The tree

The tree that Augeas maintains is very similar to the tree of files and directories in a filesystem: the components of a path are strings that can, and nodes have an optional value associated with them, which can be an arbitrary string.

Augeas' tree deviates from the files-in-a-filesystem model in one important way: multiple children in a tree can have the same label. For example, the schema for /etc/hosts uses that to map multiple aliases for the same host to multiple nodes, all with the path /files/etc/hosts/N/alias, where N is the number of the host entry in the file. All numbering in Augeas starts with one, not zero.

With that, a tree node consists of three pieces of data:

a label
a string that is part of the path of the node. The string can contain any characters except for /, *, or [. Strictly speaking, nodes can also have NULL as a label, but such nodes are not accessible through the public API — they are sometimes used by the lenses processing the files to store important formatting information.
a value
an arbitrary string or NULL
the children
a list of tree nodes, where multiple children can have the same label. Since files are inherently sequential, ordering in the list of children matters.

The tree can be searched in all kinds of ways by using path expressions

Important paths

The Augeas tree consist of two separate hierarchies: /augeas used for reporting important metadata, in particular information about how files were processed, and /files containing the result of parsing configuration files.

Configuration data in /files

By convention, all configuration files get parsed into the /files hierarchy. The contents of a file that has the absolute path /PATH in the file system appears at /files/PATH in the tree. For example, the contents of /etc/hosts appear at /files/etc/hosts.

Metainformation in /augeas

The /augeas hierarchy reports important metadata about Augeas' inner workings. Most entries under /augeas are readonly — it is possible to change them by issuing set commands, but those changes have no influence on how Augeas goes about its business.

The entries /augeas/root and /augeas/save report the file system root from which files are read, and how files are saved. The save modes, corresponding to starting augtool with the -n, the -b, or neither of these options are newfile, backup, and overwrite. Changing /augeas/save before saving the tree changes how updated files are saved.

For each file /PATH that was processed, some metadata is presented in /augeas/files/PATH. That information is due to be revised soon; the entries mentioned here will remain, but other entries may be removed.

Underneath /augeas/files/PATH, the entry error has no value if processing of the file was successful. Values that indicate errors are

Reading the file failed; this means that the file exists, but can not be read, for example, because of insufficient permissions
Parsing the file into the tree failed. This usually means that the file does not match the lens that was applied to it.
Reading the file before saving changes of it failed, for similar reasons as read_failed indicates. Before Augeas writes a changed file, it has to read the original file back in to extract comments etc.
Renaming the original file to one ending with .augsave failed; this can only happen when the save mode is backup
Renaming the new file with extension .augnew to the original file name failed; this can only happen when the save mode is backup

The entry lens/info underneath /augeas/files/PATH indicates the lens that is used to process the file. The value contains the path to the .aug schema definition, and the line and column number where that lens was defined. For example, /augeas/files/etc/hosts/lens/info might have the value /usr/share/augeas/lenses/hosts.aug:18.12-.34.

Features under /augeas/version

The node /augeas/version contains the version number of the Augeas library. Underneath it are a few entries that indicate a few features of the API:

The behavior of aug_save can be changed dynamically at runtime by changing the entry /augeas/save. The possible values that can be assigned to /augeas/save are listed in /augeas/version/save/mode
Variables can be defined for later use in path expressions
The expression used to define a variable var is stored in /augeas/variables/var