Shell Builtins

Shell Builtins

Crush's shell (internal/shell/) uses mvdan.cc/sh/v3 for POSIX shell emulation. Commands can be intercepted before they reach the OS by adding builtins — functions handled in-process.

How Builtins Work

Builtins live in builtinHandler() in internal/shell/run.go. This is an interp.ExecHandlerFunc middleware registered in standardHandlers() before the block handler, so builtins run even for commands that would otherwise be blocked. The same handler chain is shared by the stateful Shell type and the stateless Run entrypoint used by the hook runner, so builtins are available identically in the bash tool and in hooks.

The handler is a switch on args[0]. Each case either handles the command inline or delegates to a helper function.

Adding a New Builtin

1. Add the case to the switch in builtinHandler() in run.go.
2. Get I/O from the handler context, not from os.Stdin/os.Stdout. This ensures the builtin works with pipes and redirections:

   case "mycommand":
       hc := interp.HandlerCtx(ctx)
       return handleMyCommand(ctx, args, hc.Stdin, hc.Stdout, hc.Stderr)
   

3. Implement the handler in its own file (e.g., internal/shell/mycommand.go). The function signature must accept a context.Context as the first parameter, plus args, stdin, stdout, and stderr:

   func handleMyCommand(ctx context.Context, args []string, stdin io.Reader, stdout, stderr io.Writer) error {
       // args[0] is the command name ("mycommand"), args[1:] are arguments.
       // Write output to stdout, errors to stderr.
       // Return nil on success, or interp.ExitStatus(n) for non-zero exit codes.
   }
   

4. Poll ctx in every unbounded loop. Builtins that iterate over input, emit values in a generator-style loop, or do any other work that can exceed a few milliseconds MUST check ctx.Err() on each iteration and return it verbatim when non-nil. Hook timeouts rely on this: an unbounded builtin that never polls ctx cannot be interrupted by a hook's timeout_sec, and the hook runner will have to abandon the goroutine (see internal/hooks/runner.go). Returning ctx.Err() (not interp.ExitStatus(n)) lets callers distinguish "command exited non-zero" from "we ran out of time".

   for _, item := range items {
       if err := ctx.Err(); err != nil {
           return err
       }
       // ... process item
   }
   

5. Return values: return nil for success, interp.ExitStatus(n) for non-zero exit codes, or ctx.Err() on cancellation. Write error messages to stderr before returning.
6. No extra wiring needed — builtinHandler() is already registered in standardHandlers().

Existing Builtins