Add Linux URI scheme registration

Extends URI scheme registration to Linux via .desktop file manipulation.
The LinuxSchemeInstaller modifies the MimeType field in the .desktop file
to add x-scheme-handler entries, then registers them using xdg-mime.

Key components:
- DesktopFile: Simple parser/writer for .desktop files
- LinuxSchemeInstaller: Adds URI schemes to existing .desktop files
- Updates DefaultLinkService to support both Windows and Linux

The .desktop file path is specified via the scijava.app.desktop-file
system property.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ctrueden

src/main/java/org/scijava/desktop/links/DefaultLinkService.java

@@ -31,6 +31,7 @@
 import org.scijava.event.ContextCreatedEvent;
 import org.scijava.event.EventHandler;
 import org.scijava.desktop.links.SchemeInstaller;
+import org.scijava.desktop.platform.linux.LinuxSchemeInstaller;
 import org.scijava.desktop.platform.windows.WindowsSchemeInstaller;
 import org.scijava.log.LogService;
 import org.scijava.plugin.AbstractHandlerService;
@@ -64,15 +65,15 @@ private void onEvent(final ContextCreatedEvent evt) {
 			}
 		}
 
-		// Register URI schemes with the operating system (Windows only).
+		// Register URI schemes with the operating system.
 		installSchemes();
 	}
 
 	/**
 	 * Installs URI schemes with the operating system.
 	 * <p>
 	 * This method collects all schemes supported by registered {@link LinkHandler}
-	 * plugins and registers them with the OS (currently Windows only).
+	 * plugins and registers them with the OS (Windows and Linux supported).
 	 * </p>
 	 */
 	private void installSchemes() {
@@ -111,13 +112,23 @@ private void installSchemes() {
 	/**
 	 * Creates the appropriate {@link SchemeInstaller} for the current platform.
 	 * <p>
-	 * Currently only Windows is supported. macOS uses Info.plist in the .app bundle
-	 * (configured at build time), and Linux .desktop file management belongs in
-	 * scijava-plugins-platforms.
+	 * Windows and Linux are supported via runtime registration. macOS uses Info.plist
+	 * in the .app bundle (configured at build time, not at runtime).
 	 * </p>
 	 */
 	private SchemeInstaller createInstaller() {
-		return new WindowsSchemeInstaller(log);
+		final String os = System.getProperty("os.name");
+		if (os == null) return null;
+
+		final String osLower = os.toLowerCase();
+		if (osLower.contains("linux")) {
+			return new LinuxSchemeInstaller(log);
+		}
+		else if (osLower.contains("win")) {
+			return new WindowsSchemeInstaller(log);
+		}
+
+		return null; // macOS or other unsupported platforms
 	}
 
 	/**

src/main/java/org/scijava/desktop/platform/linux/DesktopFile.java

@@ -0,0 +1,232 @@
+/*-
+ * #%L
+ * Desktop integration for SciJava.
+ * %%
+ * Copyright (C) 2010 - 2026 SciJava developers.
+ * %%
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ * 
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ * 
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDERS OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ * #L%
+ */
+
+package org.scijava.desktop.platform.linux;
+
+import java.io.BufferedReader;
+import java.io.BufferedWriter;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Simple parser and writer for Linux .desktop files.
+ * <p>
+ * Supports reading and writing key-value pairs within the [Desktop Entry] section.
+ * This implementation is minimal and focused on URI scheme registration needs.
+ * </p>
+ *
+ * @author Curtis Rueden
+ */
+public class DesktopFile {
+
+	private final Map<String, String> entries;
+	private final List<String> comments;
+
+	public DesktopFile() {
+		this.entries = new LinkedHashMap<>();
+		this.comments = new ArrayList<>();
+	}
+
+	/**
+	 * Parses a .desktop file from disk.
+	 *
+	 * @param path Path to the .desktop file
+	 * @return Parsed DesktopFile
+	 * @throws IOException if reading fails
+	 */
+	public static DesktopFile parse(final Path path) throws IOException {
+		final DesktopFile df = new DesktopFile();
+		boolean inDesktopEntry = false;
+
+		try (final BufferedReader reader = Files.newBufferedReader(path, StandardCharsets.UTF_8)) {
+			String line;
+			while ((line = reader.readLine()) != null) {
+				final String trimmed = line.trim();
+
+				// Track section
+				if (trimmed.equals("[Desktop Entry]")) {
+					inDesktopEntry = true;
+					continue;
+				}
+				if (trimmed.startsWith("[") && trimmed.endsWith("]")) {
+					inDesktopEntry = false;
+					continue;
+				}
+
+				// Only process [Desktop Entry] section
+				if (!inDesktopEntry) continue;
+
+				// Skip empty lines and comments
+				if (trimmed.isEmpty() || trimmed.startsWith("#")) {
+					df.comments.add(line);
+					continue;
+				}
+
+				// Parse key=value
+				final int equals = line.indexOf('=');
+				if (equals > 0) {
+					final String key = line.substring(0, equals).trim();
+					final String value = line.substring(equals + 1);
+					df.entries.put(key, value);
+				}
+			}
+		}
+
+		return df;
+	}
+
+	/**
+	 * Writes the .desktop file to disk.
+	 *
+	 * @param path Path to write to
+	 * @throws IOException if writing fails
+	 */
+	public void writeTo(final Path path) throws IOException {
+		// Ensure parent directory exists
+		final Path parent = path.getParent();
+		if (parent != null && !Files.exists(parent)) {
+			Files.createDirectories(parent);
+		}
+
+		try (final BufferedWriter writer = Files.newBufferedWriter(path, StandardCharsets.UTF_8)) {
+			writer.write("[Desktop Entry]");
+			writer.newLine();
+
+			// Write key-value pairs
+			for (final Map.Entry<String, String> entry : entries.entrySet()) {
+				writer.write(entry.getKey());
+				writer.write('=');
+				writer.write(entry.getValue());
+				writer.newLine();
+			}
+
+			// Write comments at the end
+			for (final String comment : comments) {
+				writer.write(comment);
+				writer.newLine();
+			}
+		}
+	}
+
+	/**
+	 * Gets the value for a key.
+	 *
+	 * @param key The key
+	 * @return The value, or null if not present
+	 */
+	public String get(final String key) {
+		return entries.get(key);
+	}
+
+	/**
+	 * Sets a key-value pair.
+	 *
+	 * @param key The key
+	 * @param value The value
+	 */
+	public void set(final String key, final String value) {
+		entries.put(key, value);
+	}
+
+	/**
+	 * Checks if a MimeType entry contains a specific MIME type.
+	 *
+	 * @param mimeType The MIME type to check (e.g., "x-scheme-handler/fiji")
+	 * @return true if the MimeType field contains this type
+	 */
+	public boolean hasMimeType(final String mimeType) {
+		final String mimeTypes = entries.get("MimeType");
+		if (mimeTypes == null || mimeTypes.isEmpty()) return false;
+
+		final String[] types = mimeTypes.split(";");
+		for (final String type : types) {
+			if (type.trim().equals(mimeType)) {
+				return true;
+			}
+		}
+		return false;
+	}
+
+	/**
+	 * Adds a MIME type to the MimeType field.
+	 * <p>
+	 * The MimeType field is a semicolon-separated list. This method appends
+	 * the new type if it's not already present.
+	 * </p>
+	 *
+	 * @param mimeType The MIME type to add (e.g., "x-scheme-handler/fiji")
+	 */
+	public void addMimeType(final String mimeType) {
+		if (hasMimeType(mimeType)) return; // Already present
+
+		String mimeTypes = entries.get("MimeType");
+		if (mimeTypes == null || mimeTypes.isEmpty()) {
+			// Create new MimeType field
+			entries.put("MimeType", mimeType + ";");
+		}
+		else {
+			// Append to existing
+			if (!mimeTypes.endsWith(";")) {
+				mimeTypes += ";";
+			}
+			entries.put("MimeType", mimeTypes + mimeType + ";");
+		}
+	}
+
+	/**
+	 * Removes a MIME type from the MimeType field.
+	 *
+	 * @param mimeType The MIME type to remove
+	 */
+	public void removeMimeType(final String mimeType) {
+		final String mimeTypes = entries.get("MimeType");
+		if (mimeTypes == null || mimeTypes.isEmpty()) return;
+
+		final List<String> types = new ArrayList<>();
+		for (final String type : mimeTypes.split(";")) {
+			final String trimmed = type.trim();
+			if (!trimmed.isEmpty() && !trimmed.equals(mimeType)) {
+				types.add(trimmed);
+			}
+		}
+
+		if (types.isEmpty()) {
+			entries.remove("MimeType");
+		}
+		else {
+			entries.put("MimeType", String.join(";", types) + ";");
+		}
+	}
+}