Provide a service for executing commands by third-party applications

Re-implementation of https://github.com/termux/termux-app/pull/1029.

If Termux has property "allow-external-apps" set to "true", a third-party
program will be able to send intents for executing custom commands
within Termux environment.

Third-party program must declare permission "com.termux.permission.RUN_COMMAND".

app/src/main/AndroidManifest.xml

@@ -8,6 +8,12 @@
     <uses-feature android:name="android.hardware.touchscreen" android:required="false" />
     <uses-feature android:name="android.software.leanback" android:required="false" />
 
+    <permission android:name="com.termux.permission.RUN_COMMAND"
+        android:label="@string/run_command_permission_label"
+        android:description="@string/run_command_permission_description"
+        android:icon="@drawable/ic_launcher"
+        android:protectionLevel="dangerous" />
+
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
@@ -114,6 +120,15 @@
             android:name="com.termux.app.TermuxService"
             android:exported="false" />
 
+        <service
+            android:name=".app.RunCommand"
+            android:exported="true"
+            android:permission="com.termux.permission.RUN_COMMAND" >
+            <intent-filter>
+                <action android:name="com.termux.RUN_COMMAND" />
+            </intent-filter>
+        </service>
+
         <receiver android:name=".app.TermuxOpenReceiver" />
 
         <provider android:authorities="com.termux.files"

app/src/main/java/com/termux/app/RunCommand.java

@@ -0,0 +1,88 @@
+package com.termux.app;
+
+import android.app.Service;
+import android.content.Intent;
+import android.net.Uri;
+import android.os.Binder;
+import android.os.Build;
+import android.os.IBinder;
+import android.util.Log;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
+import java.util.Properties;
+
+/**
+ * When allow-external-apps property is set to "true", Termux is able to process execute intents
+ * sent by third-party applications.
+ *
+ * Third-party program must declare com.termux.permission.RUN_COMMAND permission.
+ *
+ * Intent expects following configuration:
+ *
+ * Action: com.termux.RUN_COMMAND
+ * Program/script path (string): com.termux.RUN_COMMAND_PATH
+ * Program/script arguments (string): com.termux.RUN_COMMAND_ARGUMENTS
+ * Session working directory (string): com.termux.RUN_COMMAND_WORKDIR
+ *
+ * Sample intent for launching program "top" in equivalent of ADB command:
+ * am startservice -a com.termux.RUN_COMMAND --es com.termux.RUN_COMMAND_PATH /data/data/com.termux/files/usr/bin/top
+ */
+public class RunCommand extends Service {
+
+    public static final String RUN_COMMAND_ACTION = "com.termux.RUN_COMMAND";
+    public static final String RUN_COMMAND_PATH = "com.termux.RUN_COMMAND_PATH";
+    public static final String RUN_COMMAND_ARGUMENTS = "com.termux.RUN_COMMAND_ARGUMENTS";
+    public static final String RUN_COMMAND_WORKDIR = "com.termux.RUN_COMMAND_WORKDIR";
+
+    class LocalBinder extends Binder {
+        public final RunCommand service = RunCommand.this;
+    }
+
+    private final IBinder mBinder = new RunCommand.LocalBinder();
+
+    @Override
+    public IBinder onBind(Intent intent) {
+        return mBinder;
+    }
+
+    public int onStartCommand(Intent intent, int flags, int startId) {
+        if (allowExternalApps() && RUN_COMMAND_ACTION.equals(intent.getAction())) {
+            Uri programUri = new Uri.Builder().scheme("com.termux.file").path(intent.getStringExtra(RUN_COMMAND_PATH)).build();
+
+            Intent execIntent = new Intent(TermuxService.ACTION_EXECUTE, programUri);
+            execIntent.setClass(this, TermuxService.class);
+            execIntent.putExtra(TermuxService.EXTRA_ARGUMENTS, intent.getStringExtra(RUN_COMMAND_ARGUMENTS));
+            execIntent.putExtra(TermuxService.EXTRA_CURRENT_WORKING_DIRECTORY, intent.getStringExtra(RUN_COMMAND_WORKDIR));
+
+            if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+                this.startForegroundService(execIntent);
+            } else {
+                this.startService(execIntent);
+            }
+        }
+
+        return Service.START_NOT_STICKY;
+    }
+
+    private boolean allowExternalApps() {
+        File propsFile = new File(TermuxService.HOME_PATH + "/.termux/termux.properties");
+        if (!propsFile.exists())
+            propsFile = new File(TermuxService.HOME_PATH + "/.config/termux/termux.properties");
+
+        Properties props = new Properties();
+        try {
+            if (propsFile.isFile() && propsFile.canRead()) {
+                try (FileInputStream in = new FileInputStream(propsFile)) {
+                    props.load(new InputStreamReader(in, StandardCharsets.UTF_8));
+                }
+            }
+        } catch (Exception e) {
+            Log.e("termux", "Error loading props", e);
+        }
+
+        return props.getProperty("allow-external-apps", "false").equals("true");
+    }
+}

app/src/main/res/values/strings.xml

@@ -2,6 +2,8 @@
 <resources>
     <string name="application_name">Termux</string>
     <string name="shared_user_label">Termux user</string>
+    <string name="run_command_permission_label">Run commands in Termux environment</string>
+    <string name="run_command_permission_description">Allow third-party applications to execute arbitrary commands within Termux environment.</string>
     <string name="new_session">New session</string>
     <string name="new_session_failsafe">Failsafe</string>
     <string name="toggle_soft_keyboard">Keyboard</string>