On Fri, 2009-05-15 at 14:41 -0400, Rob McBroom wrote:
> However, what seems like a majority of files are broken up into an
> "array" of values. Take the `/etc/exports` [example from the wiki][1].
> Existing items are assigned numbers 1-4 by Augeas when the file is
> loaded.
> If you add an item with an index of 10000,
I started the '10000' idiom because I couldn't think of anything better,
and because in TeX 10000 is infinity, but in practice it's of course a
very finite value.
Thinking about that I realize that there's a very easy way to create
labels that are (a) numbers and (b) unique that you can use instead of
10000: prefix them with one or more '0's. The label itself has no real
meaning, it just has to be a valid number (more precisely: it needs to
match the regexp /[0-9]+/) and Augeas will never use leading zeroes when
it reads in a file.
> the tree will
> contain items 1, 2, 3, 4 and 10000 until you save the changes. But on
> the next Puppet run, Augeas will load the file and number the items
> 1-5. There won't be an item 10000, so what's to stop it from being
> added over and over again on every run? The same can be said for
> inserting things. Say I want to insert something after /2 in `/etc/
> inittab`. There will be a /2 in the tree on every single run, so
> what's to stop it from inserting again and again?
Think of the subtree under /2 etc. as a record - in almost all cases,
these records have some notion of primary key; for /etc/hosts, that's
the ipaddr. If the '127.0.0.1' line is the first in /etc/hosts, you can
access the rest of the line as /files/etc/hosts/1/... _or_ by using
XPath notation to go by the primary key: /files/etc/hosts/*[ipaddr =
'127.0.0.1'] The latter will always give you the localhost line,
regardless of where it is in the file.
> I realize the numbering of items in certain files is unavoidable, but
> is there a good way to handle these situations in Puppet? Here are
> some real-world examples from my manifests that seem to work.
>
> # adds rh:06:wait:/etc/rc.shutdown
> augeas { "shutdown":
> require => File["shutdown"],
> context => "/files/etc/inittab",
> changes => [
> "ins 3 after 2",
You don't need the explicit 'ins' to append - set will do that when you
access a nonexistant node automatically.
> "set 3[1]/id rh",
> "set 3[1]/runlevels 06",
> "set 3[1]/action wait",
> "set 3[1]/process /etc/rc.shutdown",
> ],
> onlyif => "get 3/id != rh",
> }
Putting all the above together, you can write this as
# adds rh:06:wait:/etc/rc.shutdown
augeas { "shutdown":
require => File["shutdown"],
context => "/files/etc/inittab",
changes => [
"set 01/id rh",
"set 01/runlevels 06",
"set 01/action wait",
"set 01/process /etc/rc.shutdown",
],
onlyif => "match *[id = 'rh'] size == 0",
}
> This inserts a line after the `/etc/rc.d/rc.sysinit` line. Works fine
> now, but what if the sysinit line isn't item 2 in every future version
> of the OS? Maybe I could add "only if 2's id is 'si'" to make sure,
> but what if it's *not* 'si'?
If you always want your entry to show up after the 'si' line, you ned to
do the explicit 'ins' as the first thing in your changes, something like
'ins 01 after */[id = 'si']'
> It won't get added to the wrong place,
> but it won't get added anywhere else either. Clearly not what I'm
> shooting for.
>
> # boot systems to runlevel 3
> augeas { "runlevel":
> context => "/files",
> changes => [
> "set /etc/inittab/1/runlevels 3",
> ],
> onlyif => "get /etc/inittab/1/action == initdefault",
> }
>
> Again, this works, but what if initdefault was *not* item 1?
Use "set /etc/inittab/*[action = 'initdefault']/runlevels 3" - for that,
you don't even need an onlyif. If runlevels is already 3, Augeas will
notice that nothing has changed when the tree is saved, and won't touch
the file at all.
The XPath notation for Augeas is described in more detail at [1]
> On a related note, there doesn't seem to be a good way to add comments
> to a particular spot to explain the changes you've made. This is true
> even for the "straightforward" files mentioned above. Any advice?
Most lenses now map comments as nodes labelled '#comment', and you can
do something like the following to add a comment just before a given
node:
ins #comment before /files/etc/hosts/*[ipaddr = '127.0.0.1']
set /files/etc/hosts/#comment[. = ''] "My new comment"
though the 'set' is a bit of a kludge (it looks for comment nodes with
an empty comment) - the Augeas path expressions clearly need a better
way to refer to newly created nodes, or to refer to nodes by a more
complicated criterion, i.e. so that you can say 'the #comment node just
before the one with an ipaddr child with value 127.0.0.1'.
> Please view this as a question and not a complaint.
This is great feedback.
David
[1] http://augeas.net/page/Path_expressions
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