diff --git a/background.js b/background.js
index d9982df..26793c7 100644
--- a/background.js
+++ b/background.js
@@ -1,12 +1,73 @@
 let port = browser.runtime.connectNative("shellfox");
 
 
+// Return the command-string associated with a URL, if any.
+function getUrlCommand(url) {
+	let matchCommand = undefined;
+	let matchRegex = "";
+
+	try {
+		let savedCommands = JSON.parse(localStorage.getItem("commands"));
+		for (regexCommandPair of savedCommands) {
+			let regex = regexCommandPair[0];
+			let match = url.match(regex);
+			let compared = compareRegexComplexity(matchRegex, regex);
+			console.log(matchRegex, "v", regex, "=", compared);
+			if (match && (compared == 0 || compared == 1)) {
+				matchCommand = regexCommandPair[1];
+				matchRegex = regex;
+			}
+		}
+	} catch {};
+	return matchCommand;
+}
+
+
+// Execute the shell command associated with the given URL, if any.
+function runUrlCommand(url) {
+	console.log("Executing…");
+	let command = getUrlCommand(url);
+	if (command && port) {
+		port.postMessage(command);
+	}
+}
+
+
+// Compare two regular expressions, returning which one is more specific.
+// Returns -1 if a is more specific, 1 if b is, and 0 if they are equal.
+// It’s a simple (and unreliable) algorithm, for now — purely based on length.
+function compareRegexComplexity(a, b) {
+	if (a && ((a && !b) || (a.length > b.length)))
+		return -1;
+	else if (b && ((b && !a) || (a.length < b.length)))
+		return 1
+	return 0;
+}
+
+
 port.onDisconnect.addListener((port) => {
 	console.log(port.error);
 	port = undefined;
 });
 
 
-browser.pageAction.onClicked.addListener(() => {
-	port.postMessage("emacs /tmp/f");
+browser.pageAction.onClicked.addListener((tab) => {
+	console.log("onClicked");
+	runUrlCommand(tab.url);
 });
+
+
+browser.tabs.onUpdated.addListener((tabId, changeInfo, tab) => {
+	console.log("onUpdated");
+	if (getUrlCommand(tab.url))
+		browser.pageAction.show(tabId);
+	else
+		browser.pageAction.hide(tabId);
+});
+
+
+browser.tabs.onActivated.addListener((activeInfo) => {
+	console.log("activated");
+	let url = browser.tabs.get(activeInfo.tabId).url;
+});
+
diff --git a/manifest.json b/manifest.json
index 74a7a7f..c2983e3 100644
--- a/manifest.json
+++ b/manifest.json
@@ -15,12 +15,13 @@
 			"19": "res/shellfox-19.png",
 			"38": "res/shellfox-38.png"
 		},
-		"default_title": "Shellfox",
-		"show_matches": ["<all_urls>"]
+		"default_title": "Shellfox"
 	},
 
 	"permissions": [
-		"nativeMessaging"
+		"nativeMessaging",
+		"activeTab",
+		"tabs"
 	],
 
 	"browser_specific_settings": {
diff --git a/options.js b/options.js
index 1bcfebc..befb0bc 100644
--- a/options.js
+++ b/options.js
@@ -16,10 +16,13 @@ function saveCommands() {
 // command-table with them.
 function populateCommandTable() {
 	let commandTable = document.getElementById("commandTable");
-	for (cmdRegex of JSON.parse(localStorage.getItem("commands"))) {
-		let commandTr = createCommandTr(cmdRegex[0], cmdRegex[1]);
-		commandTable.appendChild(commandTr);
-	}
+	try {
+		let savedCommands = JSON.parse(localStorage.getItem("commands"));
+		for (cmdRegex of savedCommands) {
+			let commandTr = createCommandTr(cmdRegex[0], cmdRegex[1]);
+			commandTable.appendChild(commandTr);
+		}
+	} catch { };
 	// Always add a spare entry.
 	commandTable.appendChild(createCommandTr("", ""));
 }