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When I open up a terminal emulator and run the mount command, it indicates that my Android device's root filesystem (/) is of filesystem type rootfs. What is rootfs? Can I write to it? Will my changes persist even after I shut the device down?
Note: If you're an Android ROM developer who wants to add files to your ROM's rootfs, please see the relevant Unix.SE question instead.
rootfs?It's an initramfs. Basically, it's a prepopulated RAM drive. It's prepopulated with some content at boot time, usually from a cpio archive which has been compiled into the kernel.
One way to make changes to your rootfs is to unpack the cpio archive, make your changes, and repack the archive. Your device will almost surely have to be rooted before you can do this. As well, you'll need the necessary tools.
Alternatively, you can make changes to the rootfs copy which is loaded into RAM; but, if you do so, there are four gotchas.
You must root your device first.
Then, you must remount the filesystem as read-write.
Don't write too much to the rootfs; if you do, you'll take up too much RAM and your device will slow down.
The biggest gotcha is that the files you create will disappear when you shut down or reboot. Please keep on reading.
No. If you create or modify files in rootfs, your changes will be undone when you shut down or reboot.
Store your files somewhere under /storage or /data or /system instead. You can choose any of the three. Somewhere under /storage is probably best, since that's the most common choice.
/?I don't know.
I suspect that the answer is as follows: Android isn't a full Linux distribution. It doesn't include a full glibc, it's probably not fully LSB-compliant, and it doesn't feel obligated to provide the niceties that most Linux distributions provide (e.g. a disk-backed root filesystem).
Elsewhere, Gilles writes:
Most desktop Linux systems have a small initramfs which contains just enough programs and configuration files to mount the real root filesystem, which is then mounted on
/, replacing the initramfs. Android, like some embedded Linux systems, keeps the initramfs mounted forever. Android's initramfs contains only/init,adbdand a few configuration files.
References:
boot.imgfor ROM porting?: answers there explain how to fetch and edit theboot.imgfile, allowing to persistently change device's root directory content.