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dagger/docs/core-concepts/1221-action.md
Solomon Hykes 7f38a6c3e4 Docs: core concepts: actions
Signed-off-by: Solomon Hykes <solomon@dagger.io>
2022-04-07 17:58:10 +00:00

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---
slug: /1221/action
displayed_sidebar: europa
---
# Dagger Actions
Actions are the basic building block of the Dagger platform.
An action encapsulates an arbitrarily complex automation into a simple
software component that can be safely shared, and repeatably executed by any Dagger engine.
Actions can be executed directly with `dagger do`, or integrated as a component of a more complex action.
There are two types of actions: *core actions* and *composite actions*.
## Core Actions
Core Actions are primitives implemented by the Dagger Engine itself. They can be combined into higher-level composite actions. Their definitions can be imported in the `dagger.io/dagger/core` package.
To learn more about core actions, see [the core action reference](https://github.com/dagger/dagger/tree/main/pkg/dagger.io/dagger/core).
## Composite Actions
Composite Actions are actions made of other actions. Dagger supports arbitrary nesting of actions, so a composite action can be assembled from any combination of core and composite actions.
One consequence of arbitrary nesting is that Dagger doesn't need to distinguish between "pipelines" and "steps": everything is an action. Some actions are just more complex and powerful than others. This is a defining feature of Dagger.
## Lifecycle of an Action
A composite action's lifecycle has 4 stages:
1. Definition
2. Integration
3. Discovery
4. Execution
## Definition
A new action is *defined* in a declarative template called a [CUE definition](https://cuetorials.com/overview/foundations/#definitions). This definition describes the action's inputs, outputs, sub-actions, and the wiring between them.
Here is an example of a simple action definition:
```cue
package hello
import (
"dagger.io/dagger/core"
)
// Write a greeting to a file, and add it to a directory
#AddHello: {
// The input directory
dir: dagger.#FS
// The name of the person to greet
name: string | *"world"
write: core.#WriteFile & {
input: dir
path: "hello-\(name).txt"
contents: "hello, \(name)!"
}
// The directory with greeting message added
result: write.output
}
```
Note that this action includes one sub-action: `core.#WriteFile`. An action can incorporate any number of sub-actions.
Also note the free-form structure: an action definition is not structured by a rigid schema. It is simply a CUE struct with fields of various types.
* "inputs" are simply fields which are not complete, and therefore can receive an external value at integration. For example, `dir` and `name` are inputs.
* "outputs" are simply fields which produce a value that can be referenced externally at integration. For example, `result` is an output.
* "sub-actions" are simply fields which contain another action definition. For example, `write` is a sub-action.
There are no constraints to an action's field names or types.
## Integration
Action definitions cannot be executed directly: they must be integrated into a plan.
A plan is an execution context for actions. It specifies:
* What actions to present to the end user
* Dependencies between those tasks, if any
* Interactions between the tasks and the client system, if any
Actions are integrated into a plan by *merging* their CUE definition into the plan's CUE definition.
Here is an example of a plan:
```cue
package main
import (
"dagger.io/dagger"
)
dagger.#Plan & {
// Say hello by writing to a file
actions: hello: #AddHello & {
dir: client.filesystem.".".read.contents
}
client: filesystem: ".": {
read: contents: dagger.#FS
write: contents: actions.hello.dir.result
}
}
```
Note that `#AddHello` was integrated *directly* into the plan, whereas `core.#WriteFile` was integrated *indirectly*, by virtue of being a sub-action of `#AddHello`.
To learn more about the structure of a plan, see [it all begins with a plan](./1202-plan).
## Discovery
Once integrated into a plan, actions can be discovered by end users, by using the familiar convention of usage messages:
```bash
$ dagger do --help
Execute a dagger action.
Available Actions:
hello Say hello by writing to a file
Usage:
dagger do [OPTIONS] ACTION [SUBACTION...] [flags]
Flags:
[...]
```
## Execution
Once the end user has discovered the action that they need, they can execute it with `dagger do`. For example:
```bash
dagger do hello
```