shred
overwrite a file to hide its contents, and optionally delete it
Synopsis
shred
[OPTION]... FILE...
add an example, a script, a trick and tips
examples
source
Why is GNU shred faster than dd when filling a drive with random data?
Shred uses an internal pseudorandom generator
By default these commands use an internal pseudorandom
generator initialized by a small amount of entropy, but can be
directed to use an external source with the
--random-source=file option. An error is reported if file does
not contain enough bytes.
For example, the device file /dev/urandom
could be used as the source of random data. Typically, this
device gathers environmental noise from device drivers and
other sources into an entropy pool, and uses the pool to
generate random bits. If the pool is short of data, the device
reuses the internal pool to produce more bits, using a
cryptographically secure pseudorandom number generator. But be
aware that this device is not designed for bulk random data
generation and is relatively slow.
I'm not persuaded that random data is any more effective than
a single pass of zeroes (or any other byte value) at obscuring
prior contents.
To securely decommission a drive, I use a big magnet and a large
hammer.
source
Why is GNU shred faster than dd when filling a drive with random data?
Shred
uses an internal pseudorandom generator
By default these commands use an internal pseudorandom
generator initialized by a small amount of entropy, but can be
directed to use an external source with the
--random-source=file option. An error is reported if file does
not contain enough bytes.
For example, the device file /dev/urandom
could be used as the source of random data. Typically, this
device gathers environmental noise from device drivers and
other sources into an entropy pool, and uses the pool to
generate random bits. If the pool is short of data, the device
reuses the internal pool to produce more bits, using a
cryptographically secure pseudorandom number generator. But be
aware that this device is not designed for bulk random data
generation and is relatively slow.
I'm not persuaded that random data is any more effective than
a
single pass of
zeroes (or any other byte value) at obscuring prior contents.
To securely decommission a drive, I use a big magnet and a large
hammer.
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 (3)
--random-source=FILE
get random bytes from FILE
-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;
this is the
default for non-regular files
-z,
--zero
add a final overwrite with
zeros to hide shredding
--help
display this help and exit
--version
output version information and
exit
If FILE is
-, shred standard output.
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
file system overwrites data in place. This is the
traditional way to do things, but many modern file system
designs do not satisfy this assumption. The following are
examples of file systems on which shred is not effective, or
is not guaranteed to be effective in all file system
modes:
*
log-structured or journaled file systems, such as
those supplied with AIX and Solaris (and JFS, ReiserFS, XFS,
Ext3, etc.)
* file systems
that write redundant data and carry on even if some writes
fail, such as RAID-based file systems
* file systems
that make snapshots, such as Network Appliance’s NFS
server
* file systems
that cache in temporary locations, such as NFS version 3
clients
* compressed
file systems
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).
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.
copyright
Copyright © 2012 Free Software Foundation, Inc. License GPLv3+:
GNU GPL version 3 or later
<http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute
it. There is NO WARRANTY, to the extent permitted by law.
reporting bugs
Report shred bugs to bug-coreutils[:at:]gnu[:dot:]org
GNU coreutils home page:
<http://www.gnu.org/software/coreutils/>
General help using GNU software:
<http://www.gnu.org/gethelp/>
Report shred translation bugs to
<http://translationproject.org/team/>
see also
The full
documentation for shred is maintained as a Texinfo
manual. If the info and shred programs are
properly installed at your site, the command
info
coreutils 'shred invocation'
should give you
access to the complete manual.
author
Written by
Colin Plumb.