Android - How to mount /system rewritable or read-only? (RW/RO)

There are a few methods how you can mount your /system directory RW or RO. However, it will require root.

Method 1:

1. Connect your phone to the computer. (Make sure USB debugging is enabled on your phone)

2. Open CMD/Terminal on your PC.

   • Windows: CTRL + R, then type cmd.

   • Ubuntu: CTRL + ALT + T.

   • Mac: Navigate to /Applications/Utilities/ and double-click on Terminal.

3. Type this:

   1. adb shell

   2. su

   3. Choose one: (for security mount /system back to RO when finished)

      • Mount system RW: mount -o rw,remount /system
      • Mount system RO: mount -o ro,remount /system

Method 2:

1. Open terminal on your android phone (download here):

2. Type this in the terminal:

   1. su

   2. Choose one: (for security mount /system back to RO when finished)

      • Mount system RW: mount -o rw,remount /system
      • Mount system RO: mount -o ro,remount /system

Android 2.3

For people running Android 2.3 and the command fails, look at this answer: https://android.stackexchange.com/a/125437/95577

-writable-system for the emulator

When launching the emulator after a build with, you must use:

. build/envsetup.sh
lunch aosp_x86_64-eng
emulator -show-kernel -verbose -writable-system

Then, for future runs, you must keep the -writable-system option, or else image changes will not be visible:

emulator -show-kernel -verbose -writable-system

-verbose shows us that the emulator switches from the default -drive:

if=none,index=0,id=system,file=/path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/system-qemu.img,read-only

to:

if=none,index=0,id=system,file=/path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/system-qemu.img.qcow2,overlap-check=none,cache=unsafe,l2-cache-size=1048576

Therefore it:

• removes ,read-only

• uses system-qemu.img.qcow2 instead of system-qemu.img.

  This implies that changes will only be visible afterwards if you pass -writable-sytem on future boots after the change was made!

  We can see that the qcow2 image is just a small overlay on top of the base image since:

  qemu-img info /path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/system-qemu.img.qcow2
  

  contains:

  backing file: /path/to/aosp/8.1.0_r60/out/target/product/generic_x86_64/system-qemu.img
  

The emulator -help also confirms this:

emulator -help

contains:

-writable-system     make system & vendor image writable after 'adb remount'

adb remount + adb root

I think this is just a shortcut for mount as mentioned at https://android.stackexchange.com/a/110928/126934 , but it is very convenient:

adb root
adb remount
adb shell

adb help contains:

 root                     restart adbd with root permissions
 remount
     remount /system, /vendor, and /oem partitions read-write

Restore the original system image

Same as for userdata: remove the .qcow2 overlay, and re-generate it manually: https://stackoverflow.com/questions/54446680/how-to-reset-the-userdata-image-when-building-android-aosp-and-running-it-on-the