Elektra: Plugin: augeas

Elektra 0.8.24

Plugin: augeas

- infos = Information about augeas plugin is in keys below
- infos/author = Felix Berlakovich elekt.nosp@m.ra@b.nosp@m.erlak.nosp@m.ovic.nosp@m.h.net
- infos/licence = BSD
- infos/provides = storage
- infos/needs =
- infos/recommends = glob keytometa
- infos/placements = getstorage setstorage
- infos/status = maintained unittest libc configurable
- infos/metadata = order
- infos/description = reading and writing configurations via libaugeas

This is a plugin for reading and writing configuration files with help from Augeas. The plugin should be able to read all configuration files for which an Augeas lens exists. However, not all stock lenses of Augeas have been tested yet. A detailed description of the lens language and a tutorial on how to write new lenses can be found at http://augeas.net/

## Dependencies

- `libaugeas-dev`: You need version 0.16 or higher

## Installation

If you have installed Augeas manually, it may be necessary to update the ld configuration. This is especially true if an older version of Augeas is installed also. Such a situation may lead to an error similar to this:

    /usr/lib/libaugeas.so.0: version `AUGEAS_0.16.0` not found (required by kdb)

This is because `ld` tries to link `/usr/lib/libaugeas.so.0` which is an older version of Augeas. Simply add the path to the newer library to your ld search paths (consult your system documentation on how to do this).

## Mounting and Configuration

In order to mount the hosts file with the augeas plugin, issue the following command:

    kdb mount /etc/hosts system/hosts augeas lens=Hosts.lns

The value of the plugin configuration option "lens" should be the module name of the lens (Hosts in the example) with a '.lns' suffix. Depending on your distribution and kind of installation, lenses can be found at `/usr/share/augeas/lenses/dist`, `/usr/local/share/augeas/lenses/dist`, or something similar. The lens module name is equal to the filename without extension in pascal notation. For example, the lens `/usr/share/augeas/lenses/dist/hosts.aug` contains the module Hosts.

Note that, without configuring the plugin to use a lens, the plugin will print an error message on the first usage:

    kdb ls system/hosts
    The command ls terminated unsuccessfully with the info: Error (#85) occurred!
    Description: an Augeas error occurred
    Ingroup: plugin
    Module: storage
    At: /path/augeas.c:166
    Reason: Lens not found

This happens because the plugin does not know which lens it should use to read and write the configuration. For that reason, the lens configuration option was supplied together with the mount command.

## Restrictions

### Inner node values

Currently no Augeas lens supports values for inner nodes. Unfortunately no validation plugin exists yet that would prevent such modifications early:

    kdb set system/hosts/1 somevalue
    The command set terminated unsuccessfully with the info: Error (#85) occurred!
    Description: an Augeas error occurred
    Ingroup: plugin
    Module: storage
    At: /path/augeas.c:166
    Reason: Malformed child node '1'

The operation simply fails with an undescriptive error.

### Leaky abstraction of order

Most Augeas lenses require subtrees to be in a specific order. For example the hosts lens requires the ipaddr node of an entry to precede the canonical node. Unfortunately the Augeas storage plugin has no knowledge about this required order. Therefore the correct order must be ensured via order metakeys. Otherwise saving the KeySet may fail. As an example consider the following kdb shell script:

    kdbGet system/hosts
    keySetName system/hosts/6
    ksAppendKey
    keySetName system/hosts/6/ipaddr
    keySetString 14.14.14.14
    ksAppendKey
    keySetName system/hosts/6/canonical
    keySetString newhost
    ksAppendKey
    kdbSet system/hosts

This fails with an error similar to this

    Description: an Augeas error occurred
    Ingroup: plugin
    Module: storage
    At: /path/augeas.c:179
    Reason: Failed to match
    some augeas match expression
    with tree
    { \"canonical\" = \"newhost\" } { \"ipaddr\" = \"14.14.14.14\" }

Whereas the following script succeeds due to the correct order

    kdbGet system/hosts
    keySetName system/hosts/6
    ksAppendKey
    keySetName system/hosts/6/ipaddr
    keySetString 14.14.14.14
    keySetMeta order 100
    ksAppendKey
    keySetName system/hosts/6/canonical
    keySetString newhost
    keySetMeta order 110
    ksAppendKey
    kdbSet system/hosts

## Planned Improvements

- a validation plugin preventing inner node values