How do I recursively shred an entire directory tree?

Question

I have a directory tree that I would like to shred with the Linux 'shred' utility. Unfortunately, shred has no -R option for recursive shredding.

How can I shred an entire directory tree recursively?

Answer 1

Use the find command to execute shred -uvz recursively:

find <dir> -type f -exec shred -uvz {} \;

-u ensures that after the shred operation is completed, the file is unallocated and removed.

-v enables verbose output for tracking the shred progress

-z performs a final zero-ization of the file to hide that the allocation on disk was shredded.

It should also be noted that shred assumes that the file system overwrites data in-place on disk, which excludes journaled (ext3/4, XFS, etc) and copy-on-write filesystems (btrfs, zfs, etc). This means that shred may not work on a per-file basis for your filesystem.

Answer 2

Beware of shred!

From the shred-manpage:

CAUTION: Note that shred relies on a very important assumption: that the file system overwrites data in place. This is the traditional way to do things, but many modern file system designs do not satisfy this assumption. The following are examples of file systems on which shred is not effective, or is not guaranteed to be effective in all file system modes:

• log-structured or journaled file systems, such as those supplied with AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)

• file systems that write redundant data and carry on even if some writes fail, such as RAID-based file systems

• file systems that make snapshots, such as Network Appliance's NFS server

• file systems that cache in temporary locations, such as NFS version 3 clients

• compressed file systems

In the case of ext3 file systems, the above disclaimer applies (and shred is thus of limited effectiveness) only in data=journal mode, which journals file data in addition to just metadata. In both the data=ordered (default) and data=writeback modes, shred works as usual. Ext3 journaling modes can be changed by adding the data=something option to the mount options for a particular file system in the /etc/fstab file, as documented in the mount man page (man mount).

In addition, file system backups and remote mirrors may contain copies of the file that cannot be removed, and that will allow a shredded file to be recovered later.

Also, SSDs might thwart your attempts of overwriting data.

Solution: Use an encrypted filesystem, and just delete your files.

Answer 3

Use secure delete instead.

sudo apt-get install secure-delete
srm -r pathname

Done. Secure delete is a lot more paranoid than shred, using 38 passes instead of 3. To do a fast single pass, use

srm -rfll pathname

fll gets you a less random data generator, and only a single pass.

Answer 4

Combining this answer with the best known options for shred using this stack overflow link 'Deleting Files Permanently and Securely on CentOS':

find <directory> -depth -type f -exec shred -v -n 1 -z -u {} \;

Edit: Be aware that best answer for shredding a single file forces a sync which writes changes to the media before deleting the file because some or all journaled filesystems have a buffer.

If possible, the find command should call a shell script on the file which runs:

shred -v -n 1 /path/to/your/file #overwriting with random data
sync #forcing a sync of the buffers to the disk
shred -v -n 0 -z -u /path/to/your/file #overwriting with zeroes and remove the file

on each file.

Answer 5

find /your/directory -exec shred {} \;

Answer 6

find [dirname] -depth -type f -exec shred -n1 {} \;

This performs a depth-first search for files in directory [dirname], then runs the shred -n1 command on each file. When removing files and/or directories, adding -depth as default is a good habit, even though it's not strictly needed for this case. When running this sort of command with rm -rf instead of shred, -depth is needed to ensure that directories are not deleted before the contents of the directories are attempted to be deleted (thus causing errors).

Answer 7

Note that shred is an utility from GNU coreutils, it has nothing to do with Linux, other than Linux being just a kernel, operating systems using that kernel need to pick other software from elsewhere, and the GNU project has historically been the primary source for most. For basic utilities, busybox and toybox are other options and both happen to also include a shred utility with an interface designed after the GNU implementation though with a reduced feature set.

Now, several things to take into consideration:

Files on Unix-like systems can be of several different types: regular, fifo, directory, symlink, device, socket... All except socket can be opened, most can be read and written, but not all of them you'd want shred to shred when going through the list. In particular,

• opening a symlink opens the target of a symlink, so letting shred shred arbitrary symlinks could lead you to shred any file on the filesystem.
• Shredding /dev/sda would shred the whole contents of a disk
• a fifo is an inter-process communication tool, writing random data there would not make sense. The GNU implementation of shred rejects files of type fifo.

You likely want to only shred files of type:

• regular
• directory (if the file names are also sensitives).

But for the latter, while GNU shred has a --remove=wipe for that, it can only do it for one directory entry, so couldn't do it for the other entries (including the ones for files of different types, or the entries that have been removed without being wiped before).

It's also not possible to selectively shred files of a given type in a race-free way. For instance, you could determine a file is a regular file, but that file be replaced with a symlink by the time shred is invoked to wipe it.

Also note that if some of the files are being actively written, shred will fail to wipe the parts that have been written after it was invoked.

So, the best you can do with GNU shred is:

find dir -depth ! -type f -delete -o -exec shred -f --remove=wipe {} ';'

So besides all the limitations with shredding already mentioned by others, the limitations with this recursive shredding approach are also:

• it will not fully shred directory contents (the name of the files within).
• it may not full shred files that are still being written (even if they are deleted)
• it could end up shredding files of the wrong type. You'd want not to do that on directories writable by others as that could introduce security vulnerabilities.

Answer 8

The most thorough shred method I've found, which includes directory removal too, is to have find call a script to have shred:

• overwrite the file
• sync
• then delete
• and finally call rm to remove the directory names.

This method also properly handles filenames with spaces in them.

First - the shred script (I've named mine dirShredder.sh and stored it in the /root directory:

shred -v -n 1 "$1" #overwriting with random data
sync #forcing a sync of the buffers to the disk
shred -v -n 0 -z -u "$1" #overwriting with zeroes and remove the file
rm -rvf "$1" # call rm to remove the directories

Then, call the script like this:

find /volume1/pathToShred/ -mindepth 1 -depth -exec /root/dirShredder.sh "{}" \;

Make sure to mark the killit.sh file executable (chmod +x) and of course update the path for the dir you want to shred and to dirShredder.sh if you store it somewhere else.

NOTA BENE - shred has issues on Copy-on-Write filesystems (ZFS, BTRFS, et al) and even on Journaling file systems. There is no real accepted "best" way to deal with this that I've found other than "encrypted filesystems" but I'm not sure how effective this is after-the-fact.
The closest it seems you can get is to overwrite all empty space on the drive with random data after your shredding ops (not zeros, seems this is not always reliable.) Also, SSDs may have other considerations too (like TRIM.)

I'm not going into those here, there are other Stack answers (@user unknown's answer in this question for example) and plenty of discussions throughout the 'net that cover these topics so search them out if you need that level of security.

Answer 9

Use

shred yourDirectory/*

Go check your file, they're unreadable :)

rm -r yourDirectory