Puppet

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# Using Puppet with Augeas

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[Augeas](http://augeas.net/index.html) is a lovely tool that treats

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config files (well, anything really, but it's mostly about config

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files) as trees of values. You then modify the tree as you like,

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and write the file back.

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This is basically the solution to the problem of dealing with

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upstream configuration changes combined with local modifications:

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you can allow the upstream changes through and then apply changes

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with Augeas to the new version.

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Assuming you want to work with Augeas, this is a description of how

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to perform Augeas changes using Puppet. You'll need Puppet >=

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0.24.7 for this. The basic usage is [[Type\_Reference#augeas|Type

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Reference]] .

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The somewhat more important, and unfortunately complicated, part is

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figuring out what the tree for a file looks like so you can

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manipulate it properly. The definition that Augeas uses to turn a

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file into a tree is called a lens, and understanding the trees is

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more difficult than it should be, because many lenses are not

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documented sufficiently, or at all. The documentation for those

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that are

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[has its own surprisingly hard to find page](http://augeas.net/docs/references/lenses/index.html)

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on the Augeas site. You can see what lenses are available by

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looking in /usr/share/augeas/lenses/ (or

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/usr/local/share/augeas/lenses/ , or possibly somewhere else,

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depending on your setup).

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You can also see which files Augeas has successfully parsed by

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entering "ls /files/" in augtool and drilling down from there. If a

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file hasn't been properly parsed by Augeas, it simply won't show

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up. This could mean that the file has a syntax error, or it could

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imply a failure in the lense itself.

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Here's an example of how to determine the tree structure of a file,

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in this case /etc/exports.

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The easiest thing to do is to set up the file with some examples (I

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pulled examples from the bottom of "man 5 exports") and see what

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they look like:

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    $ augtool

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    augtool> ls /files/etc/exports/

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    comment[1] = /etc/exports: the access control list for filesystems which may be exported

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    comment[2] = to NFS clients.  See exports(5).

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    comment[3] = sample /etc/exports file

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    dir[1]/ = /

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    dir[2]/ = /projects

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    dir[3]/ = /usr

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    dir[4]/ = /home/joe

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From here you can investigate the structure, like so:

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    augtool> ls /files/etc/exports/dir[1]

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    client[1]/ = master

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    client[2]/ = trusty

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The corresponding line in the file is:

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    /               master(rw) trusty(rw,no_root_squash)

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Digging further:

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    augtool> ls /files/etc/exports/dir[1]/client[1]

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    option = rw

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So, if you want to add a new entry, you'd do something like this:

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    augtool> set /files/etc/exports/dir[last()+1] /foo

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    augtool> set /files/etc/exports/dir[last()]/client weeble

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    augtool> set /files/etc/exports/dir[last()]/client/option[1] ro

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    augtool> set /files/etc/exports/dir[last()]/client/option[2] all_squash

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    augtool> save

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    Saved 1 file(s)

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Which creates the line:

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    /foo weeble(ro,all_squash)

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Now that you've played around in augtool, you can make changes

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using Puppet:

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    augeas{ "export foo" :

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        context => "/files/etc/exports",

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        changes => [

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            "set dir[last()+1] /foo",

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            "set dir[last()]/client weeble",

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            "set dir[last()]/client/option[1] ro",

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            "set dir[last()]/client/option[2] all_squash",

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],

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This adds the line described above.

Puppet Augeas