is a command on

Unix-like A Unix-like (sometimes referred to as UN*X or *nix) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Unix-li ...

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s that can be used to securely delete files and devices so that it is extremely difficult to recover them, even with specialized hardware and technology; assuming it's even possible to recover the file at all. It is a part of

GNU Core Utilities The GNU Core Utilities or coreutils is a package of GNU software containing implementations for many of the basic tools, such as cat, ls, and rm, which are used on Unix-like operating systems. In September 2002, the ''GNU coreutils'' were cr ...

. Being based on the

Gutmann method The Gutmann method is an algorithm for securely erasing the contents of computer hard disk drives, such as files. Devised by Peter Gutmann and Colin Plumb and presented in the paper ''Secure Deletion of Data from Magnetic and Solid-State Memory'' ...

paper, it suffers from the same criticisms and possible shortcomings.

Background

For efficiency, the process of erasing a file from storage using the command usually only erases the file's file-system entry while keeping the content of the file intact. This frequently allows commonly available software to recover the "erased" file's data. If the file data is stored on magnetic media such as a HDD, even if the file is overwritten, residual magnetic fields may allow data recovery using specialist hardware equipment (this claim is disputed; see ). To prevent this, overwrites the contents of a file multiple times, using patterns chosen to maximize destruction of the residual data.

Features

can be invoked either on ordinary files or on devices (such as hard disk partitions), which are represented as files in Unix. By default, the command overwrites the file three times with multiple patterns, but the number is user configurable. It has an option to do an additional final overwrite with zeroes, which may help to hide the fact that it was used. By default, shred also shreds

file slack In computer storage, fragmentation is a phenomenon in which storage space, main storage or secondary storage, is used inefficiently, reducing capacity or performance and often both. The exact consequences of fragmentation depend on the specific ...

(unused space in file allocations). For example, a 5 KB file on a file system with 4 KB clusters actually requires 8 KB of storage. Shred has an option to overwrite only the file itself, as well as an option to delete the file after it has completed operation.

Limitations

A limitation of when invoked on ordinary files is that it only overwrites the data in place without overwriting other copies of the file. Copies can manifest themselves in a variety of ways, such as through manual and automatic backups, file system snapshots, copy-on-write filesystems,

wear leveling Wear leveling (also written as wear levelling) is a technique Wear leveling techniques for flash memory systems. for prolonging the service life of some kinds of erasable computer storage media, such as flash memory, which is used in solid-state d ...

flash drive A flash drive is a portable computer drive that uses flash memory. Flash drives are the larger memory modules consisting of a number of flash chips. A flash chip is used to read the contents of a single cell, but it can write entire block of cell ...

s, caching such as NFS caching, and journaling. All limitations imposed by the file system can be overcome by shredding every device on which the data resides instead of specific files. However, since wear leveled devices do not guarantee a fixed relationship between logical blocks addressable through the interface and the physical locations in which the data is stored, shredding may not provide adequate security. If available, the SATA secure erase command, issued through

hdparm hdparm is a command line program for Linux to set and view ATA hard disk drive hardware parameters and test performance. It can set parameters such as drive caches, sleep mode, power management, acoustic management, and DMA settings. GParted ...

or a similar utility, may be helpful in this situation. Even for magnetic devices, SATA secure erase will be faster and more reliable than shredding. Physical destruction may be necessary to securely erase devices such as

memory card A memory card is an electronic data storage device used for storing digital information, typically using flash memory. These are commonly used in digital portable electronic devices. They allow adding memory to such devices using a card in a soc ...

s and unusable hard disks. Gutmann method, which is based on, is known to fail on

USB stick A USB flash drive (also called a thumb drive) is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than . Since first ...

Alternatives

The command overwrite sequence of data being compliant with a Dec. 2005 draft of National Nuclear Security Administration Policy Letter NAP-14.x. Written by Jim Garlick of

LLNL Lawrence Livermore National Laboratory (LLNL) is a federal research facility in Livermore, California, United States. The lab was originally established as the University of California Radiation Laboratory, Livermore Branch in 1952 in response ...

, it offers a selection of overwrite sequences, though all ultimately suffer from the same limitations outlined above.

References

shred invocation
GNU Coreutils. Accessed February 3, 2009.
Shred and secure-delete
Free Software Magazine. November 29, 2008. Accessed February 3, 2009.

, About.com. Accessed April 17, 2012.
How To Completely Erase a Hard Drive
, About.com. Accessed April 17, 2012.

Notes

External links

* {{Data erasure Unix file system-related software Unix security-related software Free system software GNU Project software Data erasure software