$ cue help filetypes
The cue tools supports the following file types:

    Tag         Extensions      Description
    cue         .cue            CUE source files.
    json        .json           JSON files.
    yaml        .yaml/.yml      YAML files.
    jsonl       .jsonl/.ldjson  Line-separated JSON values.
    jsonschema                  JSON Schema.
    openapi                     OpenAPI schema.
	pb                          Use Protobuf mappings (e.g. json+pb)
    textproto    .textproto     Text-based protocol buffers.
    proto        .proto         Protocol Buffer definitions.
    go           .go            Go source files.
    text         .txt           Raw text file; the evaluated value
                                must be of type string.
    binary                      Raw binary file; the evaluated value
                                must be of type string or bytes.

OpenAPI, JSON Schema and Protocol Buffer definitions are
always interpreted as schema. YAML and JSON are always
interpreted as data. CUE and Go are interpreted as schema by
default, but may be selected to operate in data mode.

The cue tool will infer a file's type from its extension by
default. The user my override this behavior by using qualifiers.
A qualifier takes the form

    <tag>{'+'<tag>}':'

For instance,

	cue eval json: foo.data

specifies that 'foo.data' should be read as a JSON file. File
formats that do not have a default extension may be represented
in any data format using the same notation:

   cue def jsonschema: bar.cue foo.yaml openapi+yaml: baz.def

interprets the files bar.cue and foo.yaml as data in the
respective formats encoding an JSON Schema, while 'baz.def' is
defined to be a YAML file which contents encode OpenAPI
definitions.

A qualifier applies to all files following it on the command line
until the next qualifier. The cue tool does not allow a ':' in
filenames.

The following tags can be used in qualifiers to further
influence input or output. For input these act as
restrictions, validating the input. For output these act
as filters, showing only the requested data and picking
defaults as requested.

    Tag         Description
    data        Require concrete input and output that does
                not require any evaluation.
    graph       Like data, but allow references.
    schema      Export data and definitions.

Many commands also support the --out and --outfile/-o flags.
The --out flag specifies the output type using a qualifier
(without the ':'). The -o flag specifies an output file
possibly prefixed with a qualifier.

Examples:

# Interpret bar.cue and foo.yaml as OpenAPI data.
$ cue def openapi: bar.cue foo.yaml

# Write a CUE package as OpenAPI encoded as YAML, using
# an alternate file extension.
$ cue def -o openapi+yaml:foo.openapi

# Print the data for the current package as YAML.
$ cue export --out=yaml

# Print the string value of the "name" field as a string.
$ cue export -e name --out=text

# Write the string value of the "name" field to a text file.
$ cue export -e name -o=foo.txt

# Write the string value of the "name" field to a file foo.
$ cue export -e name -o=text:foo