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Recently, I've figured out that if I delete files from /sdcard/Download it deletes files from /storage/emulated/0/Download. And if I add the files into /sdcard/Download it duplicates them in /storage/emulated/0/Download.

So what is /storage/emulated/0/? For what purposes do we have it in our android file system?

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/storage/emulated/0/Download is the actual path to the files.

/sdcard/Download is a symlink to the actual path of /storage/emulated/0/Download

However, the actual files are located in the filesystem in /data/media, which is then mounted to /storage/emulated/0 (and often other mountpoints as well)

A Symlink In computing, a symbolic link is a term for any file that contains a reference to another file or directory in the form of an absolute or relative path and that affects pathname resolution. Symbolic links were already present by 1978 in minicomputer operating systems from DEC and Data General's RDOS.

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    This answer would be better if it explained a little bit about why it's "emulated". I believe Android does some hack to fake a FAT fs that's actually backed by something better, but I don't know the details and clicked this question hoping to learn something new.
    – R.. GitHub STOP HELPING ICE
    Dec 20, 2018 at 19:02
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    @R.. the "emulated" IMHO points to the fact that it's an "emulated SD card" (not a real one).
    – Izzy ♦
    Dec 20, 2018 at 19:38
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    @R.. It uses SDCardFS. Here's an excellent article about it: xda-developers.com/… (archive)
    – Nonny Moose
    Dec 21, 2018 at 3:16
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/storage/emulated/0/ is actually /data/media/0/ exposed through an emulated / virtual filesystem, not the actual one.

This is with reference to my previous answer here, but with more relevant details.

ANDROID STORAGE:

On Android 5:

/sdcard >S> /storage/emulated/legacy >S> /mnt/shell/emulated/0
/mnt/shell/emulated >E> /data/media

On Android 6+:

# for (Java) Android apps (running inside zygote virtual machine)
# "/storage to VIEW" bind mount is inside a separate mount namespace for every app
/sdcard >S> /storage/self/primary
/storage/self >B> /mnt/user/USER-ID
/mnt/user/USER-ID/primary >S> /storage/emulated/USER-ID
/storage/emulated >B> /mnt/runtime/VIEW/emulated
/mnt/runtime/VIEW/emulated >E> /data/media

# for services/daemons/processes in root/global namespace (VIEW = default)
/sdcard >S> /storage/self/primary
/storage >B> /mnt/runtime/default
/mnt/runtime/default/self/primary >S> /mnt/user/USER-ID/primary
/mnt/user/USER-ID/primary >S> /storage/emulated/USER-ID
/storage/emulated >B> /mnt/runtime/default/emulated
/mnt/runtime/default/emulated >E> /data/media

* >S> for symlink, >E> for emulated and >B> for bind mount
* USER-ID of current user in case of Multiple Users or Work Profile, normally 0 i.e. that of device owner
* VIEW is one of read (for apps with permission.READ_EXTERNAL_STORAGE) or write (permission.WRITE_EXTERNAL_STORAGE) or default (for processes running in root/global mount namespace i.e. outside zygote)
* There were minor differences on previous Android versions but the concept of emulation was same ever since implemented.
* For a little bit more details on Android's mount namespace implementation, see this answer.

In short, /sdcard and /storage/emulated/0 - which represent a FAT/vFAT/FAT32 filesystem - point towards /data/media/0 (or /mnt/expand/[UUID]/media/0 in case of Adoptable Storage) through FUSE or sdcardfs emulation.

Being not Android specific but generally Linux related, symlink and bind mount (see "Creating a bind mount") are out of the scope of this question, as the question is about emulation part mainly.

EMULATION:

Why the emulation is here? Emulated filesystem is an abstraction layer on actual filesystem (ext4 or f2fs) that serves basically two purposes:

  • Retain USB connectivity of Android devices to PCs (implemented through MTP now a days)
  • Restrict unauthorized access of apps/processes to user's private media and other apps' data on SD card.

Read Android's Storage Journey for details, the summary is:

Early Android devices were short on internal storage and relied on (physically) external SD cards that traditionally use FAT family of filesystem to ensure compatibility with most of the PCs (refer to Microsoft's dominance on PC world).
When the internal storage grew in size, same filesystem was shifted to internal (still called "external") SD card.
But the FAT/vFAT implementation had two major issues which were addressed by Google gradually:

  • Android devices were connected to PCs directly (USB Mass Storage) just as we connect a USB drive these days. UMS exposes the device at block level and disconnects the SD card from Android framework (un-mounts), thus making whole data unavailable to apps and possibly breaking many functionalities.
  • FAT (being Windows' favorite in development days) was never designed to enforce UNIX permissions (mode, uid, gid and likewise symlinks, and ioctls like FS_IOC_FIEMAP). So, all data on SD card was available to all apps (since every Android app is a UNIX/Linux user and has a uid) with no restrictions, hence raising serious privacy and security concerns.

Both of these issues were addressed through emulation:

  • Actual SD card storage was moved to /data partition (or independent /sdcard partition on some devices previously) which holds ext4 filesystem (gradually being replaced by f2fs), fully implementing UNIX permissions.
  • This design made using UMS impossible because whole /data partition could not be exposed to PC for 2 more reasons: (1) it contains a lot of settings and apps' data which is to be protected from other apps as well as human users. (2) Linux filesystems are not supported by Windows.
    So UMS was replaced with Media Transfer Protocol which is a client-server type extension to PTP - an already established protocol. MTP doesn't expose block device but works through software stack. MTP host runs on Android as an app (android.process.media) fully sandboxed in Android framework, not capable of doing any escalated tasks.

Now the apps (and MTP, which is also an app) interact with emulated storage instead of /data/media, achieving both purposes at the same time i.e. enforcing permission checks underneath and looking like FAT filesystem on upper surface.

Google is now implementing emulation through sdcardfs to overcome shortcomings of FUSE; one major being the input/output overhead i.e. to improve read/write speeds.

EXTERNAL STORAGE PERMISSIONS:
Concept of Public and Private files on external storage can be demonstrated using an example:
Install Termux app.
Create directories /sdcard/Android/data/com.termux/test_dir and /sdcard/test_dir.
Create files /sdcard/Android/data/com.termux/test_file and /sdcard/test_file.
Execute following commands:

without_storage_perm * You should have WhatsApp installed or select some other app's private folder.

Now Force Stop the Termux app and grant Storage permission. Execute the commands again:

with_storage_perm

See the difference in permissions of same files and directories. This seems not to be simply possible without emulation on a native Linux filesystem when there are hundreds of apps (users) to be dealt with simultaneously. This is the filesystem emulation that lets the same file to be exposed with three different sets of permissions at same time independent of it's original permissions on actual filesystem:

# touch /data/media/0/test_file

# stat -c '%a %u %g %n' /data/media/0/test_file
644 1023 1023 /data/media/0/test_file

# stat -c '%a %u %g %n' /mnt/runtime/*/emulated/0/test_file
660 0 1015 /mnt/runtime/default/emulated/0/test_file
640 0 9997 /mnt/runtime/read/emulated/0/test_file
660 0 9997 /mnt/runtime/write/emulated/0/test_file

Also see What is the “u#_everybody” UID?

Related:

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    +1. I think I misunderstand the part about MTP. Does MTP require a FAT filesystem on the target device to work with? If not, couldn't Google use ext4 filesystem for FUSE implementation as that could also enforce permissions checks needed for an app to access only their data in shared storage?
    – Firelord ♦
    Dec 22, 2018 at 6:10
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    @Firelord when discussing emulation, focus isn't on MTP implementation. Google already made changes to MTP protocol to meet certain Android needs and possibly they could implement it through some native Linux filesystem. But the point is we need a FAT-like permission-less filesystem that used to be in the early days of Android to ensure backward compatibility and that meets the design of Android's External Storage concept. I've made an edit to elaborate my point.
    – Irfan Latif
    Dec 22, 2018 at 10:41
  • I didn't see in your sources where you get the symbolic link, emulated mount, and bind mount correspondences from. You should also add that the "emulated" mount is actually a special filesystem mount that behaves like a bind mount.
    – Melab
    Aug 6, 2021 at 16:58
  • You should rephrase Actual SD card storage was moved to /data partition (or independent /sdcard partition on some devices previously). It implies that there was no /data directory before the move and implies that the external storage in devices got moved to internal storage, which is not possible.
    – Melab
    Aug 6, 2021 at 17:01
  • The phrasing of internal (still called "external") SD card implies the existence of a non-removable SD card in devices.
    – Melab
    Aug 6, 2021 at 17:03

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