Augeas
I’ve been writing a lot about Augeas and all the process i go through to get it included in debian/ubuntu. It seems that a lot of people was following it, but some of them don’t actually understand the whole picture and what is augeas for, so i’ve been asked to write a post explaining what it is, so here we go:
From the upstream homepage:
Augeas is a configuration editing tool. It parses configuration files in their native formats and transforms them into a tree. Configuration changes are made by manipulating this tree and saving it back into native config files.
Actually, it is a library which does all that stuff and can be manipulated using its public API. But what does this actually means? I will explain it using augtool and /etc/hosts as an example. As you might know it has the information of the host names and their IP addresses, so let’s take this hosts as an example:
127.0.0.1 localhost localhost.localdomain host.domain
After parsing it on augeas we will end with:
/files/etc/hosts/
- – - – - – - – - – - 1/
- – - – - – - – - – - – ipaddr = 127.0.0.1
- – - – - – - – - – - – canonical = localhost
- – - – - – - – - – - – alias = localhost.localdomain
- – - – - – - – - – - – alias = host.domain
So, then we might want to change some values:
augtool> set /files/etc/hosts/1/alias[2] myhost.domain
Ending with:
/files/etc/hosts/
- – - – - – - – - – - 1/
- – - – - – - – - – - – ipaddr = 127.0.0.1
- – - – - – - – - – - – canonical = localhost
- – - – - – - – - – - – alias = localhost.localdomain
- – - – - – - – - – - – alias = myhost.domain
or add new values:
augtool> ins alias after /files/etc/hosts/1/alias[1]
what will turn into:
/files/etc/hosts/
- – - – - – - – - – - 1/
- – - – - – - – - – - – ipaddr = 127.0.0.1
- – - – - – - – - – - – canonical = localhost
- – - – - – - – - – - – alias = localhost.localdomain
- – - – - – - – - – - – alias
- – - – - – - – - – - – alias = myhost.domain
then we need to set a value since now it’s NULL:
augtool> set /files/etc/hosts/1/alias[2] myhost
ending like:
/files/etc/hosts/
- – - – - – - – - – - 1/
- – - – - – - – - – - – ipaddr = 127.0.0.1
- – - – - – - – - – - – canonical = localhost
- – - – - – - – - – - – alias = localhost.localdomain
- – - – - – - – - – - – alias = myhost
- – - – - – - – - – - – alias = myhost.domain
Then you can save it using:
augtool> save
Also you can add a new host, or whatever you want, just need to play with the tree.
Ok, but how this Black Magic work and how can i expand it to read new configuration files? Augeas uses lenses which are a Meta Data type using regular expressions that is being used for parsing, reading and writing configuration files. If you want to play around with lenses you can check this step-by-step tutorial written by Raphael Pinson (Thank you!)
nxvl @ July 18, 2008
Finally I understand
:D 
Sounds cool, how does it deal with comments?
Does it share parsing code with the apps? Hmm, perhaps the projects parsing all the files could be convinced to export libraries for use by augeas!
foo: Comments are defined in lenses also, they are not showed on the tree, but are stored for writing them back into the file on save.
And yes, it is a library with a Public API, augtool is just an example tool so you can play with the library and see what is being done.
Comments can actually also be taken as normal fields. This is what I do in my lenses usually, so that I can see them, and add or modify comments in the file.
For example represent :
“field1-1
# comment 1
field2-4
# another comment
”
with
[...]/field1 = “1″
[...]/comment[1] = “comment 1″
[...]/field2 = “2″
[...]/comment[2] = “another comment”
Lately, I’ve also been busy trying to find a way to comment/uncomment in lenses (see the mailing list for details). In that case (which is still experimental), I would make a distinction between a comment and a commented value. So a file like :
“field1=1
# a random comment
# field2=2
”
Would be parsed as
[...]/field1 = “1″
[...]/comment = “a random comment”
[...]/field2 = “2″
[...]/field2/commented
There is still discussion on the usability of such a parsing, whether it is considered useful that developers have to check for the existence of the “commented” flag before using the parsed value. I believe it could help to write nice interfaces that allow to comment/uncomment values.
As for Augeas being used in applications, it is the goal, just like nxvl pointed out. Augeas is a C library with bindings for python, ruby, ocaml and perl. Dominique Dumont, the author of the Config::Model Perl module (cf. CPAN) is also porting Augeas a new engine for his module.
For more info on comments management, see the mailing list :
https://www.redhat.com/archives/augeas-devel/2008-July/msg00045.html
How is it going,
Apparently this guy
Is giving away accounts
http://runescapegiveaway.com
this is not my site
Howdy
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