Add ability to execute commands

Using RunCommand you can execute any binary on a system with whatever
arguments. Commands are constrained to running for up to 1sec.

Commands will be looked up in path, unless an absolute path is given.

This does not execute a shell and so cannot directly execute shell
scripts. For that, you can call it with `sh -c`.

command.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"context"
+	"os/exec"
+	"time"
+)
+
+// RunCommand runs a command, not a shell, governed by the
+// passed in context.
+//
+// Commands are given 1s to complete.
+func RunCommand(ctx context.Context, name string, args ...string) (string, error) {
+	cctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+	defer cancel()
+
+	cmd := exec.CommandContext(cctx, name, args...)
+	output, err := cmd.CombinedOutput()
+	return string(output), err
+}

examples/scripts/notification.expr

@@ -0,0 +1,5 @@
+let mixer = "MIXER_ID";
+let fader = "A";
+let oldState = status.Old.Mixers[mixer].FaderStatus[fader].MuteState;
+let newState = status.New.Mixers[mixer].FaderStatus[fader].MuteState;
+oldState != newState ? RunCommand("notify-send", "-a", "gopherxlr", "-e", "state changed", "toggle") : nil

expr.go

@@ -22,8 +22,22 @@ func (Env) PlayPause(ctx context.Context, name string) error {
 	return dbus.ToggleMediaPlayback(ctx, name)
 }
 
-func LoadPrograms(dir string) ([]*vm.Program, error) {
-	res := []*vm.Program{}
+func (Env) RunCommand(ctx context.Context, name string, args ...string) error {
+	_, err := RunCommand(ctx, name, args...)
+	if err != nil {
+		return err
+	}
+	return nil
+}
+
+type Program struct {
+	file string
+	prog *vm.Program
+}
+
+func LoadPrograms(dir string) ([]Program, error) {
+	res := []Program{}
+
 	err := filepath.WalkDir(dir, func(path string, d fs.DirEntry, err error) error {
 		if err != nil {
 			return err
@@ -37,7 +51,7 @@ func LoadPrograms(dir string) ([]*vm.Program, error) {
 			if err != nil {
 				return err
 			}
-			res = append(res, prog)
+			res = append(res, Program{file: path, prog: prog})
 		}
 		return nil
 	})

main.go

@@ -71,9 +71,12 @@ Main:
 				Context:      ctx,
 			}
 			for _, prog := range programs {
-				_, err := expr.Run(prog, env)
+				output, err := expr.Run(prog.prog, env)
 				if err != nil {
-					fmt.Println("error: ", err)
+					fmt.Printf("error executing %s: %s\n", prog.file, err)
+				}
+				if output != nil {
+					fmt.Printf("error from %s: %s\n", prog.file, output)
 				}
 			}
 		}