Section: scrub (1)
Updated: 2009-07-29
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scrub - write patterns on disk/file  


scrub [OPTIONS] special-file
scrub [OPTIONS] file
scrub -X [OPTIONS] directory  


Scrub iteratively writes patterns on files or disk devices to make retrieving the data more difficult. Scrub operates in one of three modes:

1) The special file corresponding to an entire disk is scrubbed and all data on it is destroyed. This mode is selected if file is a character or block special file. This is the most effective method.

2) A regular file is scrubbed and only the data in the file (and optionally its name in the directory entry) is destroyed. The file size is rounded up to fill out the last file system block. This mode is selected if file is a regular file. See CAVEATS below.

3) directory is created and filled with files until the file system is full, then the files are scrubbed as in 2). This mode is selected with the -X option. See CAVEATS below.

Scrub accepts the following options:

-v, --version
Print scrub version and exit.
-r, --remove
Remove the file after scrubbing.
-p, --pattern nnsa|dod|bsi|old|fastold|gutmann|random|random2
Select the patterns to write. nnsa selects patterns compliant with NNSA Policy Letter NAP-14.x; dod selects patterns compliant with DoD 5220.22-M; bsi selects patterns recommended by the German Center of Security in Information Technologies (; old selects pre-version 1.7 scrub patterns; and fastold is old without the random pass. gutmann is a 35-pass sequence described in Gutmann's paper cited below. See STANDARDS below for more detail. random is a single random pass. random2 is two random passes. Default: nnsa.
-b, --blocksize blocksize
Perform read(2) and write(2) calls using the specified blocksize (in bytes). K, M, or G may be appended to the number to change the units to KiBytes, MiBytes, or GiBytes, respectively. Default: 1M.
-f, --force
Scrub even if target contains signature indicating it has already been scrubbed.
-S, --no-signature
Do not write scrub signature. Scrub will not be able to ascertain if the disk has already been scrubbed.
-X, --freespace
Create specified directory and fill it with files until write returns ENOSPC (file system full), then scrub the files as usual. The size of each file can be set with -s, otherwise it will be the maximum file size creatable given the user's file size limit or 1g if umlimited.
-D, --dirent newname
After scrubbing the file, scrub its name in the directory entry, then rename it to the new name. The scrub patterns used on the directory entry are constrained by the operating system and thus are not compliant with cited standards.
-s, --device-size size
Override the device size (in bytes). Without this option, scrub determines media capacity using OS-specific ioctl(2) calls. K, M, or G may be appended to the number to change the units to KiBytes, MiBytes, or GiBytes, respectively.


Scrub may be insufficient to thwart heroic efforts to recover data in an appropriately equipped lab.

Scrub nnsa patterns are reasonable for sanitizing modern PRML/EPRML encoded disk devices.

The effectiveness of scrubbing regular files through a file system will be limited by the OS and file system. File systems that are known to be problematic are journaled, log structured, copy-on-write, versioned, and network file systems. If in doubt, scrub the raw disk device.

Scrubbing free blocks in a file system with the -X method is subject to the same caveats as scrubbing regular files, and in addition, is only useful to the extent the file system allows you to reallocate the target blocks as data blocks in a new file. If in doubt, scrub the raw disk device.

[MacOS X HFS file system] Scrub attempts to overwrite a file's resource fork if it exists. Although MacOS X will support additional named forks in the future, scrub is only aware of the traditional data and resource forks.  


The dod scrub sequence is compliant with the DoD 5220.22-M procedure for sanitizing removeable and non-removeable rigid disks which requires overwriting all addressable locations with a character, its complement, then a random character, and verify. Please refer to the DoD document for additional constraints.

The nnsa (default) scrub sequence is compliant with a Dec. 2005 draft of NNSA Policy Letter NAP-14.x (see reference below) for sanitizing removable and non-removable hard disks, which requires overwriting all locations with a pseudorandom pattern twice and then with a known pattern. Please refer to the NNSA document for additional constraints.

Please consult local authorities regarding your site policy for disk sanitization.  


Jim Garlick <[email protected]>

This work was produced at the University of California, Lawrence Livermore National Laboratory under Contract No. W-7405-ENG-48 with the DOE. Designated UCRL-CODE-2003-006, scrub is licensed under terms of the GNU General Public License.  


DoD 5220.22-M, "National Industrial Security Program Operating Manual", Chapter 8, 01/1995.

NNSA Policy Letter: NAP-14.x, "Clearing, Sanitizing, and Destroying Information System Storage Media, Memory Devices, and other Related Hardware", Unpublished Draft, 2005

"Secure Deletion of Data from Magnetic and Solid-State Memory", by Peter Gutmann, Sixth USENIX Security Symposium, San Jose, CA, July 22-25, 1996.

"Gutmann Method", wikiedia,

Darik's boot and Nuke FAQ:





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