nushell

name: nushell description: "Write and edit Nushell (.nu) scripts as a replacement for non-trivial bash scripts. Use when the task involves writing a shell script with logic, data manipulation, structured output, or anything beyond a few simple command invocations. Nushell is the preferred scripting language for complex automation; bash is fine for trivial one-liners and shellHooks. Triggers on: .nu files, requests to 'write a script', or when a bash script is getting complex. Also applies when adding nushell to a nix devshell." disable-model-invocation: true

Nushell Scripting

Nushell (nu) replaces non-trivial bash scripts. Use bash for trivial glue (shellHook, one-liners); use nu when a script needs logic, data processing, or grows past ~10 lines.

Current version: 0.112.2 (pre-1.0; breaking changes happen between releases — verify syntax against the book if something seems off).

Nix DevShell Integration

Add nushell to packages in flake.nix. The shellHook stays bash (it always does); call .nu scripts from just recipes or directly with nu:

packages = with pkgs; [ just nushell git ];

build *args:
    nu scripts/build.nu {{args}}

Note: If you exec nu in shellHook, bash trap EXIT handlers won't fire (bash process is replaced). Prefer calling scripts explicitly.

Script Entry Point

#!/usr/bin/env nu

def main [
    target: string               # required positional
    --release (-r)               # boolean flag
    --output (-o): path = "."    # flag with default
] {
    if $release { print $"Building ($target) in release mode" }
    # ...
}

# Subcommands:
def "main build" [] { ... }
def "main test" [] { ... }
def main [] { print "usage: script.nu build|test" }

Run: nu script.nu build --release

Tooling Caveats

With nu 0.112.2 and nufmt 0.1.0, formatter/parser tooling can emit spurious compile_block_with_id called with parse errors messages for valid module-based scripts.

Observed cases:

• Calling an imported module command inside a command body can trigger parser noise in nufmt, even when use ./module.nu [...] itself formats cleanly.
• where {|x| ... } predicate closures can also trigger parser noise.

Guidance:

• Treat nu-check plus a real runtime smoke test as the correctness gate.
• Do not switch to deprecated filter just to quiet nufmt.
• If a formatter-friendly filter is needed, prefer an each { if ... { ... } } | compact pattern over where {|...|}.

Critical Bash→Nu Differences

> is comparison, not redirection. File output uses save.

Key Patterns

Variables

let x = "immutable"
mut count = 0
$count += 1
const MAX = 100    # parse-time constant (required for source/use)

Closures cannot capture mut variables — use for loops instead of each when mutating:

mut total = 0
for n in [1 2 3] { $total += $n }   # OK
[1 2 3] | each { $total += $in }    # ERROR

String Interpolation

$"Hello, ($name)!"
$"Items: ($list | length)"   # any expression in parens

Error Handling

try {
    let data = open missing.json
} catch { |err|
    print $"Failed: ($err.msg)"
}

# External commands — most reliable pattern:
let result = (^some-cmd | complete)
if $result.exit_code != 0 {
    error make { msg: $"Failed: ($result.stderr)" }
}

Running External Commands

^git status                            # ^ forces external (skips nu builtins)
let branch = (^git rev-parse --abbrev-ref HEAD | str trim)
let result = (^risky-cmd | complete)   # captures stdout, stderr, exit_code

File I/O

open config.toml          # auto-parsed by extension → record
open data.json            # → table or record
open data.csv             # → table
"content" | save file.txt
$data | save output.json  # auto-serialized by extension
$data | save --force output.json
"line\n" | save --append log.txt

Environment

$env.PATH = ($env.PATH | prepend "/my/bin")
$env.FOO?                              # null if not set (safe access)
with-env { FOO: "bar" } { ^cmd }       # scoped to block only

Use def --env when a command needs to mutate the caller's environment.

Standard Library

Always use slash form imports — they load only the named submodule. Space form (use std log) loads the entire stdlib first and is much slower.

use std/log       # log debug, log info, log warning, log error, log critical
use std/assert    # assert, assert equal, assert error, assert length, ...
use std/iter      # iter find, iter scan, iter filter-map, iter zip-with, ...
use std/formats * # from ndjson, to ndjson, from jsonl, to jsonl, ...

Logging

All log output goes to stderr (intentional — keeps stdout clean for pipelines).

use std/log
log debug "detailed trace"   # hidden by default — see NU_LOG_LEVEL below
log info "normal operation"
log warning "something off"
log error "something failed"
log critical "unrecoverable"

NU_LOG_LEVEL defaults to 20 (INFO), so log debug is silently suppressed unless you set:

$env.NU_LOG_LEVEL = 10   # show DEBUG and above

Gotcha — use std/log inside modules: The log module exports env vars via an export-env block that doesn't always propagate correctly at module scope. Place use std/log inside each function that uses it, not at the top of the module file.

# GOOD
export def my-cmd [] {
    use std/log
    log info "hello"
}

# BAD — may error: "Cannot find column 'NU_LOG_FORMAT'"
use std/log
export def my-cmd [] { log info "hello" }

Assertions

use std/assert
assert (1 == 1)
assert equal $a $b "values must match"
assert not equal $a $b
assert length $list 3
assert error { risky-operation }   # closure must throw

Iter extras

use std/iter
$list | iter find {|e| $e > 5 }                         # first match or null
$list | iter scan 0 {|acc, x| $acc + $x}                # running totals
$list | iter filter-map {|e| $e ** 2 }                  # map + drop errors/nulls
[1 2 3] | iter zip-with [4 5 6] {|a, b| $a + $b}        # => [5 7 9]

Parallelism

par-each is the only built-in parallel primitive. It's backed by Rayon and uses all CPUs by default.

# Basic — results arrive in arbitrary order
$list | par-each {|e| heavy-work $e }

# Preserve input order (buffers and reorders)
$list | par-each --keep-order {|e| heavy-work $e }

# Limit thread count
$list | par-each --threads 4 {|e| heavy-work $e }

No ordering guarantees by default. Sort after if order matters:

$list | par-each {|e| process $e } | sort-by name

Cannot capture mut variables — same closure rule as each. Collect results and aggregate after:

# WRONG
mut total = 0
$list | par-each {|e| $total += $e }   # parse error

# RIGHT — return values, aggregate after
let total = ($list | par-each {|e| compute $e } | math sum)

Very large lists — use chunks to bound concurrency and memory:

$large_list | chunks 100 | par-each {|chunk|
    $chunk | each {|item| process $item }
} | flatten

Env var mutations inside par-each closures don't propagate out — each closure gets its own env copy. This is consistent with nushell's normal env scoping.

Full Syntax Reference

For comprehensive syntax (types, control flow, tables, modules, etc.) see references/syntax.md.

For formatting, linting, and CI tooling see references/tooling.md.