refactor: Use build tags and optimize Darwin implementation

Author: jsbroks

internal/ptysession/shell.go

@@ -0,0 +1,28 @@
+//go:build darwin
+
+package ptysession
+
+import (
+	"fmt"
+	"os/exec"
+	"strings"
+)
+
+func getUserShell(username string) (string, error) {
+	cmd := exec.Command("dscl", ".", "-read", fmt.Sprintf("/Users/%s", username), "UserShell")
+	output, err := cmd.Output()
+	if err != nil {
+		return "", fmt.Errorf("failed to get user shell on macOS: %v", err)
+	}
+
+	// Parse the output
+	for line := range strings.SplitSeq(string(output), "\n") {
+		if strings.HasPrefix(line, "UserShell:") {
+			fields := strings.Fields(line)
+			if len(fields) >= 2 {
+				return fields[1], nil
+			}
+		}
+	}
+	return "", fmt.Errorf("shell not found in dscl output")
+}

internal/ptysession/shell_darwin.go

@@ -0,0 +1,34 @@
+//go:build linux
+
+package ptysession
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strings"
+)
+
+func getUserShell(username string) (string, error) {
+	file, err := os.Open("/etc/passwd")
+	if err == nil {
+		defer file.Close()
+		scanner := bufio.NewScanner(file)
+		for scanner.Scan() {
+			line := scanner.Text()
+			fields := strings.Split(line, ":")
+			if len(fields) < 7 {
+				continue
+			}
+			if fields[0] == username {
+				shell := fields[6]
+				return shell, nil
+			}
+		}
+		if err := scanner.Err(); err != nil {
+			return "", err
+		}
+	}
+
+	return "", fmt.Errorf("user %s not found in /etc/passwd: %w", username, err)
+}

internal/ptysession/shell_linux.go

@@ -0,0 +1,12 @@
+//go:build !linux && !darwin
+
+package ptysession
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func getUserShell(username string) (string, error) {
+	return "", fmt.Errorf("operating system %s not supported", runtime.GOOS)
+}