, neither you want to browse the documentation, find the needed configuration files and copy-paste the lines there once again, nor you want to click through all the YaST modules for hundredth time over.
For example, you want to set up a shiny new workstation, with your home directory on local NFSv4 server with Kerberos authentication:

<nfs config:type="list">
   <nfs_entry>
     <mount_point>/home</mount_point>
     <nfs_options>sec=krb5i,intr</nfs_options>
     <server_path>nfs.myserver.net:/home</server_path>
     <vfstype>nfs4</vfstype> 
     </nfs_entry>
</nfs>

Your company stores user identities in LDAP, so you'll need a working LDAP client as well:

<ldap>
    <base_config_dn></base_config_dn>
    <bind_dn></bind_dn>
    <create_ldap config:type="boolean">false</create_ldap>
    <file_server config:type="boolean">false</file_server>
    <ldap_domain>ou=users,dc=myserver,dc=net</ldap_domain>
    <ldap_server>ldap.myserver.net</ldap_server>
    <ldap_tls config:type="boolean">false</ldap_tls>
    <ldap_v2 config:type="boolean">false</ldap_v2>
    <login_enabled config:type="boolean">true</login_enabled>
    <member_attribute>member</member_attribute>
    <pam_password>crypt</pam_password>
    <start_autofs config:type="boolean">false</start_autofs>
    <start_ldap config:type="boolean">true</start_ldap>
</ldap>

And, last but not least, you are behind a proxy and you want to set it up, too:

<proxy>
    <enabled config:type="boolean">true</enabled>
    <ftp_proxy>http://proxy.myserver.net:3128</ftp_proxy>
    <http_proxy>http://proxy.myserver.net:3128</http_proxy>
    <https_proxy>http://proxy.myserver.net:3128</https_proxy>
    <no_proxy>localhost</no_proxy>
    <proxy_password></proxy_password>
    <proxy_user></proxy_user>
</proxy>

Now we gathered all the pieces of configuration we want to transfer to the new machine, so let's save it, e.g. in /var/lib/autoyast/repository/mycoolprofile.xml

ayast_setup CLI in practice

ayast_setup is a commandline client designed exactly for the purpose of applying configuration stored in a small custom Autoyast XML profile to already running installed system. Let's see what it can do (some lines from the listing have been omitted for the sake of simplicity):

tux@pleczka:~> /sbin/yast ayast_setup help

YaST Configuration Module ayast_setup
--------------------------------------

Client for autoyast configuration on the running system

Basic Syntax:
    yast2 ayast_setup interactive
    yast2 ayast_setup <command> [verbose] [options]
    ....
    yast2 ayast_setup <command> help

Commands:
    setup  Configure the system using given autoyast profile

We can see it supports only one possible command - setup. Let's have a look how to use it (just substitute 'setup' for <command> keyword above):

tux@pleczka:~> /sbin/yast ayast_setup setup help

YaST Configuration Module ayast_setup
--------------------------------------

Command 'setup'
    Configure the system using given autoyast profile

    Options:
        filename  [string]  Path to autoyast profile
        ....

    Options of the [string] type must be written in the form 'option=value'.

    Example:
        setup filename=/path/to/profile

We need to specify a path to profile as setup's command filename option. So let's do it:

tux@pleczka:~> /sbin/yast ayast_setup setup filename=/var/lib/autoinstallation/repository/mycoolprofile.xml

And we're done. Depending on the size of your custom Autoyast profile, you can now leave and have a coffee and/or something else while the system is being configured. It usually won't take more than one portion of the brain-stimulator of your choice :)

Some more notes

ayast_setup has been written by Uwe and polished by /me during HackWeek 3. Most of the time I use it for testing Autoyast-related bugs in my modules.

And for those not feeling comfortable in CLI, since openSUSE 11.1 also a clickety stuff can be used to do the same :) Just run 'yast autoyast', open the profile of your choice and then click on 'Apply to System'