Shred (Unix)
Encyclopedia
shred is a Unix
command that can be used to securely delete
files and devices so that they can be recovered only with great difficulty with specialised hardware, if at all. It is a part of GNU Core Utilities
.
and data remanence
have additional information.
By default, shred also shreds file slack (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.
on flash drive
s, caching such as NFS
caching, and journaling
. Some of these limitations can be overcome by shredding the entire device instead of specific files.
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
command that can be used to securely delete
Data remanence
Data remanence is the residual representation of data that remains even after attempts have been made to remove or erase the data. This residue may result from data being left intact by a nominal file deletion operation, by reformatting of storage media that does not remove data previously written...
files and devices so that they can be recovered only with great difficulty with specialised hardware, if at all. It is a part of GNU Core Utilities
GNU Core Utilities
The GNU Core Utilities or coreutils is a package of GNU software containing many of the basic tools, such as cat, ls, and rm, needed for Unix-like operating systems...
.
Background
For efficiency, erasing a file usually only erases the file system entry and keeps the content of the file intact. This frequently allows the file to be recovered using commonly available software. Even if the file is overwritten, residual magnetic fields may allow data recovery using specialist hardware equipment. To prevent this, shred overwrites the file multiple times using patterns chosen to maximize destruction of the residual data. The articles data erasureData erasure
Data erasure is a software-based method of overwriting data that completely destroys all electronic data residing on a hard disk drive or other digital media. Permanent data erasure goes beyond basic file deletion commands, which only remove direct pointers to data disk sectors and make data...
and data remanence
Data remanence
Data remanence is the residual representation of data that remains even after attempts have been made to remove or erase the data. This residue may result from data being left intact by a nominal file deletion operation, by reformatting of storage media that does not remove data previously written...
have additional information.
Features
shred can be invoked either on ordinary files or on devices (such as hard disk partitions), which are represented as files in Unix. By default, shred overwrites the file 25 times with multiple patterns, but the number is user configurable. Shred has an option to do an additional final overwrite with zeroes, which may help to hide the fact that shred was used.By default, shred also shreds file slack (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 shred 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 levelingWear leveling
Wear leveling is a technique for prolonging the service life of some kinds of erasable computer storage media, such as Flash memory used in solid-state drives and USB Flash drives...
on flash drive
Solid-state drive
A solid-state drive , sometimes called a solid-state disk or electronic disk, is a data storage device that uses solid-state memory to store persistent data with the intention of providing access in the same manner of a traditional block i/o hard disk drive...
s, caching such as NFS
Network File System
Network File System is a network file system protocol originally developed by Sun Microsystems in 1984, allowing a user on a client computer to access files over a network in a manner similar to how local storage is accessed. NFS, like many other protocols, builds on the Open Network Computing...
caching, and journaling
Journaling file system
A journaling file system is a file system that keeps track of the changes that will be made in a journal before committing them to the main file system...
. Some of these limitations can be overcome by shredding the entire device instead of specific files.