Linux / Unix Command: shred

NAME

shred - delete a file securely, first overwriting it to hide its contents

SYNOPSIS

shred [OPTIONS] FILE [...]

DESCRIPTION

Overwrite the specified FILE(s) repeatedly, in order to make it harder for even very expensive hardware probing to recover the data.

Mandatory arguments to long options are mandatory for short options too.

-f, --force

change permissions to allow writing if necessary

-n, --iterations=N

Overwrite N times instead of the default (25)

-s, --size=N

shred this many bytes (suffixes like K, M, G accepted)

-u, --remove

truncate and remove file after overwriting

-v, --verbose

show progress

-x, --exact

do not round file sizes up to the next full block

-z, --zero

add a final overwrite with zeros to hide shredding

shred standard output

--help

display this help and exit

--version

output version information and exit

Delete FILE(s) if --remove (-u) is specified. The default is not to remove the files because it is common to operate on device files like /dev/hda, and those files usually should not be removed. When operating on regular files, most people use the --remove option.

CAUTION: Note that shred relies on a very important assumption: that the filesystem overwrites data in place. This is the traditional way to do things, but many modern filesystem designs do not satisfy this assumption. The following are examples of filesystems on which shred is not effective:

* log-structured or journaled filesystems, such as those supplied with

AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)

* filesystems that write redundant data and carry on even if some writes

fail, such as RAID-based filesystems

* filesystems that make snapshots, such as Network Appliance's NFS server

* filesystems that cache in temporary locations, such as NFS

version 3 clients

* compressed filesystems

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.