ast-grep Rule Crafter

ast-grep uses tree-sitter to parse code into AST, enabling precise pattern matching. Rules are defined in YAML for linting, searching, and rewriting code.

Quick Start

id: no-console-log
language: JavaScript
rule:
  pattern: console.log($$$ARGS)
fix: logger.log($$$ARGS)
message: Replace console.log with logger

Project Configuration

Project-level scanning requires an sgconfig.yml configuration file:

# sgconfig.yml (project root)
ruleDirs:
  - rules          # Rule directory, recursively loads all .yml files

Typical project structure:

my-project/
├── sgconfig.yml
├── rules/
│   ├── no-console.yml
│   └── custom/
│       └── team-rules.yml
└── src/

Running project scan:

ast-grep scan              # Automatically finds sgconfig.yml
ast-grep scan --config path/to/sgconfig.yml  # Specify config

Note: ast-grep scan command requires sgconfig.yml, while ast-grep run -p can be used standalone.

Rule Workflow

Lint Rule (Common)

Check only without fixing, used for CI/editor hints:

# rules/no-console-log.yml
id: no-console-log
language: JavaScript
severity: warning
message: Avoid console.log in production code
rule:
  pattern: console.log($$$ARGS)

Verify:

ast-grep scan -r rules/no-console-log.yml src/

Rewrite Rule (Optional)

Add fix when auto-fix is needed:

id: no-console-log
language: JavaScript
severity: warning
message: Replace console.log with logger
rule:
  pattern: console.log($$$ARGS)
fix: logger.log($$$ARGS)

Apply fix:

ast-grep scan -r rules/no-console-log.yml --update-all src/

Development Workflow

- [ ] 1. Explore pattern with CLI: ast-grep -p 'pattern' src/
- [ ] 2. Create rule file (.yml)
- [ ] 3. Verify: ast-grep scan -r rule.yml src/
- [ ] 4. If false positives → add constraints → re-verify

Debug AST structure:

ast-grep -p 'console.log($ARG)' --debug-query ast

Essential Syntax

Element	Syntax	Example
Single node	$VAR	console.log($MSG)
Multiple nodes	$$$ARGS	fn($$$ARGS)
Same content	Use same name	$A == $A
Non-capturing	$_VAR	$_FN($_FN)

Core Rules Quick Reference

Type	Purpose	Example
pattern	Match code structure	pattern: if ($COND) {}
kind	Match AST node type	kind: function_declaration
all	Match ALL conditions	all: [pattern: X, kind: Y]
any	Match ANY condition	any: [pattern: var $A, pattern: let $A]
not	Exclude matches	not: {pattern: safe_call()}
has	Must have child	has: {kind: return_statement}
inside	Must be in ancestor	inside: {kind: class_body}

Detailed References

Complete syntax guide: See rule-syntax.md

• Atomic rules (pattern, kind, regex, nthChild, range)
• Composite rules (all, any, not, matches)
• Relational rules (has, inside, follows, precedes)
• Transform and fixConfig

Language-specific patterns: See common-patterns.md

• JavaScript/TypeScript examples
• Python examples
• Go and Rust examples

Supported Languages

Bash, C, Cpp, CSharp, Css, Elixir, Go, Haskell, Hcl, Html, Java, JavaScript, Json, Kotlin, Lua, Nix, Php, Python, Ruby, Rust, Scala, Solidity, Swift, Tsx, TypeScript, Yaml