Working with system files of mobile devices often requires intervention in the firmware structure. One of the most common data storage formats in the ecosystem Android is a file with the extension .img. This is an image of a partition that may contain the bootloader, system, recovery or user data.
Many enthusiasts and developers are faced with the need to extract the contents of such an image to modify, analyze or restore the device. Conventional archivers like WinRAR or 7-Zip are powerless here, since the file structure differs from standard archives. To complete the task, specialized utilities are required that can correctly interpret file systems ext4, yaffs2 or erofs.
In this material we will look in detail at what tools can be used on different operating systems. You will learn about the nuances of working with images of different versions Android and understand how to avoid fatal errors that can turn your device into a “brick.”
⚠️ Attention: Any manipulations with system images are carried out at your own peril and risk. Incorrect assembly or modification of critical files can lead to complete inoperability of the device.
Android image file structure
Before you start unpacking, you need to understand what exactly you are dealing with. File system.img or boot.img is not just an archive, but an exact copy of the device’s memory section. Depending on the version Android and manufacturer, a different file system may be used internally.
Modern firmware often uses the erofs (Enhanced Read-Only File System) format, which provides better reading performance compared to outdated ext4. Old devices, especially those of the Android 4-6era, were often used yaffs2. Understanding the file system type is critical to choosing the right tool.
It is also worth considering that the image can be a “sparse image”. This is a special optimization in which empty blocks do not take up disk space. Standard tools will not be able to read such a file without first converting it to a “raw” format.
To determine the type of image, you can use the utility file in Linux or specialized scripts. If you see a message about sparse image, the first step is always to convert. Without this stage, further work is impossible.
Tools for working on Windows
Users of the operating system Windows are in the most advantageous position due to the presence of graphical interfaces. The most popular and time-tested tool is the app Android Image Kitchen. It allows you not only to unpack, but also to pack images back while maintaining checksums.
The process of working in Android Image Kitchen is maximally simplified. You just need to drag the file .img to the app window or use the file selection button. The utility will automatically detect the image type and suggest available actions. If the image is sparse, the app itself will offer to perform the conversion.
Another powerful utility is Yaffs2Image. It specializes in working with older formats, which are often found on devices with processors MediaTek. The interface here is less friendly, but the functionality for specific tasks is higher.
- 🛠️ Android Image Kitchen - a universal harvester for most tasks.
- 💾 Yaffs2Image - the best choice for old firmware and specific formats.
- 🖥️ Ext4Unpacker - a lightweight utility for working exclusively with ext4 partitions.
This A common mistake made by beginners that leads to failure of unpacking scripts. Place the files in the root of the disk, for example, in a folder C:\Firmware.
☑️ Preparing the workplace
Unpacking via the command line in Linux and macOS
For professionals and developers, the environment Linux or macOS provides the most flexible possibilities. Here the work is carried out mainly through the terminal, which allows you to automate processes and use powerful scripts.
The first stage is often the conversion of the sparse image into a raw format. To do this, use the utility simg2img, which is part of the package Android Platform Tools. The command looks simple, but requires care when specifying paths.
simg2img system_sparse.img system_raw.imgAfter receiving the raw image, it must be mounted on the system in order to access the files. In Linux this is done through the command mount. However, if the image contains encrypted sections or specific metadata, simple installation may not work.
An alternative method is to use a utility ext4_utils or scripts from the repository OpenAndroidKitchen. These tools allow you to extract content without mounting it, which is safer for the host system. You simply specify the input file and the destination directory.
⚠️ Attention: When working in a terminal, superuser rights (sudo) are not always required. Avoid running commands as root unnecessarily to avoid accidentally changing permissions on extracted files.
The process is similar, but you may need to install additional dependencies through the package manager. macOS the process is similar, but you may need to install additional dependencies through a package manager Homebrew. Problems often arise with the encoding of file names if the firmware was created for a region with a different encoding than yours.
Working with the bootloader and the Boot section
The file boot.imgdeserves special attention. Unlike the system partition, it contains the kernel (kernel) and ramdisk (ramdisk). Unpacking this image is required to modify the kernel, add drivers or change boot parameters.
Standard ext4 unpacking methods do not work here, since the structure of the boot image is completely different. For these purposes, the utility abootimg or built-in functions Android Image Kitchenare used. The process splits the image into three key components.
- 🧠 Kernel —the system kernel that manages the hardware.
- 📦 Ramdisk —a temporary file system loaded into RAM.
- ⚙️ Second Stage —additional data used on some devices.
The ramdisk is usually a compressed archive cpio, which is additionally packaged in gzip or lz4. To access the files inside, you need to apply unpackers sequentially. First, the ramdisk is extracted from the image, then it is unpacked as an archive.
What to do if the ramdisk does not unpack?
If standard utilities give an error, a non-standard compression algorithm may be used. Try to determine the compression type with the file command and select the appropriate decoder, for example lz4 or xz.
Changing content boot.img is one of the most risky operations. An error in the kernel configuration or the absence of a critical driver in the ramdisk will result in the device being unable to complete booting. Always save the original copy before editing.
Comparison of popular unpacking utilities
The choice of tool depends on your tasks, operating system and image type. Below is a table to help you navigate the variety of available software.
| Utility | OS | Sparse support | Interface |
|---|---|---|---|
| Android Image Kitchen | Windows | Automatic | Graphical |
| simg2img + mount | Linux/macOS | Requires conversion | Command line |
| OpenAndroidKitchen | Cross-platform | Yes | Graphical (Java) |
| Yaffs2Image | Windows | No | Graphical |
As can be seen from the table, for one-time tasks on Windows the best choice would be Android Image Kitchen. If you plan to engage in firmware modding professionally, it is worth mastering the command line tools in the environment Linux.
The utility OpenAndroidKitchen is a cross-platform solution written in Java. It is convenient because it works the same on all systems, but may require more resources and proper environment settings Java Runtime.
Before unpacking, always check the checksum (MD5 or SHA1) of the downloaded image. A damaged file can lead to reading errors and data loss during the extraction process.
Frequent errors and ways to solve them
In the process of work, users often encounter errors that at first glance seem fatal. However, most of them have simple solutions. The most common problem is a message that the image is not valid.
This often happens when the file was not downloaded completely or was damaged during transfer. Another reason is an attempt to open an image encrypted by the manufacturer. Some vendors, for example Huawei or Xiaomi, use their own partition encryption methods.
Another difficulty arises when working with the file system erofs. Older versions of utilities simply do not understand this format. In this case, you need to update the tools to the latest version or use specialized scripts for erofs.
⚠️ Attention: app interfaces and encryption methods are constantly changing. Support information for specific formats may be out of date. Always check the documentation for the selected utility on the official website of the developer.
If you receive an access rights error in Linux, make sure that you have rights to read the file and write to the target directory. Sometimes running a command as a superuser helps, but this should be done with caution.
The success of unpacking 90% depends on the correct determination of the file system type and image format (sparse/raw) before starting work.
Packaging and verification of changes
After making changes to the image files, the reverse stage begins assemblies. This process is no less important than unpacking. Incorrect packaging can cause the device's bootloader to reject the firmware due to an incorrect checksum or structure.
Most graphics utilities, such as Android Image Kitchen, take care of this step. They automatically recalculate the headers and form the correct file .img. However, when working manually in the console, you will have to use the utilities yourself mkuserimg or make_ext4fs.
A critical step is checking integrity. Make sure that the size of the resulting image does not exceed the size of the partition on the device. It is also worth comparing the folder structure with the original so as not to lose hidden files or files with special access rights.
For advanced users, it is useful to know that some devices require firmware signing. In this case, simply copying files is not enough - you need to use signing tools, such as AVB (Android Verified Boot) tools, otherwise the phone will not boot.
Is it possible to unpack an img image with a standard archiver?
No, standard archivers like WinRAR do not work with partition images directly. The .img file is a sector-by-sector copy of the disk, not an archive. Preliminary conversion or specialized software is required.
What to do if the files cannot be read after unpacking?
Perhaps the image is encrypted or a file system is used that your utility does not support (for example, erofs on older versions of software). Try updating the tools or determining the file system type with the file command.
Is it safe to modify boot.img?
This is a risky operation. An error in the kernel or ramdisk will result in a bootloop (cyclic reboot). Always have a working recovery image on hand and access to Fastboot mode.
Why do you need to convert sparse image to raw?
Sparse image contains empty blocks to save space. File system unpacking tools require a continuous data stream (raw) to correctly read the inode table and directory structure.