Working with Android's File System: Remounting and Writing to System Files

Android devices have a file system that is designed to be secure and protected from unauthorized modifications. By default, the system files are mounted as read-only, which can prevent users from making changes to these files. However, there are situations where developers or power users may need to modify system files, such as when testing custom builds or applying patches.

In this tutorial, we will explore how to remount the Android file system and write to system files using various methods. We will cover the use of ADB (Android Debug Bridge) commands, terminal emulator commands, and emulator flags to achieve this goal.

Understanding the File System

Before we dive into the methods for remounting the file system, it’s essential to understand how Android’s file system works. The system files are stored in the /system directory, which is mounted as read-only by default. To make changes to these files, we need to remount the file system with write permissions.

Method 1: Using ADB Remount Command

The simplest way to remount the file system and write to system files is by using the adb remount command. This command will remount the /system directory with write permissions, allowing you to push files to this location using adb push.

To use this method, follow these steps:

1. Connect your device to your computer using a USB cable.
2. Open a terminal or command prompt on your computer and navigate to the platform-tools directory of your Android SDK installation.
3. Type adb remount and press Enter.

Once you have remounted the file system, you can use adb push to copy files to the /system directory.

Method 2: Using Terminal Emulator Command

Alternatively, you can use a terminal emulator on your device to remount the file system. This method requires root access and a terminal emulator app installed on your device.

To use this method, follow these steps:

1. Open the terminal emulator app on your device.
2. Type mount -o rw,remount /system (or mount -o rw,remount -t yaffs2 /dev/block/mtdblock3 /system for some devices) and press Enter.

This command will remount the /system directory with write permissions, allowing you to make changes to system files.

Method 3: Using Emulator Flags

If you are using an emulator, you can launch it with a flag that allows you to write to system files. The -writable-system flag enables write access to the /system directory.

To use this method, follow these steps:

1. Launch the emulator from the command line using the following command: ./emulator -writable-system -avd <your_avd_name>.
2. Once the emulator is launched, type adb root and press Enter.
3. Type adb remount and press Enter.

This method allows you to write to system files on the emulator without requiring a physical device.

Conclusion

In this tutorial, we have explored various methods for remounting the Android file system and writing to system files. Whether you are using ADB commands, terminal emulator commands, or emulator flags, it’s essential to understand the risks involved with modifying system files. Always make sure to backup your data before making any changes, and be cautious when working with system files to avoid causing damage to your device.

Additional Tips

• When remounting the file system, make sure to remount it as read-only (ro) when you are finished making changes to prevent unauthorized access.
• Be aware of the device’s security features, such as verity mode, which may prevent you from writing to system files even after remounting.
• Always test your changes on a development device or emulator before applying them to a production device.