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// Copyright 2020 CUE Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
package cmd
import (
// TODO: intersperse the examples at the end of the texts in the
// body of text to make things more concrete for the user early on?
// The current approach works will if users just print the text without
// "more" or "less", in which case the examples show more prominently.
// The user can then scroll up to get a more in-depth explanation. But is
// this how users use it?
func newHelpTopics(c *Command) []*cobra.Command {
return []*cobra.Command{
var inputsHelp = &cobra.Command{
Use: "inputs",
Short: "package list, patterns, and files",
Long: `Many commands apply to a set of inputs:
cue <command> [inputs]
The list [inputs] may specify CUE packages, CUE files, non-CUE
files or some combinations of those. An empty list specifies
the package in the current directory, provided there is a single
named package in this directory.
CUE packages are specified as an import path. An import path
that is a rooted path --one that begins with a "." or ".."
element-- is interpreted as a file system path and denotes the
package instance in that directory.
Otherwise, the import path P denotes and external package found
in cue.mod/{pkg|gen|usr}/P.
An import path may contain one or more "..." to match any
subdirectory: pkg/... matches all packages below pkg, including
pkg itself, while foo/.../bar matches all directories named bar
within foo. In all cases, directories containing cue.mod
directories are excluded from the result.
A package may also be specified as a list of .cue files.
The special symbol '-' denotes stdin or stdout and defaults to
the cue file type for stdin. For stdout, the default depends on
the cue command. A .cue file package may not be combined with
regular packages.
Non-cue files are interpreted based on their file extension or,
if present, an explicit file qualifier (see the "filetypes"
help topic). By default, all recognized files are unified at
their root value. See the "filetypes" and "flags" help topics
on how to treat each file individually or how to combine them
If a data file has multiple values, such as allowed with JSON
Lines or YAML, each value is interpreted as a separate file.
If the --schema/-d is specified, data files are not merged, and
are compared against the specified schema within a package or
non-data file. For OpenAPI, the -d flag specifies a schema name.
For JSON Schema the -d flag specifies a schema defined in
"definitions". In all other cases, the -d flag is a CUE
expression that is evaluated within the package.
Examples (also see also "flags" and "filetypes" help topics):
# Show the definition of each package named foo for each
# directory dir under path.
$ cue def ./path/.../dir:foo
# Unify each document in foo.yaml with the value Foo in pkg.
$ cue export ./pkg -d Foo foo.yaml
# Unify data.json with schema.json.
$ cue export data.json schema: schema.json
var flagsHelp = &cobra.Command{
Use: "flags",
Short: "common flags for composing packages",
Long: `Non-CUE files are merged at their roots by default.
The can be combined differently or treated as different files
by using a combination of the following flags.
Individual files
To treat non-cue files as individual files, use --no-merge flag.
This is the default for vet. This flag only applies to data files
when used in combination with the --schema/-d flag.
Assigning values to a CUE path
The --path/-l flag can be used to specify a CUE path at which to
place a value. Each -l flag specifies either a CUE expression or
a CUE field (without the value following the colon), both of
which are evaluated within the value. Together, the -l flags
specify the path at increasingly deeper nesting. In the path
notation, path elements that end with a "::", instead of ":",
are created as definitions. An expression may refer to builtin
packages as long as the name can be uniquely identified.
The --with-context flag can be used to evaluate the label
expression within a struct of contextual data, instead of
within the value itself. This struct has the following fields:
// data holds the original source data
// (perhaps one of several records in a file).
data: _
// filename holds the full path to the file.
filename: string
// index holds the 0-based index element of the
// record within the file. For files containing only
// one record, this will be 0.
index: uint & <recordCount
// recordCount holds the total number of records
// within the file.
recordCount: int & >=1
Handling multiple documents or streams
To handle multi-document files, such as JSON Lines or YAML
files with document separators (---), the user must specify
a the --path, --list, or --files flag.
The --path flag merges each element into a single package as
if each element was defined in a separate file. The --list flag
concatenates each entry in a file into a list.
Using --list flag in combination with the --path flag
concatenates entries with the same path into a list, instead of
unifying them.
Finally, the --files option causes each entry to be written to
a different file. The -files flag may only be used in
combination with the import command.
# Put a value at a path based on its "kind" and "name" fields.
$ cue eval -l 'strings.ToLower(kind)' -l name foo.yaml
# Include a schema under the "myschema" field using the path notation.
$ cue eval -l myschema: schema: foo.json
# Base the path values on its kind and file name.
$ cue eval --with-context -l 'path.Base(filename)' -l data.kind foo.yaml
var filetypeHelp = &cobra.Command{
Use: "filetypes",
Short: "supported file types and qualifiers",
Long: `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.
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
For instance,
cue eval json:
specifies that '' 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
A qualifier applies to all files following it on the command line
until the next qualifier. The cue tool does not allow a ':' in
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.
# 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
var injectHelp = &cobra.Command{
Use: "injection",
Short: "inject files or values into specific fields for a build",
Long: `Many of the cue commands allow injecting values or
selecting files from the command line using the --inject/-t flag.
Injecting files
A "build" attribute defines a boolean expression that causes a file
to only be included in a build if its expression evaluates to true.
There may only be a single @if attribute per file and it must
appear before a package clause.
The expression is a subset of CUE consisting only of identifiers
and the operators &&, ||, !, where identifiers refer to tags
defined by the user on the command line.
For example, the following file will only be included in a build
if the user includes the flag "-t prod" on the command line.
// File prod.cue
package foo
Injecting values
The injection mechanism allows values to be injected into fields
that are not defined within the scope of a comprehension, list, or
optional field and that are marked with a "tag" attribute. For any
field of the form
field: x @tag(key)
an "--inject key=value" flag will modify the field to
field: x & "value"
By default, the injected value is treated as a string.
Alternatively, the "type" option allows a value to be interpreted
as an int, number, or bool. For instance, for a field
field: x @tag(key,type=int)
the flag "-t key=2" modifies the field to
field: x & 2
Valid values for type are "int", "number", "bool", and "string".
A tag attribute can also define shorthand values, which can be
injected into the fields without having to specify the key. For
instance, for
environment: string @tag(env,short=prod|staging)
"-t prod" sets the environment field to the value "prod". It is
still possible to specify "-t env=prod" in this case.
Use the usual CUE constraints to limit the possible values of a
field. For instance
environment: "prod" | "staging" @tag(env,short=prod|staging)
ensures the user may only specify "prod" or "staging".
var commandsHelp = &cobra.Command{
Use: "commands",
Short: "user-defined commands",
Long: `Commands define actions on instances. For example, they may
specify how to upload a configuration to Kubernetes. Commands are
defined directly in tool files, which are regular CUE files
within the same package with a filename ending in _tool.cue.
These are typically defined at the module root so that they apply
to all instances.
Each command consists of one or more tasks. A task may, for
example, load or write a file, consult a user on the command
line, fetch a web page, and so on. Each task has inputs and
outputs. Outputs are typically filled out by the task
implementation as the task completes.
Inputs of tasks my refer to outputs of other tasks. The cue tool
does a static analysis of the configuration and only starts tasks
that are fully specified. Upon completion of each task, cue
rewrites the instance, filling in the completed task, and
reevaluates which other tasks can now start, and so on until all
tasks have completed.
Available tasks can be found in the package documentation at
More on tasks can be found in the commands help topic.
In this simple example, we define a command called "hello",
which declares a single task called "print" which uses
"tool/exec.Run" to execute a shell command that echos output to
the terminal:
$ cat <<EOF > hello_tool.cue
package foo
import "tool/exec"
city: "Amsterdam"
who: *"World" | string @tag(who)
// Say hello!
command: hello: {
print: exec.Run & {
cmd: "echo Hello \(who)! Welcome to \(city)."
We run the "hello" command like this:
$ cue cmd hello
Hello World! Welcome to Amsterdam.
$ cue cmd --inject who=Jan hello
Hello Jan! Welcome to Amsterdam.
In this example we declare the "prompted" command which has four
tasks. The first task prompts the user for a string input. The
second task depends on the first, and echos the response back to
the user with a friendly message. The third task pipes the output
from the second to a file. The fourth task pipes the output from
the second to standard output (i.e. it echos it again).
package foo
import (
city: "Amsterdam"
// Say hello!
command: prompter: {
// save transcript to this file
var: file: *"out.txt" | string @tag(file)
ask: cli.Ask & {
prompt: "What is your name?"
response: string
// starts after ask
echo: exec.Run & {
cmd: ["echo", "Hello", ask.response + "!"]
stdout: string // capture stdout
// starts after echo
append: file.Append & {
filename: var.file
contents: echo.stdout
// also starts after echo
print: cli.Print & {
text: echo.stdout
The types of the commands and tasks are defined in CUE itself at
command: [Name]: Command
Command: {
// Tasks specifies the things to run to complete a command. Tasks are
// typically underspecified and completed by the particular internal
// handler that is running them. Tasks can be a single task, or a full
// hierarchy of tasks.
// Tasks that depend on the output of other tasks are run after such tasks.
// Use $after if a task needs to run after another task but does not
// otherwise depend on its output.
// Example:
// mycmd [-n] names
$usage?: string
// short is short description of what the command does.
$short?: string
// long is a longer description that spans multiple lines and
// likely contain examples of usage of the command.
$long?: string
// Tasks defines a hierarchy of tasks. A command completes if all
// tasks have run to completion.
Tasks: Task | {
[name=Name]: Tasks
// Name defines a valid task or command name.
Name: =~#"^\PL([-](\PL|\PN))*$"#
// A Task defines a step in the execution of a command.
Task: {
$type: "tool.Task" // legacy field 'kind' still supported for now.
// kind indicates the operation to run. It must be of the form
// packagePath.Operation.
$id: =~#"\."#
// $after can be used to specify a task is run after another one, when
// it does not otherwise refer to an output of that task.
$after?: Task | [...Task]
// TODO: tags
// - doc/nodoc
// - attr/noattr
// - id=<url>
// TODO: filetypes:
// - binpb
// TODO: cue.mod help topic