ast-grep

ast-grep Code Search

Translate a natural-language query into an ast-grep rule, verify it against a sample, then run it on the codebase. Full rule syntax (atomic/relational/composite rules, metavariables, troubleshooting, worked examples) lives in references/rule_reference.md — read it when writing anything beyond a simple pattern.

Prerequisite

ast-grep --version must work. If missing: brew install ast-grep (macOS), npm i -g @ast-grep/cli, or cargo install ast-grep. Invoke as ast-grep (the sg alias collides with the system sg).

Workflow

1. Clarify — language? exact structure to match? edge cases to include/exclude?
2. Make a sample — write a tiny snippet that should match, save to a temp file.
3. Write the rule — start with the simplest thing that could work, then add complexity:
   • pattern for direct code shapes → kind for node types → relational (has/inside) → composite (all/any/not).
   • Always add stopBy: end to relational rules so they traverse the whole subtree instead of stopping at the first non-match.
4. Test against the sample until it matches (see Debugging below).
5. Run on the codebase once it matches.

Commands

Goal	Command
Simple pattern search	ast-grep run --pattern 'console.log($ARG)' --lang js .
Rule-file search	ast-grep scan --rule rule.yml /path
Inline rule, no file	ast-grep scan --inline-rules "$YAML" /path
Test a rule on a snippet	echo '<code>' | ast-grep scan --inline-rules "$YAML" --stdin
Inspect AST to find kind values	ast-grep run --pattern '<code>' --lang js --debug-query=cst

Add --json to any of the above for structured output. --debug-query accepts cst (all nodes), ast (named nodes only), or pattern (how ast-grep reads your pattern).

Use run --pattern for simple single-node matches; use scan (rule file or inline) when you need relational or composite logic.

Escaping in inline rules

The shell expands $, so in double-quoted --inline-rules write \$VAR, or wrap the YAML in single quotes:

ast-grep scan --inline-rules "rule: {pattern: 'console.log(\$ARG)'}" .
ast-grep scan --inline-rules 'rule: {pattern: "console.log($ARG)"}' .

Debugging a rule that won't match

1. Simplify — strip sub-rules down to the smallest failing piece.
2. Confirm relational rules have stopBy: end.
3. --debug-query=cst to read the real node structure and verify your kind values.
4. Check metavariables are detected (see the reference's metavariable notes).

Resources

• references/rule_reference.md — full rule syntax and worked examples.
• ast-grep docs · playground for testing patterns online.