diff --git a/cmdlang/ast.go b/cmdlang/ast.go
index c3a076d..0943a95 100644
--- a/cmdlang/ast.go
+++ b/cmdlang/ast.go
@@ -11,7 +11,8 @@ type astLiteral struct {
 }
 
 type astCmdArg struct {
-	Literal astLiteral `parser:"@@"`
+	Literal *astLiteral  `parser:"@@"`
+	Sub     *astPipeline `parser:"| '[' @@ ']'"`
 }
 
 type astCmd struct {
diff --git a/cmdlang/builtins.go b/cmdlang/builtins.go
index 76b76a2..46fd5ba 100644
--- a/cmdlang/builtins.go
+++ b/cmdlang/builtins.go
@@ -4,6 +4,7 @@ import (
 	"bufio"
 	"context"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"strings"
@@ -16,7 +17,9 @@ func echoBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 
 	var line strings.Builder
 	for _, arg := range args.args {
-		line.WriteString(arg)
+		if s, ok := arg.(fmt.Stringer); ok {
+			line.WriteString(s.String())
+		}
 	}
 
 	return asStream(line.String()), nil
@@ -41,7 +44,12 @@ func catBuiltin(ctx context.Context, args invocationArgs) (object, error) {
 		return nil, err
 	}
 
-	return &fileLinesStream{filename: args.args[0]}, nil
+	filename, err := args.stringArg(0)
+	if err != nil {
+		return nil, err
+	}
+
+	return &fileLinesStream{filename: filename}, nil
 }
 
 type fileLinesStream struct {
diff --git a/cmdlang/eval.go b/cmdlang/eval.go
index 81b0f70..2785872 100644
--- a/cmdlang/eval.go
+++ b/cmdlang/eval.go
@@ -3,8 +3,10 @@ package cmdlang
 import (
 	"context"
 	"errors"
+	"fmt"
 	"github.com/lmika/gopkgs/fp/slices"
 	"strconv"
+	"strings"
 )
 
 type evaluator struct {
@@ -36,7 +38,7 @@ func (e evaluator) evalCmd(ctx context.Context, ec *evalCtx, ast *astCmd) (objec
 		return nil, err
 	}
 
-	args, err := slices.MapWithError(ast.Args, func(a astCmdArg) (string, error) {
+	args, err := slices.MapWithError(ast.Args, func(a astCmdArg) (object, error) {
 		return e.evalArg(ctx, ec, a)
 	})
 	if err != nil {
@@ -49,20 +51,49 @@ func (e evaluator) evalCmd(ctx context.Context, ec *evalCtx, ast *astCmd) (objec
 	})
 }
 
-func (e evaluator) evalArg(ctx context.Context, ec *evalCtx, n astCmdArg) (string, error) {
-	return e.evalLiteral(ctx, ec, n.Literal)
+func (e evaluator) evalArg(ctx context.Context, ec *evalCtx, n astCmdArg) (object, error) {
+	switch {
+	case n.Literal != nil:
+		return e.evalLiteral(ctx, ec, n.Literal)
+	case n.Sub != nil:
+		return e.evalSub(ctx, ec, n.Sub)
+	}
+	return nil, errors.New("unhandled arg type")
 }
 
-func (e evaluator) evalLiteral(ctx context.Context, ec *evalCtx, n astLiteral) (string, error) {
+func (e evaluator) evalLiteral(ctx context.Context, ec *evalCtx, n *astLiteral) (object, error) {
 	switch {
 	case n.Str != nil:
 		uq, err := strconv.Unquote(*n.Str)
 		if err != nil {
 			return "", err
 		}
-		return uq, nil
+		return strObject(uq), nil
 	case n.Ident != nil:
-		return *n.Ident, nil
+		return strObject(*n.Ident), nil
 	}
 	return "", errors.New("unhandled literal type")
 }
+
+func (e evaluator) evalSub(ctx context.Context, ec *evalCtx, n *astPipeline) (object, error) {
+	pipelineRes, err := e.evaluate(ctx, ec, n)
+	if err != nil {
+		return "", err
+	}
+
+	switch v := pipelineRes.(type) {
+	case stream:
+		// TODO: use proper lists here, not a string join
+		sb := strings.Builder{}
+		if err := forEach(v, func(o object) error {
+			// TODO: use o.String()
+			sb.WriteString(fmt.Sprint(o))
+			return nil
+		}); err != nil {
+			return "", err
+		}
+
+		return strObject(sb.String()), nil
+	}
+	return pipelineRes, nil
+}
diff --git a/cmdlang/objs.go b/cmdlang/objs.go
index dd53a3c..934da5a 100644
--- a/cmdlang/objs.go
+++ b/cmdlang/objs.go
@@ -3,13 +3,21 @@ package cmdlang
 import (
 	"context"
 	"errors"
+	"fmt"
 	"strconv"
 )
 
-type object = any
+type object interface {
+}
+
+type strObject string
+
+func (s strObject) String() string {
+	return string(s)
+}
 
 type invocationArgs struct {
-	args     []string
+	args     []object
 	inStream stream
 }
 
@@ -20,6 +28,17 @@ func (ia invocationArgs) expectArgn(x int) error {
 	return nil
 }
 
+func (ia invocationArgs) stringArg(i int) (string, error) {
+	if len(ia.args) < i {
+		return "", errors.New("expected at least " + strconv.Itoa(i) + " args")
+	}
+	s, ok := ia.args[i].(fmt.Stringer)
+	if !ok {
+		return "", errors.New("expected a string arg")
+	}
+	return s.String(), nil
+}
+
 // invokable is an object that can be executed as a command
 type invokable interface {
 	invoke(ctx context.Context, args invocationArgs) (object, error)