shred Man page

SHRED(1) User Commands SHRED(1)


shred – overwrite a file to hide its contents, and optionally delete it


shred [OPTION]… FILE…


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

If FILE is -, shred standard output.

Mandatory arguments to long options are mandatory for short options

-f, –force
change permissions to allow writing if necessary

-n, –iterations=N
overwrite N times instead of the default (3)

get random bytes from FILE

-s, –size=N
shred this many bytes (suffixes like K, M, G accepted)

-u truncate and remove file after overwriting

–remove[=HOW] like -u but give control on HOW to delete; See below

-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

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. The optional
HOW parameter indicates how to remove a directory entry: ‘unlink’ =>
use a standard unlink call. ‘wipe’ => also first obfuscate bytes in
the name. ‘wipesync’ => also sync each obfuscated byte to disk. The
default mode is ‘wipesync’, but note it can be expensive.

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 sys‐
tem 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

* file systems that cache in temporary locations, such as NFS version 3

* 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.


Written by Colin Plumb.


GNU coreutils online help:
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Copyright © 2016 Free Software Foundation, Inc. License GPLv3+: GNU
GPL version 3 or later .
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.


Full documentation at:
or available locally via: info ‘(coreutils) shred invocation’

GNU coreutils 8.25 February 2016 SHRED(1)