unbash

Fast 0-deps bash parser written in TypeScript

Install

npm install unbash

When to use unbash?

Use unbash when your input is Bash source: a pasted command, a complete script, or shell source embedded in another format, and you need to inspect its structure without executing it. It returns a typed, source-positioned AST.

Example use cases:

• Classify commands, redirects, substitutions, and background execution for permission prompts, allowlists, or audit findings
• Statically inventory executables and literal file, configuration, and dependency references in package scripts, CI steps, task-runner configs, hooks, and source-code shell calls
• Import or convert a supported command such as curl without folding neighboring pipelines, logical chains, redirects, or comments into its arguments
• Attach diagnostics to generated or pasted Bash using source-positioned errors and partial trees
• Build explanations, visualizations, or structured previews of pipelines, conditions, expansions, and nested commands
• Find or migrate command invocations and flags with source-range edits that leave surrounding formatting and comments untouched

When Bash is embedded in JSON, YAML, or source code, use the host-language parser to extract the shell string first. unbash provides Bash structure and source ranges within that string; the consumer owns command-specific argument semantics and policy.

Supported syntax

unbash supports commands, control flows, pipelines, redirects, assignments, compound statements, parameter and word expansions, process and nested substitutions, coproc, heredocs, herestrings, nested and generated syntax, etc.

Nested commands remain structured inside parameter operands and array indexes, arithmetic expressions, brace expansions, extglobs, redirect targets, and heredoc bodies. Nodes retain source positions and words retain both raw text and dequoted values.

Malformed and incomplete input returns a best-effort partial AST with detected, source-positioned errors. Recovery is bounded for deeply nested parameter and arithmetic expansions, substitutions, subshells, braces, conditionals, loops, select, case, and [[ ]] groups.

unbash does not execute code, perform shell expansion, provide a sandbox, or decide whether a command is safe. Security-sensitive consumers must inspect word parts, nested scripts, and errors on each parsed script. unbash is a tolerant parser: for malformed or incomplete input, it recovers where possible and returns a best-effort partial AST with source-positioned errors. It does not target PowerShell, cmd.exe, or other shell languages. Much POSIX sh syntax is also valid Bash.

To parse process.argv (string[]), use Node.js parseArgs or a CLI library such as yargs or citty.

Usage

import { parse } from "unbash";

const ast = parse('if [ -f "$1" ]; then cat "$1"; fi');

Result:

{
  type: "Script",
  commands: [{
    type: "Statement",
    command: {
      type: "If",
      clause: { type: "CompoundList", commands: [ /* [ -f "$1" ] */ ] },
      then: { type: "CompoundList", commands: [ /* cat "$1" */ ] }
    }
  }]
}

See the full AST at unbash.statichost.page#input=if [ -f...

Word parts

A Word holds its expansions in parts. This is a lazy getter, computed on first access (not an own enumerable property):

const source = "echo a$(id)b";
const word = parse(source).commands[0].command.suffix[0];

word.parts; // [Literal, CommandExpansion, Literal]

Object.keys(word); // ["type", "text", "pos", "end"]
({ ...word }); // same
structuredClone(word); // same

Read parts directly, or serialize with JSON.stringify, which includes it through toJSON. A generic walker driven by Object.keys finds no expansions at all, and reports no error while doing so:

import { parse } from "unbash";

const script = parse('echo "$HOME" $(mktemp)');

for (const statement of script.commands) {
  const command = statement.command;
  if (command.type !== "Command") continue;
  for (const word of [command.name, ...command.args]) {
    if (word?.type !== "Word") continue;
    for (const part of word.parts ?? []) {
      if (part.type === "CommandExpansion") console.log(part.text);
    }
  }
}
// $(mktemp)

BraceExpansion, ExtendedGlob, ArithmeticWord, and HereDocBody expose nested syntax through parts. Parameter and assignment indexes are Words with their own parts. Assignment values are Words or ArrayValues; array elements are Words. Declaration assignments and redirects are separate command items, each with its own children. Command.args and Command.redirects are lazy views over the ordered prefix and suffix items. They are getters like parts, but JSON.stringify omits them because the two lists already carry every item.

Positions are zero-based UTF-16 code-unit offsets forming half-open [pos, end) ranges in the source owned by the nearest ParsedScript. Root scripts and verbatim nested substitutions share the caller's source, so their ranges slice that source directly:

const nested = word.parts.find((part) => part.type === "CommandExpansion").script;
const command = nested.commands[0].command;

source.slice(command.pos, command.end); // exact nested command source

A legacy backtick script whose body contains backslash escapes owns its decoded string as an enumerable source property, retained by JSON serialization. Ordinary scripts nested inside it index that decoded source. Live nodes still have lazy prototype getters; use JSON.stringify when serializing their complete public syntax.

Parse errors inside a lazily parsed script surface on that script, not on the root: check errors on every nested script while traversing. A consumer that only reads the root errors array cannot tell that a substitution body failed to parse.

Print

Basic opinionated printer, does not preserve whitespace or comments (except shebang):

import { parse } from "unbash";
import { print } from "unbash/printer";

const ast = parse('if [ -f "$1" ]; then cat "$1"; fi');
const script = print(ast);

Result:

if [ -f "$1" ]; then
  cat "$1"
fi

unbash vs tree-sitter-bash

tree-sitter-bash is the right choice when you need:

• Incremental parsing
• CST output preserving all tokens and punctuation
• Error recovery with ERROR nodes and missing tokens

unbash provides:

• A typed, executable-syntax AST instead of a grammar CST
• A synchronous, zero-dependency TypeScript package with no native addon, WASM runtime, parser initialization, or query layer
• Structured word parts, arithmetic and test-expression trees, recursively parsed substitutions, and direct source positions
• Best-effort error recovery that preserves a partial AST and collects errors
• Also see tree-sitter-bash gaps covered by unbash

unbash vs sh-syntax

sh-syntax is a WASM wrapper around the robust mvdan/sh Go parser. It is highly recommended if you need:

• Support for Bash, POSIX sh, mksh, Bats, and experimental Zsh parsing
• Mature, configurable formatting and pretty-printing

unbash provides:

• A zero-dependency, synchronous TypeScript API without WASM loading
• A smaller, JSON-friendly Bash AST with lazy structured word parts, recursively parsed substitutions, and direct source positions
• Best-effort partial ASTs for malformed or incomplete editor and user input
• Also see sh-syntax gaps covered by unbash

unbash vs bash-parser

bash-parser and its fork @ericcornelissen/bash-parser (community dependency maintenance fork ❤️ now archived) provide:

• A POSIX-only mode that rejects bash-specific syntax

unbash provides:

• A zero-dependency architecture
• A typed TypeScript API (ESM-only)
• Best-effort error recovery that preserves a partial AST and collects errors
• Structured AST nodes for parameter expansions, arithmetic expressions, and [[ ]] test expressions; bash-parser treats [[ ]] as ordinary commands and (( )) as nested subshells
• Herestrings, C-style for, select, process substitution, coproc, array assignments, extglob, ;&/;;& case fallthrough, Bash 5.3 command substitutions, and {variable} file-descriptor redirects

Benchmarks

Parse throughput in MB/s; higher is better. Median per-run p75 iteration times from 11 runs per unbash version and 22 per other parser, on Apple M1 Pro/32GB with Node.js 24.19.0. Parentheses show speed relative to unbash v5.

Parser	short (1.1KiB)	advanced (0.9KiB)	medium (150KiB)	large (970KiB)
unbash v5	73.6	73.0	104.1	123.9
unbash v4	79.2 (0.93x)	71.3 (1.02x)	96.5 (1.08x)	100.7 (1.23x)
tree-sitter-bash (native)	4.58 (16x)	6.47 (11x)	15.00 (7x)	11.88 (10x)
tree-sitter-bash (WASM)	4.96 (15x)	5.61 (13x)	8.78 (12x)	8.21 (15x)
sh-syntax	0.03 (2268x)	0.04 (1728x)	8.26 (13x)	14.02 (9x)
bash-parser	0.26 (278x)	n/a	n/a	n/a
@ericcornelissen/bash-parser	0.26 (288x)	n/a	n/a	n/a

Run the benchmarks using Node.js 24+:

pnpm install
pnpm bench

Size

The parser bundle is 85KiB minified and 21KiB gzipped.

Playgrounds

• unbash.statichost.page
• ast-explorer.dev

License

ISC