fauxnix

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

One-command install for Claude Code · Codex · OpenCode · Kimi Code · Qwen Code, plus any MCP client. 109 translated commands · 400+ automated tests · 253-case differential corpus verified against real GNU coreutils · zero LLM calls at runtime.

Try it now — no install

npx fauxnix-cli@latest "ls -la src | head -3"
npx fauxnix-cli@latest translate "find . -name '*.log' -mtime +7 -delete"

$ 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

Connect your agent — one command

npm install -g fauxnix-cli
fauxnix install --claude     # or --codex / --opencode / --kimi / --qwen
fauxnix doctor               # verifies encoding, harness config, and MCP round-trip

Idempotent; prints exactly what changed. Manual configurations below if you prefer to edit config files yourself.

npm package name is fauxnix-cli (the fauxnix name on npm belongs to an unrelated 2015 websocket library); the installed command is fauxnix. Requires Windows with PowerShell 5.1+ (built-in) and Node.js ≥ 22.20.0. This is the next-release support policy for this checkout. Published v1.0.0 retains its original engine metadata. Current Node 22/24 LTS patches are tested.

claude mcp add fauxnix -- fauxnix mcp

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

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

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

fauxnix install --qwen

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, not a Windows path.

For workflows whose commands are already known, the MCP server also exposes bash_batch: it compiles every step before execution, runs the plan atomically in one session, and returns one structured result per step in a single MCP round trip (stops on first nonzero exit by default).

{
  "steps": [
    { "id": "write", "command": "printf 'a\\r\\nb' > data.txt" },
    { "id": "measure", "command": "wc -c data.txt" }
  ]
}

See compiled MCP batch plans for timeout, budget, cancellation, and preflight semantics.

Experimental: native shell facade

fauxnix facade (experimental) implements the bash process contract — facade -c "<cmd>" one-shots with bash argv semantics ($0/$1 from trailing operands) and a persistent stdin session with <bash-input> markers — so a harness can eventually point its built-in Bash tool at a fauxnix-backed bash.exe instead of an MCP sidecar, with zero protocol round trips. Status, launcher strategy, and security rules: RFC — native shell facade.

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:

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).

If Git Bash already works for you, keep it — we literally use it as our differential-testing oracle. fauxnix is for when you can't or don't want to ship one: agent fleets where the bash toolchain drifts or isn't detected (the Windows ARM64 Git-Bash detection failure is a live example), CI runners, locked-down machines, or anywhere a single npm install -g is easier than a toolchain.

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. That matters as labs train computer-use agents on Mac fleets — the agent keeps writing bash; fauxnix makes the Windows box answer like the box the agent was trained on (RFC: computer-use parity).

What's translated

109 commands, 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)
• text I/O: echo printf cat head tail wc tee nl tac md5sum sha1sum sha256sum base64 seq yes xargs
• 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

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.

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.

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.

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, decodes native output at the process boundary (UTF-8 by default or GBK(936) in ansi mode), strips CLIXML serialization and PowerShell noise from stderr, and rewrites common PowerShell errors (including zh-CN locale messages) into bash phrasing. File reads are always sniffed per file (UTF-8 strict → GBK fallback), so grep/sed/awk over GBK files works in either mode.
• 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.

PowerShell 7 is an opt-in, CI-tested tier: set FAUXNIX_PS=pwsh before starting fauxnix or its MCP harness. The default is Windows PowerShell 5.1; invalid values fail loudly rather than falling back. See PowerShell 7 support.

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 on a non-last pipeline stage are rejected with operation-specific, actionable error messages instead of misbehaving. Per-stage < remains supported. (if/then/elif/else/fi, for x in ..., while/until, case ... esac (;; only), 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.
• Gzip decompression and gzip -t use Node's strict streaming decoder, including every concatenated member. Incomplete streams and checksum errors fail without committing a destination or deleting the archive. Trailing non-gzip garbage is an error rather than GNU's warning. Text stdout is validated before emission and needs temporary disk space proportional to decoded size; -t discards output. Standalone translated scripts require supported node.exe on PATH. See the strict gzip contract for cancellation and spool limits.
• mv replaces regular files only within one volume, without predeleting the destination. Cross-volume replacement, leaf reparse-point replacement, and replacing an existing empty directory fail explicitly with originals retained; -f does not bypass these limits. Existing-target -n still skips. Ordinary absent-target moves retain PowerShell behavior and are not transactional across volumes. Identity checks and rename are separate operations, not a race-proof transaction. See the move contract.
• stat --printf omits the implicit newline and interprets text escapes, including ASCII octal/hex escapes. Numeric escapes producing bytes 0x80–0xFF are rejected before output; use literal Unicode text instead. NUL is preserved in direct output, text pipelines, and redirected files; command substitution retains fauxnix's existing text behavior rather than Bash's NUL-removal behavior.
• 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. For awk programs using printf, terminal and file-redirect output streams per formatted fragment. Pipeline and command-substitution output is coalesced into one text value to preserve adjacent fragments without adding separators. That intermediate value requires memory proportional to output size and is not capped by the caller's capture budget.
• 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.

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.

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