# Atmos Automation Language Reference

The Atmos Automation Language is based on Starlark, a Python-like language.
This reference covers its syntax, data types, and execution rules. For a first
program and instructions to run it, start with [Custom CLI Apps](/automation/standalone-cli-apps).

## What this reference covers

The language is Starlark with four options that Atmos turns on: control flow such as `if` and `for`
at the top level of a file, `while` loops, sets through the `set()` function, and recursion.
Atmos uses `.star` files for standalone scripts. Values such as integers,
strings, lists, and dictionaries are built into the language. Atmos also supplies
`sum()` and `round()` for numeric calculations. Functions for
running commands and reading component configuration are supplied by Atmos.

This reference covers the language. For the Atmos-supplied helpers and values, such as
`exec.run`, `steps.parallel`, `fs.read_file`, `ctx`, and `output`, see the
[automation function reference](/functions/automation). For how scripts run in each entry point,
see the [script step](/steps/type/script) and the overview in
[Atmos Automation Language](/automation/language).

A first script shows the pieces together:

```python
def capacity(replicas, workers_per_replica):
    if replicas < 1:
        fail("replicas must be positive")
    return replicas * workers_per_replica

print("Total workers: {}".format(capacity(3, 4)))  # Total workers: 12
```

## Topics

1. [Lexical Elements](/automation/reference/lexical-elements) covers how source text is read:
   comments, identifiers, keywords, literals, indentation, and line continuation.
2. [Data Types](/automation/reference/data-types) covers `None`, booleans, integers, floats,
   strings, bytes, lists, tuples, dictionaries, sets, functions, and how values compare and
   convert.
3. [Expressions and Operators](/automation/reference/expressions-operators) covers operator
   precedence, comparisons, slicing, comprehensions, conditional expressions, and calls.
4. [Statements](/automation/reference/statements) covers assignment, `if`, `for`, `while`, `def`,
   `return`, `load`, and the other statements.
5. [Built-in Functions](/automation/reference/builtin-functions) lists the universal constants
   and functions, and maps the names that Atmos adds.
6. [Methods](/automation/reference/methods) lists the methods of strings, lists, dictionaries,
   sets, and bytes.
7. [Execution Model](/automation/reference/execution-model) explains single-assignment globals,
   frozen values, determinism, errors and tracebacks, recursion, cancellation, module loading,
   output streams, and dry runs.
8. [Differences from Python](/automation/reference/differences-from-python) gives Python
   programmers a practical list of what works differently or is missing.

## Where to go next

- [Atmos Automation Language](/automation/language) introduces the language, why Atmos uses
  Starlark, and how each entry point supplies inputs.
- [Automation Functions](/functions/automation) documents every helper and value that Atmos adds,
  one page each.
- [Script step](/steps/type/script) describes `type: script` with `interpreter: starlark`,
  including module paths, working directory, containers, and dry runs.