Magisk ships with a feature complete BusyBox binary (including full SELinux support). The executable is located at `/data/adb/magisk/busybox`. Magisk's BusyBox supports runtime toggle-able "ASH Standalone Shell Mode". What this standalone mode means is that when running in the `ash` shell of BusyBox, every single command will directly use the applet within BusyBox, regardless of what is set as `PATH`. For example, commands like `ls`, `rm`, `chmod` will **NOT** use what is in `PATH` (in the case of Android by default it will be `/system/bin/ls`, `/system/bin/rm`, and `/system/bin/chmod` respectively), but will instead directly call internal BusyBox applets. This makes sure that scripts always run in a predictable environment and always have the full suite of commands no matter which Android version it is running on. To force a command _not_ to use BusyBox, you have to call the executable with full paths.
Every single shell script running in the context of Magisk will be executed in BusyBox's `ash` shell with standalone mode enabled. For what is relevant to 3rd party developers, this includes all boot scripts and module installation scripts.
For those who want to use this "Standalone Mode" feature outside of Magisk, there are 2 ways to enable it:
1. Set environment variable `ASH_STANDALONE` to `1`<br>Example: `ASH_STANDALONE=1 /data/adb/magisk/busybox sh <script>`
2. Toggle with command-line options:<br>`/data/adb/magisk/busybox sh -o standalone <script>`
To make sure all subsequent `sh` shell executed also runs in standalone mode, option 1 is the preferred method (and this is what Magisk and the Magisk app internally use) as environment variables are inherited down to child processes.
Please read the [Boot Scripts](#boot-scripts) section to understand the difference between `post-fs-data.sh` and `service.sh`. For most module developers, `service.sh` should be good enough if you just need to run a boot script.
In all scripts of your module, please use `MODDIR=${0%/*}` to get your module's base directory path; do **NOT** hardcode your module path in scripts.
All files you want to replace/inject should be placed in this folder. This folder will be recursively merged into the real `/system`; that is: existing files in the real `/system` will be replaced by the one in the module's `system`, and new files in the module's `system` will be added to the real `/system`.
If you place a file named `.replace` in any of the folders, instead of merging its contents, that folder will directly replace the one in the real system. This can be very handy for swapping out an entire folder.
If you want to replace files in `/vendor`, `/product`, or `/system_ext`, please place them under `system/vendor`, `system/product`, and `system/system_ext` respectively. Magisk will transparently handle whether these partitions are in a separate partition or not.
Zygisk is a feature of Magisk that allows advanced module developers to run code directly in every Android applications' processes before they are specialized and running. For more details about the Zygisk API and building a Zygisk module, please checkout the [Zygisk Module Sample](https://github.com/topjohnwu/zygisk-module-sample) project.
If your module requires some additional sepolicy patches, please add those rules into this file. Each line in this file will be treated as a policy statement. For more details about how a policy statement is formatted, please check [magiskpolicy](tools.md#magiskpolicy)'s documentation.
A Magisk module installer is a Magisk module packaged in a zip file that can be flashed in the Magisk app or custom recoveries such as TWRP. The simplest Magisk module installer is just a Magisk module packed as a zip file, in addition to the following files:
-`update-binary`: Download the latest [module_installer.sh](https://github.com/topjohnwu/Magisk/blob/master/scripts/module_installer.sh) and rename/copy that script as `update-binary`
-`updater-script`: This file should only contain the string `#MAGISK`
The module installer script will setup the environment, extract the module files from the zip file to the correct location, then finalizes the installation process, which should be good enough for most simple Magisk modules.
If you need to customize the module installation process, optionally you can create a script in the installer named `customize.sh`. This script will be _sourced_ (not executed!) by the module installer script after all files are extracted and default permissions and secontext are applied. This is very useful if your module require additional setup based on the device ABI, or you need to set special permissions/secontext for some of your module files.
If you would like to fully control and customize the installation process, declare `SKIPUNZIP=1` in `customize.sh` to skip all default installation steps. By doing so, your `customize.sh` will be responsible to install everything by itself.
For convenience, you can also declare a list of folders you want to replace in the variable name `REPLACE`. The module installer script will create the `.replace` file into the folders listed in `REPLACE`. For example:
- When your module is downloaded with the Magisk app, `update-binary` will be **forcefully** replaced with the latest [`module_installer.sh`](https://github.com/topjohnwu/Magisk/blob/master/scripts/module_installer.sh). **DO NOT** try to add any custom logic in `update-binary`.
- Due to historical reasons, **DO NOT** add a file named `install.sh` in your module installer zip.
- **DO NOT** call `exit` at the end of `customize.sh`. The module installer script has to perform some cleanups before exiting.
Since `/` is read-only on system-as-root devices, Magisk provides an overlay system to enable developers to replace files in rootdir or add new `*.rc` scripts. This feature is designed mostly for custom kernel developers.
Overlay files shall be placed in the `overlay.d` folder in boot image ramdisk, and they follow these rules:
1. Each `*.rc` file (except for `init.rc`) in `overlay.d` will be read and concatenated **AFTER**`init.rc` if it does not exist in the root directory, otherwise it will **REPLACE** the existing one.
To add additional files which you can refer to in your custom `*.rc` scripts, add them into `overlay.d/sbin`. The 3 rules above do not apply to anything in this folder; instead, they will be directly copied to Magisk's internal `tmpfs` directory (which used to always be `/sbin`).
Starting from Android 11, the `/sbin` folder may no longer exists, and in that scenario, Magisk randomly generates a different `tmpfs` folder each boot. Every occurrence of the pattern `${MAGISKTMP}` in your `*.rc` scripts will be replaced with the Magisk `tmpfs` folder when `magiskinit` injects it into `init.rc`. On pre Android 11 devices, `${MAGISKTMP}` will simply be replaced with `/sbin`, so **NEVER** hardcode `/sbin` in the `*.rc` scripts when referencing these additional files.
