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A hashbang is only valid as the first bytes of a file. The chunk header took the hashbang line out of the banner for one case only: an entry point with no hashbang of its own. A shared chunk for target bun wrote the whole banner after the `// @bun` pragma, and an entry point with its own hashbang wrote the banner's hashbang on the next line. Both files failed to parse. Each chunk now splits the banner one time. A chunk that writes bytes before the banner writes the banner's hashbang first, or leaves it out when the entry point has its own. The banner push writes the whole banner only when the chunk is still empty.
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Updated 12:41 PM PT - Oct 21st, 2026
✅ @robobun, your commit a7f6663a6446c50ae524a32046edf25324944cce passed in 🧪 To try this PR locally: bunx bun-pr 46107That installs a local version of the PR into your bun-46107 --bun |
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Status: ready for review. CI passes on a7f6663. Reproduced on a release build of main (59c1ac4) with One question for a maintainer is in the PR notes: which hashbang the file gets when the entry point and the banner both have one. |
WalkthroughChunk post-processing now separates hashbangs from banner text and applies banner content across split chunks. Tests cover Bun and Node targets, source hashbang precedence, executable permissions, source maps, CLI builds, and source and bytecode compilation. ChangesBanner hashbang handling
Suggested reviewers: Priority: ➖ Normal Merge Risk: 🔵 Low · up to
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| Check name | Status | Explanation |
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| Title check |  |
The title clearly and concisely describes the main change: ensuring a banner hashbang is written only as the first line of a chunk. |
| 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, fix, trade-offs, design decisions, and verification results. It does not use the template headings exactly, but it provides the requi… |
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The description is complete and relevant. It explains the problem, fix, trade-offs, design decisions, and verification results. It does not use the template headings exactly, but it provides the required information in equivalent sections.
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Inline comments:
Review comments at @src/bundler/linker_context/postProcessJSChunk.rs:
- Around line 39-51: Update the hashbang line-offset accounting in
`split_banner_hashbang`’s caller so a CRLF terminator advances the offset as one
generated line, not two. When the hashbang ends in `\r`, account for its content
without that byte, then advance over `\r\n` together; preserve the existing
accounting for LF-only hashbangs.
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src/bundler/linker_context/postProcessJSChunk.rstest/bundler/bundler_banner.test.tstest/bundler/bundler_compile_splitting.test.ts
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline finding, I also checked the j.len == 0 whole-banner push at postProcessJSChunk.rs:464 — only a non-bun, non-entry chunk reaches it with nothing written, so a banner #! can no longer land after other bytes, and node/browser shared chunk output is byte-identical to main. The execute bit now following is_entry_point() matches main for shared chunks (they never got a hashbang before). The remaining open point is the policy question the description raises (entry hashbang silently wins over the banner's), which is a maintainer call rather than a bug.
Extended reasoning...
The change is confined to the chunk header block of post_process_js_chunk (one new helper, is_bun hoisted, hashbang selection extended to bun shared chunks, execute bit gated on entry points) plus ~340 lines of new itBundled tests covering splitting, CLI, node target, source-vs-banner hashbang for ESM/CJS, dynamic imports, source maps, and compile with and without bytecode. No security-sensitive surface is touched and no CODEOWNER covers these files. A pre-existing CRLF source-map offset was confirmed and posted inline; the behavioral choice of which hashbang wins when both the entry file and the banner have one is a design decision the author explicitly left to a maintainer, so a human look is still needed.
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Two follow-ups for this PR are on the branch
Data for the question in Notes (which hashbang the file gets). Three public build scripts have an entry that starts with
With the entry's line kept and no message, that build exits 0 and writes |
When the entry point has its own hashbang, the banner's hashbang is not written. The part left out was the banner up to its first LF. A hashbang comment also ends at a lone CR, U+2028 or U+2029, so the text after one of those was left out with it. The part left out now ends at the first line break that `strings::find_js_line_break` finds, as in the lexer.
The hashbang line of a CRLF banner keeps its CR. `line_offset.advance` counted that CR as a line break and then counted the line feed written after it as a second one, so each mapping of the chunk was one line too low. The line feed is not counted when the hashbang line ends in CR. The bytes written do not change.
…r tests The expected execute bit of an entry point was `!isWindows`. A probe file now tells whether the file system keeps an execute bit.
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
The pushes since my last review address the CRLF line accounting I raised: LineColumnOffset::advance already counts a lone trailing \r as one line break, so skipping the extra advance(b"\n") in postProcessJSChunk.rs:451 makes the hashbang line count exactly once, and the new SourceMapWithHashbangCRLFBannerAndSplitting case pins it. Beyond the inline nit, I also traced each arm of the old 'brk block against the new selection: non-#! banners on every chunk, node/browser shared chunks, and entry points with a #! banner all produce the same bytes as before (banner_rest == banner when there is no #!, and j.len == 0 covers the untouched shared-chunk path), and is_executable = chunk.is_entry_point() is equivalent on every path main could reach. The precedence question in the description (entry hashbang silently wins over the banner's) is a product call a maintainer should still settle.
Extended reasoning...
The change rewrites the chunk-header selection in src/bundler/linker_context/postProcessJSChunk.rs so a banner hashbang is written first on bun-target shared chunks and dropped when the entry has its own, plus two helpers and a CRLF source-map line fix; it adds itBundled and subprocess tests in test/bundler/bundler_banner.test.ts and test/bundler/bundler_compile_splitting.test.ts. It touches no security-sensitive surface. Not approved because the PR itself poses an unanswered design question about hashbang precedence, a third-party inline comment from coderabbitai at :51 has no visible resolution, and the test evidence defers to CI.
Problem
bun build --splitting --target=bun --banner '#!/usr/bin/env bun' a.js b.js --outdir outwrites a shared chunk that starts with// @bun, then#!/usr/bin/env bun.bun out/a.jsfails witherror: Syntax Erroratout/a-<hash>.js:2:1.src/bundler/linker_context/postProcessJSChunk.rs:401) takes the hashbang out of the banner. A shared chunk, and an entry point with its own hashbang, write the whole banner after other bytes.Fix
test/bundler/bundler_banner.test.ts(12 new tests, 11 fail on main) and 2 inbundler_compile_splitting.test.ts.\r(bundler: count a CR-terminated banner as one line break in source maps #38452).Background
#!) is valid only as the first bytes of a JavaScript file.--splittingmoves shared code to a chunk that is not an entry point. For--target=bun, each chunk starts with a// @bunpragma.Downsides
#!still does not parse.Notes
Origin and reach. Found during work on #45748. There is no issue and no user report. The shared-chunk case has shipped since #22641 (bun-v1.2.22). On a released version, the workaround is to put the
#!line in the entry file and leave it out of the banner.--compile --splittingembeds the same chunk text. On main the executable fails withSyntaxError: Invalid character: '#', and with--bytecodethe build printsFailed to generate bytecode forthat chunk.Header shapes (
#!Bis the banner's hashbang,#!Sthe entry file's own):#!banner#!B, pragma, rest#!B, rest (does not parse)#!B, pragma, rest#!S,#!banner#!S, pragma,#!B, rest (does not parse)#!S, pragma, restThe pragma is only there for target bun. The chunk of an
import()target is an entry point chunk. Only entry point chunks get the execute bit, as on main.In the last row, only the hashbang comment of the banner is left out. As in the lexer, that comment ends at the first line break, and a lone CR, U+2028 and U+2029 are line breaks too. The text after it stays.
Question for a maintainer. When the entry point and the banner both have a hashbang, which line should the file get? This PR keeps the entry point's line, which main also writes first (
default/HashbangBannerUseStrictOrderasserts that the banner does not override it), and drops the banner's line with no message. The other options are a warning for that case, or the banner's line wins.Three public build scripts show what users expect. Each has an entry that starts with
#!/usr/bin/env bun, builds it for--target=nodewith the banner#!/usr/bin/env node, and then rewrites line 1 because the banner's line did not replace the entry's:yumazak/tap-to-md(scripts/fix-cli-shebang.ts),eloi24/verifactu-sdk(scripts/build.ts) andAnishBplayz/DiffHawk(scripts/build-npm.ts). For such a build, main writes#!/usr/bin/env bun,// @bun,#!/usr/bin/env nodeand the file fails on the first run. This PR writes#!/usr/bin/env bun,// @bun, and the build prints nothing about the banner's line.For reference: esbuild 0.21.5 and 0.28.2 write both lines and node rejects the file (
SyntaxError: Invalid or unexpected token). rolldown 1.2.14 stops the build ([PARSE_ERROR] Invalid Character !). Both keep a banner hashbang on line 1 of each chunk of a split build.One more cell of the first Downsides bullet. An entry that starts with
#!/usr/bin/env bun, built with--target=node --format=cjs --banner '#!/usr/bin/env node': on main node fails with aSyntaxError. With this PR node exits 0 and prints nothing. That output is byte-identical to what main writes for the same entry with no banner. The cause is the// @bun @bun-cjswrapper (#33859).Other open PRs that touch these lines.
\ror\nand fixes the line count of a banner that ends in\r. After this PR, its cut goes insplit_banner_hashbang, and the CR check in the hashbang push is then dead code.is_buncondition. This PR moves that binding above the hashbang selection. The pragma and the selection read the same binding.#! in file\n#! from banner\n"use strict";\nfordefault/HashbangBannerUseStrictOrder. With this PR the output is#! in file\n"use strict";\n.bannerdescription. This PR changes no docs. A sentence about the hashbang can go there when the question above has an answer.Measurements (Linux x64, release builds, main 59c1ac4 against this PR).
import()target with its own hashbang) x 8 banners (none,// x,#!x,#!xLF,#!xLF// r,#!xCRLF// r,#!xLF LF// r,#!xCRcode;) = 120 builds, 192.jsfiles. 108 files differ, all in builds whose entry fails to run on main. Node and browser shared chunks that differ: 0. Mode changes: 0.post_process_js_chunk, instructions / conditional branches for each chunk, main to PR (gdbnextifrom the first instruction of the hashbang selection to the"use strict"block, 2 runs, identical). No banner, target bun: entry 59/12 to 70/14, entry with its own hashbang 79/12 to 92/15, shared chunk 47/9 to 59/12. No banner, target node: 39/8 to 50/9, 59/8 to 72/11, 27/5 to 39/7. With a// xbanner, target bun: 82/16 to 92/18, 99/15 to 116/20, 67/12 to 81/16. Target node: 62/12 to 72/13, 79/11 to 96/16, 47/8 to 61/11.post_process_js_chunkfor a build of 3 chunks (gdb hit count onStringJoiner::push_node), main to PR. Target bun: no banner 21 to 21,// x28 to 28,#!xLF// r32 to 34 (the hashbang and its newline on the shared chunk). Target node: 18 to 18, 25 to 25, 29 to 29. Allocations underpost_process_js_chunk: 6 to 6 in all six builds.post_process_js_chunk23,994 to 24,274 bytes (nm -S). Release text 107,348,240 to 107,348,496 bytes (size).console.log(split build), main to PR. LF banner: 0 to 0 on each chunk.#!xCRLF// r: entry chunks +1 to 0, shared chunks 0 to 0.#!xCR: entry chunks +1 to 0, bun-target shared chunk +1 to 0, node-target shared chunk +1 to +1.// xCR, with no hashbang: +1 to +1 on each chunk. The two rows that stay at +1 are a banner that ends in a bare\rat the banner push (bundler: count a CR-terminated banner as one line break in source maps #38452).Tests. Each clause of the header has a test that fails without it. Checked with debug builds of five mutants: no
is_entry_point()guard on the entry file's hashbang (1 test fails), execute bit on each chunk with a hashbang (3), hoist on each chunk (1), no hoist for the pragma (4), whole banner at each push (13). Withoutbanner_without_hashbangand the CR check, exactlybanner/SourceHashbangWithHashbangBannerKeepsTheRestOfTheBannerandbanner/SourceMapWithHashbangCRLFBannerAndSplittingfail.banner/HashbangBannerWithSplittingAndTargetNodepasses on main too. It pins the node and browser shared chunk, which must not change.Also run on the debug build with this PR:
esbuild/default.test.ts,esbuild/splitting.test.ts,bundler_splitting,bundler_footer,bundler_bun,bundler_cjs,bundler_naming,bun-build-apiand all ofbundler_compile_splitting: 604 pass. 11 tests reach a timeout on the local debug build. Each of them runs a loop of many builds or a large input, and none uses a banner.no test proof · iteration 0 · platform-specific test(s) that do not run on this machine, deferring to CI, which covers all platforms: test/bundler/bundler_compile_splitting.test.ts, test/bundler/bundler_banner.test.ts