Nushell

ccusage Nushell

Nushell is a functional, structured-data language that happens to be a shell. Write it that way: pipelines of transformations over records and tables, not bash with different syntax. Root AGENTS.md says when to pick Nushell over Babashka, Rust, or TypeScript.

Scripts live in .github/scripts/, apps/ccusage/scripts/, and scripts/.

Runtime Shape

Every executable script is a .nu file with a Nix shebang and a def main; that shebang is how CI gets an interpreter, rather than a nushell profile install in the workflow. List only the tools the script directly invokes; --inputs-from is relative to the script's own directory, so its depth differs per directory — copy the header from a neighbor, such as .github/scripts/upsert-pr-comment.nu or apps/ccusage/scripts/stage-native-package.nu.

Files that exist only to be imported as modules carry no shebang and no main: .github/scripts/pricing-lock.nu, apps/ccusage/scripts/native-binary.nu.

Workflows run these scripts through the shebang, but the commit-msg hook in nix/git-hooks.nix invokes scripts/validate-commit-scope.nu as an argument to nushell instead, so that script only ever gets nu itself.

Calling external tools such as gh, jq, git, hyperfine, pnpm, node, or bun is fine when they are the right boundary — pin them through the shebang shell rather than a global install.

Docs

Read before writing. The sidebar on any of these reaches the rest of the book, plus /commands/ and /cookbook/.

https://www.nushell.sh/book/thinking_in_nu.html

https://www.nushell.sh/book/nushell_map_functional.html

https://www.nushell.sh/book/style_guide.html

Style

Functional style is the default. Treat each of these as a defect to fix:

• mut + for as an accumulator. Use reduce, each, where, group-by, zip, flatten, insert/update, generate. mut is legitimate only for genuinely sequential state that no filter expresses.
• String plumbing between steps. Pass records and tables; serialize once at the boundary (to json --raw) and parse once on the way in (from json).
• Nested if/else chains on a value's shape. Use match, including list patterns and guards.
• Shelling out for data. Native commands over ^jq, ^sed, ^awk, ^date.
• Bare each for effects. Say so with | ignore when the result is unused.

Also:

• Type custom command signatures — parameter types, --flag, and the return type. They document intent and are checked at parse time.
• run-external for commands whose flags Nushell would otherwise parse; quote short flags such as '-L', '-x', '-c', and keep arguments as lists up to the external boundary.
• complete when you need exit code, stdout, and stderr without throwing; error make rather than a sentinel return value.
• Timestamps are datetime and durations are duration; compare and subtract them directly instead of formatting to strings.
• Progress to stderr for CI scripts whose stdout is a data artifact.

Validation

Nushell renames and breaks things between minor releases, so check constructs against the pinned interpreter instead of trusting memory:

direnv exec . nu --ide-check 10 path/to/script.nu
direnv exec . nu -c 'help <command>'

just fmt runs nufmt over *.nu through treefmt. For behavior changes, invoke the script through its shebang as a smoke check and run the repo check that owns the caller.