Introduction
Android devices, particularly the developer-friendly Nexus models from Google, are especially suited for unlocking and tinkering with.
The Android OS, based upon Linux, is modular enough to allow replacement and upgrading of particular components of the system: bootloader, recovery, radio software, and both the underlying operating system and the user interface. Just as you can replace the default home screen/launcher app with one you like better, you can run a different build of the open source Android operating system (AOSP) for improved performance or bleeding edge capabilities. These are called ROMs.
Even without switching out the core OS, you can use a customized recovery partition. Team Win (TWRP) and ClockworkMod (CWM) are the most popular third party recovery replacements, and are available for many Android devices. They provide advanced maintenance functions, including the ability to install root apps that let the user and apps perform functions as the system user.
Tools
The basic tool for working with a Android device is the Android Software Development Kit (SDK), which includes tools like fastboot and adb (Android Debug Bridge). There are third party "lite" versions of the basic tools for most operating systems. Some GUI tools take the complexity out of using command line tools to perform common operations, but these actions are no less powerful.
Warning
Unlocking, flashing, rooting your device may void the warranty. You are also at risk for "bricking" (rendering inoperable and irreparable) your phone or tablet. You assume full liability and responsibility for attempting to follow any of these steps.
Some applications will not work if a device has been unlocked or rooted. E.g. Android Pay now detects if the system has been compromised this way in Android 6.0, and will refuse to work for you.
Further Information
Google Nexus
- Google Nexus: Using Hardware Devices
- Google Nexus: Booting into fastboot (bootloader)
- ADB Fastboot Install (MacOS, Linux)
XDA developer forums
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To power down most phones, hold down the power button until a prompt appears. Tap on the pop-up message. Some devices may have a "Power off" command in the software settings.
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If your phone is crashing or locking up after boot, try rebooting into safe mode. In the previous step, tap and hold on "Power off" until the prompt changes to "Reboot to safe mode." Tap OK.
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With the phone in an off state, hold down buttons in a combination to enter the bootloader. Google Nexus models have specific combinations like holding Volume Down and Power simultaneously.
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Your device's bootloader may look different from this Nexus 5 screen. That's okay. Note the essential information available to you here: phone model, product variant, serial/IMEI number, SIM and bootloader locked state.
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The bootloader often displays information like the phone model, version of fastboot, whether or not it is boot-unlocked or not.
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For Google Nexus devices, you can now connect your phone to a computer (Windows, MacOS, Linux) running the Android SDK tools, allowing some commands to be sent.
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From your bootloader screen, toggling the volume buttons will offer functions like Start, Power Down, Restart Bootloader, and usually … Recovery. Pressing the Power button generally executes the displayed function.
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The recovery partition is a rudimentary mini operating mode which allows loading updates, wiping the cache partition, and performing a factory reset to a phone that cannot boot properly (e.g. in an endless bootloop, never reaching an unlock screen). It's not called recovery for no reason -- it's an essential tool for restoring a broken phone.
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