shred

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.

Options

--force, -f

change permissions to allow writing if necessary

--iterations=<NUMBER>, -n <NUMBER>

overwrite N times instead of the default (3)

--size=<N>, -s <N>

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

-u

deallocate and remove file after overwriting

--remove=<HOW>

like -u but give control on HOW to delete; See below

--verbose, -v

show progress

--exact, -x

do not round file sizes up to the next full block;
this is the default for non-regular files

--zero, -z

add a final overwrite with zeros to hide shredding

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.

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 journal 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 journal 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.

Examples

Overwrite a file:

shred {{path/to/file}}

Overwrite a file and show progress on the screen:

shred --verbose {{path/to/file}}

Overwrite a file, leaving [z]eros instead of random data:

shred --zero {{path/to/file}}

Overwrite a file a specific [n]umber of times:

shred --iterations {{25}} {{path/to/file}}

Overwrite a file and remove it:

shred --remove {{path/to/file}}

Overwrite a file 100 times, add a final overwrite with [z]eros, remove the file after overwriting it and show [v]erbose progress on the screen:

shred -vzun 100 {{path/to/file}}

The examples are provided by the tldr-pages project under the CC BY 4.0 License.

Please note that, as uutils is a work in progress, some examples might fail.