Home » What happens when you ‘mount over’ an existing folder with contents?

What happens when you ‘mount over’ an existing folder with contents?


What happens to the actual content of /tmp when my hard drive is mounted?

Pretty much nothing. They’re just hidden from view, not reachable via normal filesystem traversal.

Is it possible to perform r/w operations on the actual content of /tmp while the hard drive is mounted?

Yes. Processes that had open file handles inside your “original” /tmp will continue to be able to use them. You can also make the “reappear” somewhere else by bind-mounting / elsewhere.

# mount -o bind / /somewhere/else
# ls /somewhere/else/tmp  

Here’s a little experiment you can run to get a better (I hope) feel for what is happening.

Note: This is not an attempt at being perfectly correct or an exhaustive description of what is really happening. Should be just accurate enough to give you the big picture though.

I created a user called me on my machine, and a random directory in his home, with a file in it:

me@home $ pwd
me@home $ echo hello > some_file
me@home $ ls  
me@home $ cat some_file 

At this point, nothing unusual – it’s just a plain directory with a plain file. I leave that session open right as it is, with its cwd inside that test directory.

As root, I create a small filesystem and mount it over /home/me/tmp.

root@home # dd if=/dev/zero of=./fs bs=1M count=10
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.00467318 s, 2.2 GB/s

root@home # mkfs -t ext2 ./fs 
mke2fs 1.42.12 (29-Aug-2014)
[... snip ...]
Writing superblocks and filesystem accounting information: done

root@home # mount ./fs /home/me/tmp

I then open a new terminal as me, and look around:

me@home #2 $ cd tmp
me@home #2 $ ls
me@home #2 $ cat some_file
cat: some_file: No such file or directory
me@home #2 $ echo bye bye > some_file
-su: some_file: Permission denied

So, that file we created is clearly not there. The lost+found directory is indicative of the root of an ext filesystem. And I lost write permission, so it’s clearly not the original directory.

Back to the first me session, let’s look at how it sees the world:

me@home $ echo something else > other_file

No problem writing.

me@home $ cat some_file other_file 
something else

Original file is still there, new file created without issue.

Huh? What’s going on?

The first session entered the directory before it was overlayed by root mounting another filesystem on it. That mount action doesn’t affect the original filesystem at all. The shell process has a perfectly valid handle to the directory in the original filesystem, and can continue interacting with it. It’s sort of running around beneath the carpet mount point.

The second session entered the directory after the mount was laid down. So it sees the new, empty filesystem. And the sysadmin borked the permissions, so it can’t use the requested space… lets fix that.

root@home # chown me:users /home/me/tmp
me@home #2 $ echo bye bye > some_file
me@home #2 $ ls 
lost+found  some_file
me@home #2 $ cat some_file 
bye bye

Can session 1 escape from under the rug? (It’s getting musty.)

Sure! If session 1 moves back up the filesystem tree out of the mount, it will lose that handle to the inside and will follow the mount like everyone else.

me@home $ cd
me@home $ pwd
me@home $ cd tmp
me@home $ cat some_file other_file
bye bye
cat: other_file: No such file or directory

Same view as session #2, we’re back to normal.

But how do you know the files didn’t disappear? No one’s looking anymore!

That’s one of the moments where bind mounts become handy. They let you mount an already mounted filesystem somewhere else.

me@home $ mkdir ~/bind
root@home # mount -o bind /home/me /home/me/bind

(Yes, you can bind-mount a filesystem “inside itself”. Cool trick, eh?)

me@home $ ls bind/tmp
other_file  some_file
me@home $ cat bind/tmp/*
something else

So they are indeed there, ready for action. It’s simply that they’re not visible/reachable at their original location, the mount hides them from normal directory traversals.

I encourage you to play around with this, it is really not complicated once you’ve understood the “trick” that is being played. And once you’ve Got It™, look into union filesystems for even more carpet pulling 🙂

One note though: mounting over /tmp or /var (or any of the core OS directories) really isn’t a good idea once the boot process is finished. A lot of applications leave state in those directories, and might get seriously confused if you play mount games around them.

Related Solutions

Joining bash arguments into single string with spaces

[*] I believe that this does what you want. It will put all the arguments in one string, separated by spaces, with single quotes around all: str="'$*'" $* produces all the scripts arguments separated by the first character of $IFS which, by default, is a space....

AddTransient, AddScoped and AddSingleton Services Differences

TL;DR Transient objects are always different; a new instance is provided to every controller and every service. Scoped objects are the same within a request, but different across different requests. Singleton objects are the same for every object and every...

How to download package not install it with apt-get command?

Use --download-only: sudo apt-get install --download-only pppoe This will download pppoe and any dependencies you need, and place them in /var/cache/apt/archives. That way a subsequent apt-get install pppoe will be able to complete without any extra downloads....

What defines the maximum size for a command single argument?

Answers Definitely not a bug. The parameter which defines the maximum size for one argument is MAX_ARG_STRLEN. There is no documentation for this parameter other than the comments in binfmts.h: /* * These are the maximum length and maximum number of strings...

Bulk rename, change prefix

I'd say the simplest it to just use the rename command which is common on many Linux distributions. There are two common versions of this command so check its man page to find which one you have: ## rename from Perl (common in Debian systems -- Ubuntu, Mint,...

Output from ls has newlines but displays on a single line. Why?

When you pipe the output, ls acts differently. This fact is hidden away in the info documentation: If standard output is a terminal, the output is in columns (sorted vertically) and control characters are output as question marks; otherwise, the output is...

mv: Move file only if destination does not exist

mv -vn file1 file2. This command will do what you want. You can skip -v if you want. -v makes it verbose - mv will tell you that it moved file if it moves it(useful, since there is possibility that file will not be moved) -n moves only if file2 does not exist....

Is it possible to store and query JSON in SQLite?

SQLite 3.9 introduced a new extension (JSON1) that allows you to easily work with JSON data . Also, it introduced support for indexes on expressions, which (in my understanding) should allow you to define indexes on your JSON data as well. PostgreSQL has some...

Combining tail && journalctl

You could use: journalctl -u service-name -f -f, --follow Show only the most recent journal entries, and continuously print new entries as they are appended to the journal. Here I've added "service-name" to distinguish this answer from others; you substitute...

how can shellshock be exploited over SSH?

One example where this can be exploited is on servers with an authorized_keys forced command. When adding an entry to ~/.ssh/authorized_keys, you can prefix the line with command="foo" to force foo to be run any time that ssh public key is used. With this...

Why doesn’t the tilde (~) expand inside double quotes?

The reason, because inside double quotes, tilde ~ has no special meaning, it's treated as literal. POSIX defines Double-Quotes as: Enclosing characters in double-quotes ( "" ) shall preserve the literal value of all characters within the double-quotes, with the...

What is GNU Info for?

GNU Info was designed to offer documentation that was comprehensive, hyperlinked, and possible to output to multiple formats. Man pages were available, and they were great at providing printed output. However, they were designed such that each man page had a...

Set systemd service to execute after fstab mount

a CIFS network location is mounted via /etc/fstab to /mnt/ on boot-up. No, it is not. Get this right, and the rest falls into place naturally. The mount is handled by a (generated) systemd mount unit that will be named something like mnt-wibble.mount. You can...

Merge two video clips into one, placing them next to each other

To be honest, using the accepted answer resulted in a lot of dropped frames for me. However, using the hstack filter_complex produced perfectly fluid output: ffmpeg -i left.mp4 -i right.mp4 -filter_complex hstack output.mp4 ffmpeg -i input1.mp4 -i input2.mp4...

How portable are /dev/stdin, /dev/stdout and /dev/stderr?

It's been available on Linux back into its prehistory. It is not POSIX, although many actual shells (including AT&T ksh and bash) will simulate it if it's not present in the OS; note that this simulation only works at the shell level (i.e. redirection or...

How can I increase the number of inodes in an ext4 filesystem?

It seems that you have a lot more files than normal expectation. I don't know whether there is a solution to change the inode table size dynamically. I'm afraid that you need to back-up your data, and create new filesystem, and restore your data. To create new...

Why doesn’t cp have a progress bar like wget?

The tradition in unix tools is to display messages only if something goes wrong. I think this is both for design and practical reasons. The design is intended to make it obvious when something goes wrong: you get an error message, and it's not drowned in...