Built-ins & Fast Paths

Niubash resolves a command word through three layers, in priority order. This page documents what lives in each layer, how the inventory is kept honest, and where the inventory overlaps with WinuxCmd.

Layer	What it is	Introspection (`type`, `command -v`, `enable`)
Real built-in	Runs in-process, no child process. Listed in the rubash whitelist (`src/executor/builtin_names.rs`).	Reports as builtin
Fast path	Hidden optimization: runs in-process only for simple argument shapes; falls back to the external WinuxCmd command otherwise. Deliberately not in the whitelist.	Reports as external — same as GNU Bash, where these commands are external too
External command	Resolved via `PATH`, provided by WinuxCmd (176 commands).	Reports as external

Real built-ins (64)

All 61 GNU Bash 5.2 built-ins are covered — none missing. The entries below that are not part of Bash are deliberate extensions:

Group	Commands
GNU Bash 61 built-ins (all aligned)	`.` `:` `[` `alias` `bg` `bind` `break` `builtin` `caller` `cd` `command` `compgen` `complete` `compopt` `continue` `declare` `dirs` `disown` `echo` `enable` `eval` `exec` `exit` `export` `false` `fc` `fg` `getopts` `hash` `help` `history` `jobs` `kill` `let` `local` `logout` `mapfile` `popd` `printf` `pushd` `pwd` `read` `readarray` `readonly` `return` `set` `shift` `shopt` `source` `suspend` `test` `times` `trap` `true` `type` `typeset` `ulimit` `umask` `unalias` `unset` `wait`
zsh compatibility extension	`setopt` `unsetopt`
In-process `env`	`env` (coreutils semantics, sorted output, `-0`)
Windows-only	`sudo` (not counted in the 64)

Notes:

declare/typeset and source/. are alias pairs; [ is implemented by the test builtin and validates the closing ].
time, [[, and (( are reserved words / grammar, not built-ins — same as Bash. time honors TIMEFORMAT.
The sudo built-in is a Windows-specific extension (#[cfg(windows)]).

Fast paths (hidden built-ins)

These run in-process for simple argument shapes and fall back to the external WinuxCmd command otherwise. They are intentionally excluded from the whitelist so that introspection keeps reporting them as external, matching GNU Bash:

Command	In-process behavior	Fallback
`sleep`	Fractional seconds supported	external `sleep`
`dirname`	Plain paths	external `dirname`
`basename`	Plain paths	external `basename`

env is the one hybrid: it is on the whitelist (reported as a built-in), but its implementation is fully in-process with coreutils semantics and no external fallback path.

Two additional reproduction built-ins exist for test compatibility only: recho and zecho (mirrors of the helper built-ins in GNU Bash's own test suite; not shipped in official Bash builds).

Overlap with WinuxCmd

Thirteen command names exist both as rubash built-ins/reserved words and as WinuxCmd external commands: echo env kill printf pwd test [ true false (real built-ins), sleep dirname basename (fast paths), and time (reserved word).

The overlap follows Bash semantics: inside a script the built-in or reserved word shadows the external command, while explicit forms such as command env or /usr/bin/env still reach the WinuxCmd external version. Both implementations were compared option-by-option (escapes, format specifiers, signals, jobspec, TIMEFORMAT) on 2026-09-08 — aligned.

How the inventory stays honest

The authoritative machine-readable lists live in docs/builtins.md on unixwin/rubash. A doc-sync test inside src/executor/builtin_names.rs parses that document and fails cargo test whenever the whitelist or the fast-path dispatch changes without the document being updated — and vice versa. Adding, removing, or reclassifying a built-in therefore requires touching both the code and the doc in the same change.