shell: open a Response over Bun.file(path) as a file redirect by robobun · Pull Request #46687 · oven-sh/bun

robobun · 2026-10-24T05:18:42Z

Problem

$`cmd < ${new Response(Bun.file(path))}` runs cmd on an empty stdin at exit 0 when the file is missing, and creates the file. The bare ${Bun.file(path)} exits 1 with bun: No such file or directory: <path>.
On main a spawned command gives the path to spawn, which opens it with O_CREAT (src/spawn_sys/spawn_process.rs:801). A builtin reads blob.shared_view(), empty for a file.
On that arm > does not truncate and >> does not append.

Fix

Interpreter::redirect_target classifies a redirect once. A Response over Bun.file(path) is a path word: both redirect setups open it with shell_openat and the flags of the operator.
The classifier only reads. The redirect takes the body after the open succeeds.
Verified: test/js/bun/shell/bunshell.test.ts, "redirect to a Response over Bun.file(path)" (46 tests, 37 fail on main), and 9 more shell suites.
Self-reviewed: 29 concerns raised, 27 addressed (Notes).

Background

A command is spawned or a builtin (echo, on Windows cat), each with its own redirect setup.
The template turns ${Bun.file(path)} into path text. ${response} stays a JS object until the command starts.
Weighed: classify in the template (never uses the body), drop O_CREAT in spawn only (cannot reach builtins).

Downsides

25 of 30 redirect cells change for such a Response, 10 from a throw to a write (ledger in Notes).
A builtin now blocks on a FIFO open (1 ms to 1452 ms in a probe), as the bare < fifo does.
.text +1,024 bytes. A redirect to another JS object runs 6 to 27 more instructions.

Notes

Origin and reach

A fuzz run found this. No user reported it, and there is no issue. The request was: "a Response whose body is a Blob over a file is opened like the bare Bun.file, so that a missing file fails the command."
The form that a person is most likely to write, < from a Response over an existing file on POSIX, already works on main.
await fetch("file:///missing") resolves with ok: true and status 200, so cat < ${await fetch(fileUrl)} is the same case. The caller has no sign that the body is a missing file.
On main, cmd < ${await response.blob()} and the bare ${Bun.file(path)} already fail as they should (measured on a release build).

Repro

import { $ } from "bun";
import { existsSync } from "node:fs";
const missing = process.cwd() + "/none.txt";
const r = await $`cat < ${new Response(Bun.file(missing))}`.quiet().nothrow();
console.log(r.exitCode, JSON.stringify(r.stderr.toString()), existsSync(missing));
// main:    0 "" true
// this PR: 1 "bun: No such file or directory: /.../none.txt" false

Behaviour changes, main to this branch

The matrix is 2 exec forms (a spawned command, builtin echo) x 5 operators (<, >, >>, 2>, &>) x 3 file states (existing, missing, in a missing directory). Target: new Response(Bun.file(path)). Linux x64, debug builds of main 5c6d4e2 and of this branch. The bare Bun.file(path) gives the same result on both builds in all 30 cells. This branch equals the bare form in 30 of 30 cells, main in 5.

Change	Cells	Which
Silent exit 0 becomes exit 1 that names the file	3	spawned `<` missing (main also creates the file), builtin `<` missing and missing directory
Wrong content at exit 0 becomes right content	3	spawned `>`, `>>`, `2>` to an existing file (main writes at offset 0 and keeps the tail)
A throw becomes a write at exit 0	10	builtin `>`, `>>`, `2>`, `&>` to an existing or missing file (8), spawned `&>` (2)
A throw becomes a shell error, exit 1	5	the same 5 pairs into a missing directory
Error text only	4	spawned `<`, `>`, `>>`, `2>` into a missing directory: main names the command, this branch names the file
Unchanged	5	spawned `<` existing, spawned `>`, `>>`, `2>` to a missing file, builtin `<` existing

The throws on main are Cannot redirect stdout/stderr to an immutable blob. Expected a file (builtin, for a file) and Blobs are immutable, and cannot be used for stdout/stderr (spawned &>, because the arm takes the body two times).

More changes outside the matrix:

After a failed open, response.bodyUsed stays false, and a stream that the caller holds stays unlocked. On main the body is used.
A new output file of a spawned command gets mode 0666 before umask, as a path word does. On main it gets 0664.
A relative path resolves against the cwd of the shell on every platform, as the bare form does. On main this was already so on Linux. On macOS and Windows main uses the process cwd (read from src/spawn_sys/posix_spawn.rs and src/spawn/process.rs, not run). response.text() keeps the process cwd.
Bun.file("") in a Response gives bun: ambiguous redirect, as the bare form does.
Windows only, read from the code and not run: > to such a Response when the file has the hidden or system attribute. On main a spawned command writes into the file, because spawn opens it with OPEN_ALWAYS. Here the open fails with EPERM, as > hidden.txt and the bare form do on main: shell_openat opens with CREATE_ALWAYS and has no fallback for a hidden file. That fallback is handed to a separate change.
The body forms that are still the file behave the same: after .body was read, new Response(Bun.file(path).stream()), .clone(), and fetch() of a file: URL.

What stays as on main

A second redirect from the same Response reads a used body: empty stdin at exit 0. The bare form reads the file again. shell: reject a stream, used, failed or S3-backed Response body as stdin #42786 turns a used body into a throw.
A sliced file (Bun.file(p).slice(1, 3)) is opened whole, in a Response or bare (Honor Bun.file().slice() bounds in Bun.write #41209, spawn: send a sliced Bun.file() at stdin in chunks of its window #44301).
Bun.file(fd) and in-memory bodies with a builtin: echo out > ${Bun.file(fd)} exits 0 and writes nothing, echo out > ${new Response(Bun.file(fd))} throws, echo out > ${new Response(new Blob(["x"]))} exits 0 and drops the output, builtin cat < ${Bun.file(fd)} reads nothing. These are other defects of the same function. shell: fix inverted file-blob check for builtin > ${Response(...)} redirect #35318 fixed them and was closed by the stale sweep. They are handed to a separate change.
Bun.spawn({ stdin: Bun.file(missing) }) still creates the file. spawn_process.rs:801 is not changed here. After this PR no shell redirect reaches that open.
A path with a NUL byte throws ERR_INVALID_ARG_VALUE, as the bare form does. The only producer on main is Bun.build with a NUL in naming or outdir, which panics in a debug build before it returns. That is why no test covers the branch. On a release build of this branch the throw is measured.

Numbers

Release builds of main 5c6d4e2 and of this change on that commit, Linux x64.

.text: 82,231,705 to 82,232,729 bytes (size -A).
Instructions inside Cmd::transition_to_exec, JIT off, single-stepped with gdb, warm runs, builtin echo: no redirect 1105 to 1101, path word 1587 to 1418, Buffer 1572 to 1588, Blob 1419 to 1446, in-memory Response 1730 to 1736.
Allocator calls per command (breakpoints on the mimalloc entry points, 220 minus 20 commands): path word 209.26 to 208.26, Buffer 201.26 to 201.26, in-memory Response 205.285 to 205.285.
Syscalls per command (ptrace count, 120 minus 20 commands), routes that do not change: open, openat, close, dup2, fcntl, fstat, read, write, socketpair are equal for no redirect, a path word, a Buffer and an in-memory Response, spawned and builtin.
Syscalls, Response over an existing file: spawned net +1 (bun: +1 openat, +1 fcntl, +1 close. The child before exec: -1 openat, -1 close). Builtin: +1 openat, +1 close (main opens nothing).
Scans of a string Response body per redirect (calls of to_utf8_from_latin1, debug builds): 2 on main, 2 on this branch.
FIFO: builtin echo hi < ${new Response(Bun.file(fifo))} with a writer that opens after 1.5 s takes 1 ms on main (it opens nothing) and 1452 ms here with 0 timer callbacks. The bare < fifo takes 1452 ms on both.
Opens of a missing read target with O_CREAT: each spawned shell form goes from 1 to 0. The 7 Bun.spawn stdin forms keep theirs.
strace, valgrind and bloaty are not installed on the machine. The counts come from a ptrace counter, gdb and size.

Related changes

shell: reject a stream, used, failed or S3-backed Response body as stdin #42786 rejects a Response body that is not in memory (a stream, a used body). It lets a file body pass, so the two do not overlap in behaviour. They edit the same two functions. The one that lands second puts throw_if_body_not_in_memory on the RedirectTarget::Object arm and merges the two docs sentences.
spawn: one open-flags table for Bun.file(path) stdio, stdout and stderr replace the file #46665 gives Bun.spawn path slots one flags table. It also sends a Response over a path file to shell_openat for spawned commands, in Cmd.rs. This PR does that for both exec forms through one classifier, so the one that lands second drops or rebases that hunk.
shell: open FIFO redirect targets on the work pool instead of the event-loop thread #40084 moves the open of a FIFO redirect off the JS thread for ast::Redirect::Atom. It does not see this route, because a Response becomes a path word only in redirect_target.
spawn: open Bun.file() stdio paths in the parent so a blocking open cannot freeze the runtime #37641 removes O_CREAT from the stdin path open in spawn. That is a Bun.spawn default change and is not part of this PR.
Refuse an S3-backed Blob at spawn stdin, in the shell, in Bun.build files and in redis #46673 throws for an S3-backed Blob in a shell command. It rewrites the Blob arm of handle_template_value as a match over the store kinds. This PR moved the path test of that arm into file_store_path, so the one that lands second merges the two.
spawn: reject ReadableStream, Request and Response for stdout/stderr #41812 rejects a Response at Bun.spawn stdout. This PR accepts a Response over a file as a shell output target, because main already writes through it for spawned >, >> and 2>.

Questions for a maintainer

Output: is a Response over Bun.file(path) an output target in the shell? Main accepts it for spawned >, >>, 2> with wrong flags and throws elsewhere. This PR makes it the path word for every operator. The other choice is to reject it for output. A sliced body is included: echo out > ${new Response(Bun.file(p).slice(1, 3))} replaces the file, as the bare ${Bun.file(p).slice(1, 3)} does on main. On main the builtin throws for it, and a spawned command writes at offset 0 and keeps the tail.
Command start or template time: this PR classifies when the command starts, so the redirect uses the body, as on main. A template-time form would make ${response} a path in every position and never use the body.
The relative path base on macOS and Windows (see the ledger).
Should Bun.spawn({ stdin: Bun.file(missing) }) stop creating the file? It is a separate change.

Verification

bun bd test test/js/bun/shell/bunshell.test.ts: 510 pass, 0 fail. The new block on a debug build of main: 9 pass, 37 fail. The 9 are controls.
Also run with the fix: bunshell-file, bunshell-instance, file-io, lazy, throw, exec, yield, pipeline_stack, assignments-in-pipeline.
test/js/web/fetch/body.test.ts: 775 or 776 of 776 pass. The string body consumption does not leak cases time out at 5 s at random in a debug build, on main too.
bun run rust:check-all x86_64-pc-windows-msvc aarch64-apple-darwin passes. The builtin cat < content test and the relative path tests run on Windows and macOS only in CI.

Self-review

Done after review: the classifier became read-only (an early draft turned a stream back into its file before the open, which locked the caller's stream), one RedirectTarget enum replaced two optional locals in each setup, and an empty path became an ambiguous redirect.
Not done: test.failing rows for the cells that stay wrong, because they are other defects and are handed off. A test for the second use of a Response, because it would pin a result that shell: reject a stream, used, failed or S3-backed Response body as stdin #42786 changes.

no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/js/bun/shell/bunshell.test.ts

A redirect to `${new Response(Bun.file(path))}` now takes the arm that opens a path word, for spawned commands and for builtins. `Interpreter::redirect_target` classifies the redirect of a command once: a path word, a `Response` over `Bun.file(path)`, or another JS object. The shell opens the file with its own cwd and the flags of the operator. It takes the body after the open succeeds, so a failed open leaves the body and its stream unread. Before, a spawned command got the path as a spawn stdio. That open carries O_CREAT for stdin too, so a missing input file was created and the command read an empty stdin at exit 0. `>` did not truncate and `>>` did not append. A builtin read an empty stdin from any file and could not write to one. On macOS and Windows a relative path in such a Response now resolves against the cwd of the shell, as the bare `Bun.file(path)` does and as Linux already did. This line is read from the code, not run.

robobun · 2026-10-24T05:18:55Z

^{Updated 12:37 AM PT - Oct 24th, 2026}

✅ @robobun, your commit cd5cbee4be6fe4243de2292022492abb667a50ec passed in Build #131683! 🎉

🧪 To try this PR locally:

bunx bun-pr 46687

That installs a local version of the PR into your bun-46687 executable, so you can run:

bun-46687 --bun

robobun · 2026-10-24T05:18:56Z

Status: ready for review.

How this was reproduced (Linux x64, 1.4.3-canary and a build of main):

import { $ } from "bun";
import { existsSync } from "node:fs";
const missing = process.cwd() + "/none.txt";
const r = await $`cat < ${new Response(Bun.file(missing))}`.quiet().nothrow();
console.log(r.exitCode, JSON.stringify(r.stderr.toString()), existsSync(missing));

main: 0 "" true (the command reads an empty stdin, and none.txt now exists with 0 bytes)
this PR: 1 "bun: No such file or directory: <cwd>/none.txt" false, the same as the bare ${Bun.file(missing)}

The new tests are in test/js/bun/shell/bunshell.test.ts, block "redirect to a Response over Bun.file(path)".

coderabbitai · 2026-10-24T05:26:57Z

Walkthrough

Bun Shell now resolves file-backed Response bodies as paths for redirects, opens those paths through redirect handling, and consumes the response body after a successful open. The change adds file-store detection, tests for redirect operators and failure cases, and documentation for stdin behavior.

Changes

File-backed Response redirects

Layer / File(s)	Summary
Detect and encode file-backed paths `src/runtime/webcore/Body.rs`, `src/runtime/shell/shell_body.rs`	`Value.file_store` and `file_store_path` identify file-backed Blob stores. `handle_template_value` appends a resolved path to the shell script.
Resolve and apply redirect targets `src/runtime/shell/interpreter.rs`, `src/runtime/shell/Builtin.rs`, `src/runtime/shell/states/Cmd.rs`	`redirect_target` returns path or object targets. Builtin and subprocess redirects use the resolved target and consume an associated response body after applying a path redirect.
Document and test redirect behavior `docs/runtime/shell.mdx`, `test/js/bun/shell/bunshell.test.ts`	The tests cover redirect operators, command types, path cases, and response-body consumption. The documentation describes file-backed `Response` bodies used as stdin.

Possibly related PRs

oven-sh/bun#42786: Covers complementary handling of supported file-backed and unavailable or unsupported Response bodies in shell redirects.
oven-sh/bun#46665: Covers shell redirection of Bun.file(path) and file-backed Response objects, including path resolution and file flags.
oven-sh/bun#46673: Covers the complementary case of rejecting S3-backed Blob inputs in shell redirects.

Suggested reviewers: dylan-conway, jarred-sumner

Priority: Normal

Merge Risk: Low · up to cd5cb

A Response backed by a sliced file may redirect the whole file instead of the selected range. This narrow issue should be fixed or explicitly accepted before merging.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary change: treating a Response over Bun.file(path) as a file redirect.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description is complete and relevant. It explains the problem, implementation, behavior changes, limitations, related work, and verification results. It uses Problem, Fix, and Verification section…

Full details: Description check

Explanation

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Inline comments:
Review comments at @src/runtime/webcore/Body.rs:
- Line 741: Update the Value::Blob handling in file_store() to return None when
the blob is sliced, before returning its store. Keep the existing store return
for unsliced blobs that need to read a file so sliced object bodies preserve
their offset and size.

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Source mode: Auto
Repository: oven-sh/bun
Branch: robobun/1d2c3231/shell-response-file-redirect
File: .coderabbit.yaml
Commit: cd5cbee
Layers: Repository YAML
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Run ID: 6381ed62-b96a-4470-bfc3-8abd238b032c

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Reviewing files that changed from the base of the PR and between a8b5e92 and cd5cbee.

Files selected for processing: 7

docs/runtime/shell.mdx
src/runtime/shell/Builtin.rs
src/runtime/shell/interpreter.rs
src/runtime/shell/shell_body.rs
src/runtime/shell/states/Cmd.rs
src/runtime/webcore/Body.rs
test/js/bun/shell/bunshell.test.ts

Included review availability: This review used your included allowance. Your plan provides up to 10 included reviews per hour; 2 remain after this review.

claude

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

Beyond the inline finding on sliced file bodies, I also checked the two redirect setups for parity and ruled out: the FIFO-open blocking in both Builtin.rs and Cmd.rs (it now matches the bare < fifo word, which already blocks the same way); the empty/missing-NUL path concern on the RedirectTarget::Path arm (Atom words arrive NUL-terminated from expansion, the Response arm appends the NUL, and the empty word still hits the ambiguous-redirect error on both paths); and the missing-global throw_mini() change (a JsBuf redirect cannot exist in mini mode, where no JS values are interpolated).

Extended reasoning...

The change reroutes Bun Shell redirects whose target is a Response over Bun.file(path) through the shell's own file open in both the builtin (Builtin.rs) and spawned (Cmd.rs) redirect setups, with a new classifier in interpreter.rs and a file_store accessor in webcore/Body.rs. It touches no auth, crypto, or network surface; the sensitive part is path handling, which reuses the existing NUL-rejecting file_store_path helper. One confirmed finding (slice offsets dropped for file-backed bodies) is posted inline, so a human look is already indicated; this note only records the additional candidate issues that were examined and ruled out.

robobun assigned Jarred-Sumner Oct 24, 2026

github-actions Bot added the claude label Oct 24, 2026

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Comment thread src/runtime/webcore/Body.rs

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shell: open a Response over Bun.file(path) as a file redirect - #46687

shell: open a Response over Bun.file(path) as a file redirect#46687
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Problem

Fix

Background

Downsides

Origin and reach

Repro

Behaviour changes, main to this branch

What stays as on main

Numbers

Related changes

Questions for a maintainer

Verification

Self-review

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