feat(mkdir): add --fail-if-exists flag to error on existing directory by Jorge-Polanco-Roque · Pull Request #19021 · nushell/nushell

Jorge-Polanco-Roque · 2026-09-13T16:59:29Z

Description

Follow-up to #19003.

The default mkdir behavior stays exactly as-is: idempotent and -p-like. #19006 tried making mkdir error by default and was reverted in #19008, since people rely on mkdir foo to ensure a directory exists, and ^mkdir -p does not error either.

Per @fdncred's suggestion in the thread, this PR adds an opt-in flag instead.

With --fail-if-exists, mkdir errors on a target that already exists with <path>: File exists, matching coreutils, while still creating any non-existing targets passed in the same call. Without the flag, behavior is unchanged.

The --verbose created: true part of the issue was already fixed in #19017, so this covers the remaining piece.

User-facing changes (Release notes)

Added mkdir --fail-if-exists <dir>, which errors if the directory already exists instead of silently succeeding. Default mkdir behavior is unchanged.

Additional notes

Verification:

Added commands::umkdir tests for erroring on an existing directory with the flag.
Added coverage that mkdir --fail-if-exists still creates new directories.
Added coverage that mkdir -v --fail-if-exists reports created: false.
Existing default-behavior tests for idempotent mkdir and intermediary directories still pass.
cargo fmt --all --check: clean
cargo clippy -p nu-command: clean
nu-command test suite: clean

Disclosure: I use AI assistance for code review and for drafting text; I review and verify every change myself before sending it.

The default mkdir stays -p-like/idempotent (making it error by default was reverted in nushell#19008 as a breaking change). Per the maintainer's suggestion on already exists ("<path>: File exists", matching coreutils) while still creating non-existing targets in the same call. Default behavior unchanged. Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

fdncred · 2026-09-15T11:26:39Z

I'm not sure I understand why this is necessary with mkdir. Why does one want to show an error if you mkdir a path that already exists?

Jorge-Polanco-Roque · 2026-09-15T13:42:53Z

Mostly two things.

The first is just parity with regular mkdir. Plain mkdir foo (without -p) already errors if the dir is there, it's only mkdir -p that's idempotent. Since nu's mkdir always behaves like -p, there's no way to get that stricter check right now, so this adds it back as an opt-in flag.

The other one is catching mistakes with generated dirs. If a script makes a fresh output folder named by timestamp or id, you usually want it to blow up when that folder already exists instead of quietly reusing it and mixing new output into old results. Right now you'd have to do a separate path exists check before the mkdir, and this folds that into the command (with the same File exists message coreutils uses).

The default doesn't change at all, so anyone using mkdir foo to just make sure a dir exists is unaffected. It came out of the #19003 thread: after the error-by-default change (#19006) got reverted in #19008, an opt-in flag seemed like the compromise that keeps both camps happy.

fdncred · 2026-09-15T13:46:38Z

I buy your second paragraph about programatic creation, meh on the first.

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🟡 Changes recommended

The preflight existence check introduces a race that can violate the flag’s promised behavior.

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Pull request overview

Adds opt-in mkdir --fail-if-exists behavior while preserving idempotent defaults.

Changes:

Adds the new switch and error handling.
Adds command examples and tests.

File summaries

File	Description
`crates/nu-command/src/filesystem/umkdir.rs`	Implements the flag and verbose error records.
`crates/nu-command/tests/commands/umkdir.rs`	Tests existing and new targets.

Review details

Files reviewed: 2/2 changed files
Comments generated: 2
Review effort level: Balanced

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-            // --verbose does not report an existing directory as created.
+            // --verbose does not report an existing directory as created and so
+            // `--fail-if-exists` can report the existing path as an error.
            let already_existed = dir.exists();


+        let actual: String = test()
+            .cwd(dirs.test())
+            .run("mkdir -v --fail-if-exists already_there | get 0.created | to text")?;
+
+        assert_eq!("false", actual.trim());


Jorge-Polanco-Roque · 2026-09-15T14:19:56Z

Yeah, fair — the parity thing isn't really the point, I'll drop that framing.

The case I actually care about: a script does something like mkdir out/$name where $name was computed upstream. If that comes out empty or stale, right now nu just quietly reuses whatever dir is already sitting there, and you don't find out until something downstream breaks in a confusing way. With --fail-if-exists the error lands right at the mkdir, where it's obvious what went wrong.

Happy to reword the docs to lead with that instead of the mkdir -p comparison if it reads better.

Determine AlreadyExists from an atomic create_dir instead of a racy exists() pre-check (TOCTOU), and assert the error field in the verbose test. Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

fdncred · 2026-09-16T12:43:53Z

Thanks

Jorge-Polanco-Roque force-pushed the fix/mkdir-fail-if-exists-flag branch from 4337c24 to f0d2e02 Compare September 15, 2026 03:25

fdncred requested a balanced review from Copilot September 15, 2026 13:47

Copilot started reviewing on behalf of fdncred September 15, 2026 13:47

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Copilot AI reviewed Sep 15, 2026

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fix(mkdir): make --fail-if-exists atomic and cover verbose error branch

ceb9b2c

Determine AlreadyExists from an atomic create_dir instead of a racy exists() pre-check (TOCTOU), and assert the error field in the verbose test. Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

fdncred merged commit f2fdb76 into nushell:main Sep 16, 2026
14 checks passed

github-actions Bot added this to the v0.116.0 milestone Sep 16, 2026

fdncred added notes:ready Indicates Ready for Release notes notes:additions Noted in "Additions" section labels Sep 16, 2026

sharmis-tp mentioned this pull request Nov 12, 2026

Error when mkdir target directory already exists #19510

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feat(mkdir): add --fail-if-exists flag to error on existing directory - #19021

feat(mkdir): add --fail-if-exists flag to error on existing directory#19021
fdncred merged 2 commits into
nushell:mainfrom
Jorge-Polanco-Roque:fix/mkdir-fail-if-exists-flag

Jorge-Polanco-Roque commented Sep 13, 2026 •

edited by cptpiepmatz

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Description

User-facing changes (Release notes)

Additional notes

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Jorge-Polanco-Roque commented Sep 13, 2026 •

edited by cptpiepmatz

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