Augeas is distributed under the GNU Lesser General Public License
(LGPL) version
2.1. A copy of the license is included in the file COPYING in the
source distribution.
Yes. The LGPL allows you to use Augeas from a proprietary application. It would be very helpful to both you and the community if you send bug fixes and improvements as patches for possible incorporation in the main development tree. It will decrease your maintenance costs anyway if you do so.
The download page explains where you can find released RPM's and tarballs and how you can check the latest development version out from Mercurial
To quote Wikipedia: "In Greek mythology, Augeas ... was King of Elis ... He is best known ... for his stables, which housed the single greatest number of cattle in the country and had never been cleaned."
The main means of communication is the
Augeas mailing list
[email protected]. For more information on contributing, see
the contribute page
Of course. Besides writing code, there are many more areas where Augeas needs help: evaluating existing tree representations of configuration files, writing descriptions for new file formats, and documenting Augeas' tree structure are all areas where help is sorely needed.
Yes, there is actually nothing specific to configuration files in Augeas. If you have some other unstructured text file that you'd like to transform into a tree, all you need to do is write a file format description for it.
Yes. One of the main design goals of Augeas is to not interfere with any other ways in which you might change your configuration files. Augeas works very hard to preserve comments and formatting details such as the number of spaces used to separate fields etc. It also does not depend on any additional information embedded in comments — you can always alternate between changing a configuration file with your favorite editor or custom script, and changing it with Augeas.
Yes, the augtool command can be used to manipulate the tree and the
underlying files with shell scripts. Run augtool help for a list of
commands augtool supports.
Bindings for Ruby, Python, OCaml, Perl, Haskell, PHP, Lua, Java, Node.js, Go and Tcl are available for download. Patches/additions for other languages are eagerly anticipated.
No, that is only necessary if you want to change how files are mapped
into the tree or map new files. For using Augeas, it is enough to
understand the functionality provided by augtool, or the equivalent
calls in C or the language bindings.
Each node in the Augeas tree consists of three pieces of data: a label
and a value, both strings, and a list of child nodes. The label and
value are both strings. The label is used in the path to the node and
child nodes, much as file names are used in the paths to files in a file
system. The label must not contain the characters / or * or end
with [N] where N is a positive number.
Yes, that is the biggest difference between the Augeas tree and files in
a filesystem (and makes it similar to DOM's for XML documents). A node
can have any number of children called foo; in paths, a specific child
can be picked out with foo[N] where N is a positive number —
the first child with label foo is picked with foo[1], and the last
can be picked with foo[last()]. Just foo in a path picks all the
children with that label. This convention follows
XPath
Augeas only stores strings in the tree. So far, there hasn't been a compelling reason to support other data types.
The contents of a file /some/path are mapped into the tree as
/files/some/path, which is usually a fine-grained tree view of that
file; additional (meta-)information about each file is available in
/augeas/files/some/path.
Lenses are the building blocks of the file <-> tree transformation; they combine parsing a file and building the tree, the get transformation, with turning the tree back into an (updated) file, the put transformation.
The name comes from the Harmony project. In Harmony's lingo, Augeas maps back and forth between a concrete view (the configuration file) and an abstract view (the tree representation). And calling something that goes back and forth between two views a 'lens' seems only natural, doesn't it ?
A bidirectional language is one where the program expresses a
transformation from input to output, and from (possibly modified) output
back to the corresponding input. They are called bidirectional rather
than bijective because there are generally many inputs for the same
output and vice versa. In Augeas case, many input files (e.g.,
/etc/hosts with varying amounts of whitespace between fields) map to
the same tree, and there are many ways to map the same tree back to a
file. Other examples of bidirectional languages are Harmony (mapping
trees to trees) and Boomerang (mapping strings to strings), both from
the Harmony project.
The lenses match regular expressions, different lenses do different things with the strings that match (create a new tree node with a certain label, store a value in a tree node, combine tree nodes into a larger tree); writing the regular expressions is the main task when describing a file format.
Augeas uses extended POSIX regexps. There are a few more esoteric features of extended POSIX regexps that are not supported, the most notable of them being POSIX named character classes, collating elements and back references. Named character classes should be supported soon (patches welcome).
The file format descriptions are written in a small subset of ML (a popular functional language.) If you don't know ML, don't despair: there are only two or three constructs that you need to know to write file format descriptions. The Augeas language is statically typed; besides the 'usual' checks of a type checker, checks also make sure that the transformations described by lenses are safe.
The Augeas language has a built-in test construct for running unit
tests; additional round-trip tests can be found in the tests/
directory in the source distribution. They are run by the augtest
command in the same directory.
File format descriptions are contained in files with the extension
.aug. In the source distribution, they are in the lenses
directory, which, by default, is installed into
/usr/share/augeas/lenses.
Yes, please. One of Augeas' goals is to collect file format descriptions, and build a library of them, so that eventually there will be a 'canonical' tree representation of configuration data.
Libfa is a library for manipulating finite automata. A finite automaton is a representation of all strings matching a regular expression. Augeas uses it for typechecking lenses and ensuring they don't contain ambiguities.
Yes. Libfa is also licensed under the LGPL, and has no dependencies
on Augeas.
A list of operations can be found here.
It supports extended POSIX regular expressions, exactly the same as Augeas.
The GNU C library (and GNU Regex) contains a much better
matcher than I would ever hope to write for libfa — being able to
use that matcher is the main reason for using extended POSIX syntax.