feat(installer): add XDG Base Directory support on Linux

Add support for XDG_DATA_HOME on Linux following the XDG Base Directory
Specification. This aligns dtvem with standard Linux conventions.

Changes:
- paths.go: Use $XDG_DATA_HOME/dtvem or ~/.local/share/dtvem on Linux
- install.sh: Detect platform and use XDG paths on Linux
- Add comprehensive documentation of path selection rationale
- Add unit tests for XDG path selection logic

Path Selection Summary:
- Linux: $XDG_DATA_HOME/dtvem (or ~/.local/share/dtvem if unset)
- macOS: ~/.dtvem (familiar to Unix CLI users)
- Windows: %USERPROFILE%\.dtvem (visible and easy to backup)
- Override: DTVEM_ROOT env var works on all platforms

Closes #91

Author: CalvinAllen

install.sh

@@ -5,7 +5,6 @@ set -e
 # Usage: curl -fsSL https://raw.githubusercontent.com/dtvem/dtvem/main/install.sh | bash
 
 REPO="dtvem/dtvem"
-INSTALL_DIR="$HOME/.dtvem/bin"
 
 # This will be replaced with the actual version during release
 # Format: DTVEM_RELEASE_VERSION="1.0.0"
@@ -36,6 +35,33 @@ warning() {
     echo -e "${YELLOW}⚠${NC} $1"
 }
 
+# Get dtvem root directory
+# On Linux, respects XDG_DATA_HOME if set (defaults to ~/.local/share/dtvem)
+# On macOS, uses ~/.dtvem
+get_dtvem_root() {
+    # Check for DTVEM_ROOT environment variable first (overrides all)
+    if [ -n "$DTVEM_ROOT" ]; then
+        echo "$DTVEM_ROOT"
+        return
+    fi
+
+    local os
+    os=$(uname -s)
+
+    if [ "$os" = "Linux" ]; then
+        # Respect XDG Base Directory specification
+        if [ -n "$XDG_DATA_HOME" ]; then
+            echo "$XDG_DATA_HOME/dtvem"
+        else
+            # XDG default: ~/.local/share
+            echo "$HOME/.local/share/dtvem"
+        fi
+    else
+        # macOS and others: use ~/.dtvem
+        echo "$HOME/.dtvem"
+    fi
+}
+
 # Detect OS
 detect_os() {
     case "$(uname -s)" in
@@ -180,6 +206,12 @@ main() {
     ARCH=$(detect_arch)
     info "Detected platform: ${OS}-${ARCH}"
 
+    # Determine install directory based on platform and XDG
+    DTVEM_ROOT=$(get_dtvem_root)
+    INSTALL_DIR="$DTVEM_ROOT/bin"
+    SHIMS_DIR="$DTVEM_ROOT/shims"
+    info "Install directory: $INSTALL_DIR"
+
     # Determine version to install
     local requested_version=""
     if [ -n "$DTVEM_VERSION" ]; then
@@ -336,7 +368,7 @@ main() {
     info "Running dtvem init to add shims directory to PATH..."
     if "$INSTALL_DIR/dtvem" init; then
         success "dtvem is ready to use!"
-        info "Both ~/.dtvem/bin and ~/.dtvem/shims have been added to PATH"
+        info "Both $INSTALL_DIR and $SHIMS_DIR have been added to PATH"
     else
         warning "dtvem init failed - you may need to run it manually"
     fi

src/internal/config/paths.go

@@ -45,9 +45,29 @@ func initPaths() *Paths {
 	}
 }
 
-// getRootDir returns the root dtvem directory
+// getRootDir returns the root dtvem directory based on platform conventions.
+//
+// Path Selection Rationale:
+//
+// Linux: Follows XDG Base Directory Specification (https://specifications.freedesktop.org/basedir-spec/)
+//   - Uses $XDG_DATA_HOME/dtvem if XDG_DATA_HOME is set
+//   - Otherwise uses ~/.local/share/dtvem (XDG default)
+//   - This is the standard location for user-specific data files on Linux
+//
+// macOS: Uses ~/.dtvem
+//   - macOS has its own conventions (~/Library/Application Support) but many CLI tools
+//     use dotfiles in home directory for better discoverability and Unix compatibility
+//   - ~/.dtvem is more familiar to users coming from tools like nvm, pyenv, rbenv
+//
+// Windows: Uses %USERPROFILE%\.dtvem
+//   - Alternatives considered: %LOCALAPPDATA% (C:\Users\<user>\AppData\Local)
+//   - Chose home directory for consistency with macOS/Linux and better visibility
+//   - Users expect CLI tool configs in their home directory
+//   - Easier to locate and backup than buried in AppData
+//
+// Override: DTVEM_ROOT environment variable overrides all platform defaults
 func getRootDir() string {
-	// Check for DTVEM_ROOT environment variable first
+	// Check for DTVEM_ROOT environment variable first (overrides all)
 	if root := os.Getenv("DTVEM_ROOT"); root != "" {
 		return root
 	}
@@ -59,9 +79,25 @@ func getRootDir() string {
 		return ".dtvem"
 	}
 
+	// On Linux, respect XDG Base Directory specification
+	if runtime.GOOS == constants.OSLinux {
+		return getXDGDataPath(home)
+	}
+
+	// On macOS and Windows, use ~/.dtvem
 	return filepath.Join(home, ".dtvem")
 }
 
+// getXDGDataPath returns the XDG-compliant data path for dtvem on Linux
+// Uses XDG_DATA_HOME if set, otherwise defaults to ~/.local/share/dtvem
+func getXDGDataPath(home string) string {
+	if xdgDataHome := os.Getenv("XDG_DATA_HOME"); xdgDataHome != "" {
+		return filepath.Join(xdgDataHome, "dtvem")
+	}
+	// XDG default: ~/.local/share
+	return filepath.Join(home, ".local", "share", "dtvem")
+}
+
 // RuntimeVersionPath returns the path to a specific runtime version
 func RuntimeVersionPath(runtimeName, version string) string {
 	paths := DefaultPaths()

src/internal/config/paths_test.go

@@ -7,6 +7,8 @@ import (
 	"strings"
 	"sync"
 	"testing"
+
+	"github.com/dtvem/dtvem/src/internal/constants"
 )
 
 func TestGetPaths(t *testing.T) {
@@ -285,3 +287,168 @@ func TestDefaultPaths_ConcurrentAccess(t *testing.T) {
 		t.Error("Root path is empty")
 	}
 }
+
+func TestGetXDGDataPath(t *testing.T) {
+	home := "/home/testuser"
+
+	tests := []struct {
+		name           string
+		xdgDataHome    string
+		expectedSuffix string
+	}{
+		{
+			name:           "XDG_DATA_HOME set",
+			xdgDataHome:    "/custom/data",
+			expectedSuffix: filepath.Join("/custom/data", "dtvem"),
+		},
+		{
+			name:           "XDG_DATA_HOME empty - use default",
+			xdgDataHome:    "",
+			expectedSuffix: filepath.Join(home, ".local", "share", "dtvem"),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Save and restore XDG_DATA_HOME
+			originalXDG := os.Getenv("XDG_DATA_HOME")
+			defer func() {
+				if originalXDG != "" {
+					_ = os.Setenv("XDG_DATA_HOME", originalXDG)
+				} else {
+					_ = os.Unsetenv("XDG_DATA_HOME")
+				}
+			}()
+
+			if tt.xdgDataHome != "" {
+				_ = os.Setenv("XDG_DATA_HOME", tt.xdgDataHome)
+			} else {
+				_ = os.Unsetenv("XDG_DATA_HOME")
+			}
+
+			result := getXDGDataPath(home)
+			if result != tt.expectedSuffix {
+				t.Errorf("getXDGDataPath(%q) = %q, want %q", home, result, tt.expectedSuffix)
+			}
+		})
+	}
+}
+
+func TestGetRootDir_XDGOnLinux(t *testing.T) {
+	// This test verifies the XDG behavior on Linux
+	// On other platforms, it verifies that XDG is NOT used
+	if runtime.GOOS != constants.OSLinux {
+		t.Skip("XDG tests only run on Linux")
+	}
+
+	// Save original environment
+	originalRoot := os.Getenv("DTVEM_ROOT")
+	originalXDG := os.Getenv("XDG_DATA_HOME")
+	defer func() {
+		if originalRoot != "" {
+			_ = os.Setenv("DTVEM_ROOT", originalRoot)
+		} else {
+			_ = os.Unsetenv("DTVEM_ROOT")
+		}
+		if originalXDG != "" {
+			_ = os.Setenv("XDG_DATA_HOME", originalXDG)
+		} else {
+			_ = os.Unsetenv("XDG_DATA_HOME")
+		}
+		resetPathsForTesting()
+	}()
+
+	// Clear DTVEM_ROOT to test XDG behavior
+	_ = os.Unsetenv("DTVEM_ROOT")
+
+	// Test with custom XDG_DATA_HOME
+	customXDG := "/tmp/custom-xdg-data"
+	_ = os.Setenv("XDG_DATA_HOME", customXDG)
+	resetPathsForTesting()
+
+	result := getRootDir()
+	expected := filepath.Join(customXDG, "dtvem")
+	if result != expected {
+		t.Errorf("getRootDir() with XDG_DATA_HOME=%q = %q, want %q", customXDG, result, expected)
+	}
+
+	// Test with XDG_DATA_HOME unset (should use default)
+	_ = os.Unsetenv("XDG_DATA_HOME")
+	resetPathsForTesting()
+
+	result = getRootDir()
+	home, _ := os.UserHomeDir()
+	expected = filepath.Join(home, ".local", "share", "dtvem")
+	if result != expected {
+		t.Errorf("getRootDir() with XDG_DATA_HOME unset = %q, want %q", result, expected)
+	}
+}
+
+func TestGetRootDir_NonLinux(t *testing.T) {
+	// On non-Linux platforms, verify that ~/.dtvem is used regardless of XDG
+	if runtime.GOOS == constants.OSLinux {
+		t.Skip("This test only runs on non-Linux platforms")
+	}
+
+	// Save original environment
+	originalRoot := os.Getenv("DTVEM_ROOT")
+	originalXDG := os.Getenv("XDG_DATA_HOME")
+	defer func() {
+		if originalRoot != "" {
+			_ = os.Setenv("DTVEM_ROOT", originalRoot)
+		} else {
+			_ = os.Unsetenv("DTVEM_ROOT")
+		}
+		if originalXDG != "" {
+			_ = os.Setenv("XDG_DATA_HOME", originalXDG)
+		} else {
+			_ = os.Unsetenv("XDG_DATA_HOME")
+		}
+		resetPathsForTesting()
+	}()
+
+	// Clear DTVEM_ROOT and set XDG_DATA_HOME
+	_ = os.Unsetenv("DTVEM_ROOT")
+	_ = os.Setenv("XDG_DATA_HOME", "/should/be/ignored")
+	resetPathsForTesting()
+
+	result := getRootDir()
+	home, _ := os.UserHomeDir()
+	expected := filepath.Join(home, ".dtvem")
+
+	if result != expected {
+		t.Errorf("getRootDir() on %s should ignore XDG_DATA_HOME, got %q, want %q",
+			runtime.GOOS, result, expected)
+	}
+}
+
+func TestGetRootDir_DTVEMRootOverridesXDG(t *testing.T) {
+	// Verify that DTVEM_ROOT takes precedence over XDG_DATA_HOME on all platforms
+	originalRoot := os.Getenv("DTVEM_ROOT")
+	originalXDG := os.Getenv("XDG_DATA_HOME")
+	defer func() {
+		if originalRoot != "" {
+			_ = os.Setenv("DTVEM_ROOT", originalRoot)
+		} else {
+			_ = os.Unsetenv("DTVEM_ROOT")
+		}
+		if originalXDG != "" {
+			_ = os.Setenv("XDG_DATA_HOME", originalXDG)
+		} else {
+			_ = os.Unsetenv("XDG_DATA_HOME")
+		}
+		resetPathsForTesting()
+	}()
+
+	// Set both DTVEM_ROOT and XDG_DATA_HOME
+	customRoot := "/custom/dtvem/root"
+	_ = os.Setenv("DTVEM_ROOT", customRoot)
+	_ = os.Setenv("XDG_DATA_HOME", "/should/be/ignored")
+	resetPathsForTesting()
+
+	result := getRootDir()
+	if result != customRoot {
+		t.Errorf("getRootDir() with DTVEM_ROOT set should return DTVEM_ROOT, got %q, want %q",
+			result, customRoot)
+	}
+}