Nushell Pro Skill | Agent Skills

Nushell Pro

Write secure, idiomatic, portable, and testable Nushell. Keep the main workflow in this file; load detailed references only when the task needs them.

Operating Workflow

Identify the task: new script, debugging, review, refactor, module design, migration, performance work, security audit, or Bash conversion.
Read the target .nu files, nearby tests, AGENTS.md, and existing project conventions before changing code.
Confirm the active version for version-sensitive behavior. From any shell, prefer the direct CLI check:
```
nu --version
```
When already inside Nushell or when structured version data is needed, use:
```
version | get version
```
Load the smallest relevant reference set:

| Task | Reference | | ---------------------------------------------------- | --------------------------------------------------------------------------------------------- | | Nu 0.114+ migration and version compatibility | Nu 0.114 Migration | | String quoting, interpolation, regex, globs | String Formats | | Security, paths, credentials, destructive operations | Security | | Script/code review | Script Review and Anti-Patterns | | Bash/POSIX conversion | Bash to Nushell | | Modules, exports, scripts, tests | Modules & Scripts | | Daemons, background jobs, E2E smoke tests | Daemon & E2E Smoke Tests | | Types, records, lists, conversions | Data & Type System | | Streaming, closures, performance, diagnostics | Advanced Patterns | | Large columnar data | Dataframes | | Common mistakes | Anti-Patterns |
Apply the cross-cutting guardrails below before style or performance cleanup.
Validate with the narrowest safe command, then run the relevant tests.
Report security/correctness findings before style and performance notes.

If a referenced file is unavailable, say so and continue with this file rather than inventing its contents.

Cross-Cutting Guardrails

These rules apply across task types. Load the routed reference before relying on syntax or behavior that changed between Nushell releases.

Types and pipeline contracts

Declare pipeline input in the I/O signature rather than as a positional parameter. Capture $in once when it must be reused because streams can be single-pass.
Type exported command parameters and input/output signatures.
Treat external/config records as untrusted; use optional access such as $record.field? and validate the resulting type/value.
Remember that default evaluates its fallback argument eagerly. Make the fallback null-safe or branch explicitly when it can fail or is expensive.
Keep return types consistent. Avoid any unless the function is genuinely polymorphic.
Prefer match for several branches on one value; use if for one-off boolean predicates.

External commands and errors

Pass external arguments as separate values, never as an interpolated command string. Use complete and check exit_code when status matters.
Prefer try/catch and $err.details for structured in-process errors.
Treat rendered nested-Nu diagnostics as presentation text, not a stable protocol. For CLI tests, normalize ANSI styling, gutters, and PTY wrapping before matching a long, domain-specific phrase.

Security stop checkpoint

Before approving code that executes commands, deletes files, reads credentials, or accepts paths/patterns, confirm these boundaries:

Never pass untrusted strings to nu -c, source, run, ^sh -c, ^bash -c, or ^cmd.exe /C.
Pass external command arguments as separate values, not an interpolated shell command string.
For existing paths restricted to a base directory, expand both paths and use path relative-to to prove containment. A string starts-with check is unsafe because /safe/base2 starts with /safe/base.
Prefer Nu's built-in mktemp/mktemp --directory; it is portable and returns a path directly. Do not use predictable temp names.
Scope secrets with with-env; do not log them or pass them in argv when a stdin/config-file mechanism exists.
Guard destructive paths against root, $nu.home-dir, unexpected types, and untrusted globs. Consider TOCTOU and partial-success behavior.

For output paths that do not exist yet, validate the existing parent directory with the same containment rule, then join only a validated leaf name.

Strings and formatting

Prefer simple literals and raw regex strings; use double quotes only when actual escapes are required.
Remember that $'...' interpolates but does not process escape sequences.
A literal ( forces $"...". Since $'...' does no escape processing, every ( inside it opens an interpolation expression and \( cannot prevent that. Write $"\(abc)($var)", never $'\(abc)($var)'. The failure is often silent rather than an error: $'(1 + 1) items' evaluates to 2 items. See String Formats.
Never build command strings for execution.
Use kebab-case for commands/flags, snake_case for variables/parameters, and SCREAMING_SNAKE_CASE for environment variables.

Data flow and performance

Prefer pipelines and immutable let bindings.
Use where, select, update, insert, items, transpose, reduce, and enumerate instead of manual parsing and mutable accumulation.
Do not capture mut variables in closures.
for is appropriate for sequential side effects but is not a transforming expression; use each when a list result is required.
Use par-each only when concurrency is safe and beneficial; preserve each when order or sequential side effects matter.
Add lines before parse when line-by-line stream parsing is intended.
Use native tables for small interactive data and Polars for large columnar group-by/join/aggregation workloads.

Modules and scripts

Export only the intended API; keep helpers private.
Use export def main when the command should match the module name.
Use def --env/export def --env for caller-visible environment changes.
source, use, and run targets must be trusted and available at parse time.
Test at the correct seam: direct functions for stable structured errors, CLI subprocesses for argument parsing/process boundaries, and both when needed.

Review Order

When reviewing code, report findings in this order:

Security: injection, traversal, credentials, destructive operations, temp files, environment poisoning.
Correctness: types, null handling, parse-time constraints, exit codes, cleanup/rollback, platform behavior, stable tests.
Maintainability: naming, module boundaries, duplication, documentation.
Performance: streaming, unnecessary collection, safe parallelism, Polars.

Skip issues already enforced by the project's formatter/linter unless the tool output shows they are currently failing.

Validation

Use the narrowest safe commands first:

nu --no-config-file --ide-check 100 path/to/script.nu
nu -c 'source path/to/module.nu'
nu path/to/test-script.nu

--ide-check emits JSON Lines on stdout and may still exit with code 0 when a record has type: "diagnostic" and severity: "Error". It also exits 0 with empty output when the target file does not exist, so verify the path exists before treating an empty result as a pass. Parse every non-empty line with from json; do not use the process exit code alone. Treat severity: "Error" diagnostics as blockers, surface other severities such as Warning without blocking, and ignore type: "hint" records. Handle a non-zero exit code or stderr separately as a CLI startup or I/O failure.
Prefer --no-config-file for reproducible standalone checks. Omit it when the script intentionally depends on commands or environment from user configuration, and document that dependency.
For scripts with side effects, source/parse-check them or run against a temp fixture.
Reproduce terminal-sensitive tests under a narrow PTY when diagnostics or tables are involved, for example stty cols 24 && nu tests/example.nu.
Check diffs for debug markers and accidental changes before finishing.
If validation fails, fix the smallest reproducible issue and rerun the exact failing command before broadening the test suite.

Agent Skills: Nushell Pro

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