Shred tip: Securely remove multiple files so no one can recover file again
Shred utility overwrites a file to hide its contents, and optionally delete it if needed. The idea is pretty simple as it overwrites the specified FILE(s) repeatedly, in order to make it harder for even very expensive hardware probing to recover the data. By default file is overwritten 25 times. I've seen cases where law enforcement agencies had successfully recovered data from 5 year old *not so* working hard disk as evidence. Also when you move your rented server you should consider running file shredding; otherwise new owner can get data including passwords.
Shred a single file
Securely delete a file called /home/vivek/login.txt:
$ shred -u ~/login.txt
You can add a final overwrite with zeros to hide shredding:
$ shred -u -x ~/login.txt
Where,
- -u : Remove file after overwriting
- -x : Add a zero to hide shredding
- -n NUM : Overwrite NUM times instead of the default 25
Shred a multiple files
Let us say you have 100 subdirectories and just wanted to get rid of all files:
$ find -t f . -exec shred -u '{}' \;
If you have 1000s of files consider a running job in background using nohup - (execute commands after you exit from a shell prompt over ssh session):
$ nohup find -t f /var/www/ -exec shred -n30 -u '{}' \; &
Shred drawbacks
- Shred doesn’t go well with log-structured or journaled file systems, such as JFS, ReiserFS, XFS, Ext3, etc.
- Compressed file systems
- RAID-based file systems
- NETApps (Network Appliance’s) NFS server
So how do I wipe on journaling file systems?
There is no simple solution. I’ve tried different techniques.
You can store sensitive data on ext2 or fat32 file system and easily delete files. According to shred man page:
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).
Someone suggested to use disk encryption to store data that needs to be wiped.
Run shred on entire partition:
# shred -n 30 -vz /dev/hdb2
On remote computer, use nohup:
# nohup shred -n 30 -vz /dev/sdb1 &
Output:
shred: /dev/sdb1: pass 1/26 (random)... shred: /dev/sdb1: pass 1/26 (random)...1013MiB/234GiB 0% shred: /dev/sdb1: pass 1/26 (random)...1014MiB/234GiB 0% shred: /dev/sdb1: pass 1/26 (random)...1.9GiB/234GiB 0% shred: /dev/sdb1: pass 1/26 (random)...2.0GiB/234GiB 0% shred: /dev/sdb1: pass 1/26 (random)...3.0GiB/234GiB 1% shred: /dev/sdb1: pass 1/26 (random)...3.1GiB/234GiB 1% shred: /dev/sdb1: pass 1/26 (random)...4.0GiB/234GiB 1% shred: /dev/sdb1: pass 1/26 (random)...4.1GiB/234GiB 1% shred: /dev/sdb1: pass 1/26 (random)...5.0GiB/234GiB 2% shred: /dev/sdb1: pass 1/26 (random)...5.1GiB/234GiB 2% shred: /dev/sdb1: pass 1/26 (random)...6.1GiB/234GiB 2% ...... .. ...
And finally you can always destroy hard disk physically, perhaps through a hard drive in hot melting metal 
If you just need to securely wipes the hard disks use dban.
Do you use any other utility for file shredding or file wiping? Do you have a better solution for file wiping on journaling file systems? Please share your experience in the comments!
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You may also be interested in other helpful articles:
- Linux : How to delete file securely
- Linux or UNIX Recover deleted files - undelete files
- How to: Linux / UNIX Delete or Remove Files With Inode Number
- Linux / UNIX Software that make retrieving the data more difficult with Department of Defense ( DoD 5520.22-M ) compliant disk / file wiping
- Linux recover deleted files with lsof command - howto
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Tags: file shredding, file wiping, multiple files, nohup, overwrite, securely removing linux files, Security, shred, ssh session
I don’t have much stuff to shred - but I am sure this will come in handy someday.
Do you know how to recover deleted files on ext3 and other file systems? There are many times I when I have deleted a file by mistake. I will really appreciate a post on that topic.
Backup is the best defense but I will address your issue later on.