20000419/fauxnix

CI npm version npm downloads license 20000419/fauxnix MCP server

Run Linux-style commands on Windows — natively, deterministically, with no VM and no WSL.

fauxnix is a bash→PowerShell translation layer built for AI agents. Your agent keeps writing the bash it already knows (ls -la | grep foo, find . -name '*.ts' | wc -l, kill -9 1234), and fauxnix deterministically translates each command into PowerShell, executes it natively, and hands back output that looks like GNU/Linux: ls -l columns, bash-style error messages, coreutils exit codes, UTF-8/GBK handled automatically.

npm install -g fauxnix-cli    # then point any MCP harness at `fauxnix mcp`

$ fauxnix "ls -la src | head -2"
-rw-r--r-- 1 me me 1204 Aug 16 09:12 ast.ts
-rw-r--r-- 1 me me 8192 Aug 16 09:12 cli.ts

$ fauxnix "cat nope.txt"
cat: nope.txt: No such file or directory        # not a PowerShell stack trace

Measured: your model is probably worse at PowerShell than you think

Same model (DeepSeek-V4-Pro), same 5 tasks, three execution modes on one Windows machine — full data in docs/benchmark-deepseek-v4-pro.md and [`docs/benchmark-ark-models.md](docs/benchmark-ark-models.md):

	PowerShell	fauxnix	Git Bash
tool calls / unexpected errors	14 / 9	7 / 0	4 / 0
time (T1–T4)	163s	66s	57s

Across 7 models on the Volcano Ark Coding Plan, the PowerShell-vs-fauxnix gap held for every model tested — worst case (kimi-k2-thinking): 3.1× slower with 24 error events writing PowerShell vs zero errors through fauxnix. fauxnix lands within ~15% of the real-bash ceiling with no bash toolchain installed.

Why

LLM agents are dramatically better at bash than at PowerShell — bash dominates training data, so models on Windows often produce "looks right, doesn't run" commands (wrong quoting, curl that isn't curl, mojibake from codepage mismatches, inscrutable CategoryInfo error dumps). Existing solutions are either a full VM (WSL — heavy, wrong filesystem, separate environment) or plain shell wrappers (still PowerShell underneath).

fauxnix takes the third road: translate, don't emulate. A large, high-value subset of the Linux command line — file ops, text processing, process management, archives, networking basics — maps cleanly onto PowerShell + .NET. fauxnix implements that subset faithfully and fails loudly and helpfully on what it can't translate, so the agent never gets silently-wrong results.

Labs now train computer-use agents on fleets of real desktops. Reporting in 2026 (The Information, widely repeated) has OpenAI buying tens of thousands of Mac mini / Mac Studio boxes — no screen, no keyboard — to reinforcement-learn agents that click, edit, test, and run bash workflows, and Anthropic renting Mac minis through AWS for the same class of work. That scoring environment is macOS. Windows users should not have to install a guest Unix to keep up: the agent keeps writing bash; fauxnix makes the Windows box answer like the box the agent was trained on. See docs/rfc-computer-use-windows.md.

Install

npm install -g fauxnix-cli

Or from source:

git clone https://github.com/20000419/fauxnix && cd fauxnix
npm ci
npm install -g .

npm package name is fauxnix-cli (the fauxnix name on npm belongs to an unrelated 2015 websocket library); the installed command is still fauxnix.

Requires: Windows with PowerShell 5.1+ (built-in) and Node.js ≥ 18.

PowerShell 7 is an opt-in, CI-tested tier:

$env:FAUXNIX_PS = 'pwsh'
fauxnix check                 # edition: Core

Set the variable before starting fauxnix or its MCP harness; restart the harness after changing it. Windows PowerShell 5.1 remains the default. Invalid values and a missing selected executable fail loudly rather than falling back. The default is pinned below SystemRoot; pwsh.exe is resolved once from absolute PATH directories, excluding the current working directory. See PowerShell 7 support.

Quick start

# one-off commands
fauxnix "ls -la"
fauxnix "grep -rn TODO src | wc -l"
fauxnix "cat log.txt | grep -i error | sort | uniq -c"

# see what a command becomes (great for debugging / learning PS)
fauxnix translate "find . -name '*.log' -mtime +7 -delete"

# check your environment
fauxnix check
fauxnix doctor                   # check + encoding, harness config, MCP

# write user-level MCP config (idempotent; also --codex/--opencode/--kimi/--qwen)
fauxnix install --claude

# run the MCP stdio server (what agent harnesses connect to)
fauxnix mcp

translate only renders a script and does not read command operands. Because sed -f needs a script file, translate-only mode asks you to use -e with inline script text; normal command execution continues to support sed -f.

Unknown commands (git, node, npm, python, cargo, gh, docker, ...) are passed through natively with argv-style quoting. Windows .cmd/.bat shims necessarily pass through cmd.exe; fauxnix preserves its supported punctuation and fails loudly for %, embedded double quotes, NUL, and line breaks rather than passing a different argument.

Use with your agent harness

fauxnix ships an MCP stdio server exposing a bash tool (plus fauxnix_translate and fauxnix_session). Point any MCP-capable harness at it with fauxnix install --claude (or --codex / --opencode / --kimi / --qwen). Idempotent; prints what changed. Manual configs below.

• Quickstarts — copy-paste config + a 10-command smoke: docs/examples/

Claude Code

claude mcp add fauxnix -- fauxnix mcp

Codex (~/.codex/config.toml or codex mcp add fauxnix -- fauxnix mcp)

[mcp_servers.fauxnix]
command = "fauxnix"
args = ["mcp"]

Note: in non-interactive codex exec mode, MCP tool calls are auto-denied by the approval layer; pass --dangerously-bypass-approvals-and-sandbox (or run interactively and approve once).

OpenCode (opencode.json)

{
  "mcp": {
    "fauxnix": { "type": "local", "command": ["fauxnix", "mcp"] }
  }
}

Kimi Code — unlike the others, MCP servers live in a JSON file, not the TOML config: ~/.kimi-code/mcp.json

{
  "mcpServers": {
    "fauxnix": { "command": "fauxnix", "args": ["mcp"] }
  }
}

Qwen Code (~/.qwen/settings.json)

fauxnix install --qwen

The installer preserves the rest of settings.json and writes an absolute Node + package-entry launcher so Qwen startup does not depend on its working directory or PATH order. See the Qwen example for the generated JSON shape.

Any MCP client — stdio server: fauxnix mcp. The tool name is bash (override with FAUXNIX_TOOL_NAME). Tool description already teaches the model the supported subset, so no system-prompt changes are required.

The MCP session persists cwd, environment variables, export/unset, cd -/OLDPWD, and positional parameters (set -- / $1 / "$@") across tool calls — it behaves like a logged-in shell, not a stateless exec. $0 is the MCP tool name (bash / FAUXNIX_TOOL_NAME), not a Windows path.

What's translated

~105 commands, all output-matched against real GNU coreutils on Windows (Git Bash) during development:

• files: ls cp mv rm mkdir rmdir touch mktemp ln readlink realpath basename dirname stat file du df find chmod chown diff
• text filters: grep egrep sed awk sort uniq cut tr — sed/awk scripts are parsed while preparing an executable plan (unsupported constructs throw named errors, never silently misbehave); inspect-only translate keeps sed -f file reads out of that path
• text I/O: echo printf cat head tail wc tee nl tac md5sum sha1sum sha256sum base64 seq yes xargs

The curated agent-daily 60 carry a CommandSpec: unknown options fail with a GNU-style usage error instead of being ignored. The generated docs/command-specs.md is the exact list, coverage count, option table, and exclusion rationale; fauxnix list --json exposes the same per-command metadata. find stays unspec'd so predicates like -name still compile; sed/awk/egrep keep their command-specific parsers; tar remains native to tar.exe so supported bsdtar options reach the executable. Implemented GNU holes include cp -n / mv -n / touch -c / tee --append / grep -m / head --lines / du --max-depth / env -u / ps -f / command -V / date --date=@SECONDS.

• shell/system: cd pwd export unset env printenv ps kill pkill pgrep sleep which type whoami id groups date uname hostname uptime free nproc clear true false test [ [[ : pushd popd dirs sudo timeout man history less more source . eval exit alias set shift
• network: curl wget ping netstat ss ip ifconfig nslookup dig host
• archives: tar gzip gunzip zcat zip unzip

Plus shell syntax: pipes, && / || / ;, redirections (> >> 2> 2>&1 < &>, /dev/null), quoting, $VAR $1 $# "$@" set -- shift, ${name:-word} ${name//pat/str} ${name:off:len} ${name[n]} ${#name[@]}, A=(x y z) array assignment, $(...) command substitution, VAR=x cmd prefixes, ~ expansion, and POSIX-style path normalization (/tmp, /d/foo → D:\foo).

Exit codes follow bash conventions: 0 ok, 1 fail, 2 usage/serious, 127 command not found, 124 timeout.

How it works

bash command ──parser──▶ AST ──translator──▶ PowerShell script ──executor──▶ selected PowerShell
                                                                              │
agent ◀── GNU-style output, bash-style errors ◀── UTF-8 framed host protocol ◀┘

• Deterministic translation, zero LLM calls at runtime.
• Each command maps to a generator that emits a self-contained PowerShell block honoring the "Fauxnix contract": string-per-line stdout, [Console]::Error.WriteLine for bash-style stderr, $script:fx_exit for exit codes, $input for stdin.
• The executor wraps every script with UTF-8 enforcement ([Console]::OutputEncoding, $OutputEncoding, chcp 65001), decodes native output at the process boundary (UTF-8 by default or GBK(936) in ansi mode), keeps host frames UTF-8, strips CLIXML serialization and PowerShell noise from stderr, and rewrites common PowerShell errors (including zh-CN locale messages) into bash phrasing.
• Scripts run via -EncodedCommand (UTF-16LE) and transparently fall back to a temp .ps1 file when the 32 KB command-line limit would be exceeded.

Known deviations (honest list)

fauxnix optimizes for the commands agents actually run. Documented deviations:

• X=1 standalone assignments follow export semantics (one session-wide environment; bash's shell-var vs exported-var distinction does not exist), and a same-segment prefix is visible to $VAR inside the command's own words (Z=in [[ $Z == in ]] is true here, false in bash where word expansion precedes the temporary environment).
• yes is capped at 65,536 lines — PS 5.1 pipelines cannot signal upstream producers to stop, so an unbounded yes | head would hang.
• tail -f, eval, alias, heredocs, env -i/--ignore-environment, background &, and output/fd redirects (> >> 2> 2>> &> &>> 2>&1 1>&2) on a non-last pipeline stage (echo hi >f | cat) are rejected with operation-specific, actionable error messages instead of misbehaving. Per-stage < remains supported; fully routed per-stage output fds are still tracked by #157. (if/then/elif/else/fi, for x in ..., while/until, case ... esac (;; only; ;&/;;& fail loud), backtick substitution, command -v, pipeline read, dotenv-style source, word-level $((...)) arithmetic expansion, A=(x y z) arrays, and ${name//pat/str} / ${name:off:len} are supported.)
• command -v <builtin> prints /usr/bin/<name> where bash prints the bare builtin name; exit codes and empty-result semantics match.
• chmod maps only the read-only bit; exec bits are no-ops on Windows. chown is a silent no-op (as in Git Bash).
• ps aux columns are approximations (no per-process CPU% accounting, USER shows ?).
• gzip -c/pipeline stdin is text-faithful, not byte-faithful; file-mode gzip f is byte-exact.
• A pipeline producing exactly one line, piped into wc -l, counts that line (bash would count 0 if the producer omitted the trailing newline). printf 'x' | md5sum stays byte-exact.
• sed/awk support the common subset; hold-space, labels, arrays, loops throw named "not supported" errors at translate time.
• curl/wget refuse loopback/private/reserved addresses (localhost, 127.x, ::1, 10.x, 172.16–31.x, 192.168.x, 169.254.x) as a safety default for agent-driven HTTP.
• Native-tool pipelines vs encoding: PS 5.1 has a single console-encoding knob, so piping localized admin tools (ipconfig, tasklist — GBK on zh-CN) and UTF-8-native dev tools (node, curl) cannot both decode cleanly mid-pipeline. Default favors UTF-8 dev tools; set FAUXNIX_NATIVE_ENCODING=ansi when your agents grep Chinese output of native Windows admin tools. File reads are always sniffed per file (UTF-8 strict → GBK fallback), so grep/sed/awk over GBK files works in either mode — unlike Git Bash, which only matches the encoding its locale assumes.

Development

npm install
npm test          # unit + real-PowerShell integration suite (Windows only, auto-skipped elsewhere)
$env:FAUXNIX_PS = 'pwsh'; npm test   # same suite through PowerShell 7
npm run build
npx tsx scratch/run.mjs "any bash command"   # quick live check

Differential vs Git Bash is opt-in (FAUXNIX_DIFF_ORACLE=1; skips if unset or bash.exe is missing — Git Bash is not required). See test/differential/README.md. The 253-case corpus enforces the RFC C-7 minimum of 200 cases and a 95% identity gate. The weekly oracle runs from .github/workflows/differential.yml; two consecutive green scheduled runs are still required release evidence after this gate lands.

Architecture map: src/parser.ts (bash subset → AST) · src/translator.ts (AST → PowerShell + executor wrapper) · src/executor.ts (spawn, redirects, session persistence) · src/commands/*.ts (per-command generators) · src/mcp.ts (MCP server) · src/cli.ts.

Roadmap: docs/rfc-roadmap-to-1.0.md — tracks, milestones, and the RFC process for proposing waves.

Security

Trust model, host protocol, kill semantics, network guard, and reporting: SECURITY.md.

License

MIT © 20000419