go.mod

@@ -7,6 +7,7 @@ require (
 	github.com/chzyer/readline v1.5.1
 	github.com/lmika/gopkgs v0.0.0-20240408110817-a02f6fc67d1f
 	github.com/stretchr/testify v1.10.0
+	lmika.dev/pkg/modash v0.0.0-20250619112300-0be0b6b35b1b
 )
 
 require (
@@ -17,5 +18,4 @@ require (
 	go.abhg.dev/goldmark/frontmatter v0.2.0 // indirect
 	golang.org/x/sys v0.0.0-20220310020820-b874c991c1a5 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
-	lmika.dev/pkg/modash v0.0.0-20250619112300-0be0b6b35b1b // indirect
 )

go.sum

@@ -22,6 +22,7 @@ github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZb
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
 github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
 github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 github.com/yuin/goldmark v1.7.8 h1:iERMLn0/QJeHFhxSt3p6PeN9mGnvIKSpG9YYorDMnic=
 github.com/yuin/goldmark v1.7.8/go.mod h1:uzxRWxtg69N339t3louHJ7+O03ezfj6PlliRlaOzY1E=

ucl/builtins/os.go

@@ -2,20 +2,32 @@ package builtins
 
 import (
 	"context"
+	"errors"
 	"os"
+	"os/exec"
+
 	"ucl.lmika.dev/ucl"
 )
 
+type OSProvider interface {
+	LookupEnv(string) (string, bool)
+	Exec(ctx context.Context, cmd string, args ...string) (*exec.Cmd, error)
+}
+
 type osHandlers struct {
+	provider OSProvider
 }
 
 func OS() ucl.Module {
-	osh := osHandlers{}
+	osh := osHandlers{
+		provider: builtinOSProvider{},
+	}
 
 	return ucl.Module{
 		Name: "os",
 		Builtins: map[string]ucl.BuiltinHandler{
 			"env":  osh.env,
+			"exec": osh.exec,
 		},
 	}
 }
@@ -26,7 +38,7 @@ func (oh osHandlers) env(ctx context.Context, args ucl.CallArgs) (any, error) {
 		return nil, err
 	}
 
-	val, ok := os.LookupEnv(envName)
+	val, ok := oh.provider.LookupEnv(envName)
 	if ok {
 		return val, nil
 	}
@@ -38,3 +50,42 @@ func (oh osHandlers) env(ctx context.Context, args ucl.CallArgs) (any, error) {
 
 	return "", nil
 }
+
+func (oh osHandlers) exec(ctx context.Context, args ucl.CallArgs) (any, error) {
+	var cmdArgs []string
+
+	for args.NArgs() > 0 {
+		var s string
+		if err := args.Bind(&s); err != nil {
+			return nil, err
+		}
+
+		cmdArgs = append(cmdArgs, s)
+	}
+
+	if len(cmdArgs) == 0 {
+		return nil, errors.New("expected command")
+	}
+
+	cmd, err := oh.provider.Exec(ctx, cmdArgs[0], cmdArgs[1:]...)
+	if err != nil {
+		return nil, err
+	}
+
+	res, err := cmd.Output()
+	if err != nil {
+		return nil, err
+	}
+
+	return string(res), nil
+}
+
+type builtinOSProvider struct{}
+
+func (builtinOSProvider) LookupEnv(key string) (string, bool) {
+	return os.LookupEnv(key)
+}
+
+func (builtinOSProvider) Exec(ctx context.Context, name string, args ...string) (*exec.Cmd, error) {
+	return exec.CommandContext(ctx, name, args...), nil
+}

ucl/builtins/os_test.go

@@ -2,8 +2,10 @@ package builtins_test
 
 import (
 	"context"
-	"github.com/stretchr/testify/assert"
 	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
 	"ucl.lmika.dev/ucl"
 	"ucl.lmika.dev/ucl/builtins"
 )
@@ -34,3 +36,25 @@ func TestOS_Env(t *testing.T) {
 		})
 	}
 }
+
+func TestOS_Exec(t *testing.T) {
+	tests := []struct {
+		descr string
+		eval  string
+		want  any
+	}{
+		{descr: "run command 1", eval: `os:exec "echo" "hello, world"`, want: "hello, world\n"},
+		{descr: "run command 1", eval: `os:exec "date" "+%Y%m%d"`, want: time.Now().Format("20060102") + "\n"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.descr, func(t *testing.T) {
+			inst := ucl.New(
+				ucl.WithModule(builtins.OS()),
+			)
+			res, err := inst.EvalString(context.Background(), tt.eval)
+			assert.NoError(t, err)
+			assert.Equal(t, tt.want, res)
+		})
+	}
+}