6.6 shred: Remove files more securely

shred overwrites devices or files, to help prevent even very expensive hardware from recovering the data.

Ordinarily when you remove a file (see section 6.5 rm: Remove files or directories), the data is not actually destroyed. Only the index listing where the file is stored is destroyed, and the storage is made available for reuse. There are undelete utilities that will attempt to reconstruct the index and can bring the file back if the parts were not reused.

On a busy system with a nearly-full drive, space can get reused in a few seconds. But there is no way to know for sure. If you have sensitive data, you may want to be sure that recovery is not possible by actually overwriting the file with non-sensitive data.

However, even after doing that, it is possible to take the disk back to a laboratory and use a lot of sensitive (and expensive) equipment to look for the faint "echoes" of the original data underneath the overwritten data. If the data has only been overwritten once, it's not even that hard.

The best way to remove something irretrievably is to destroy the media it's on with acid, melt it down, or the like. For cheap removable media like floppy disks, this is the preferred method. However, hard drives are expensive and hard to melt, so the shred utility tries to achieve a similar effect non-destructively.

This uses many overwrite passes, with the data patterns chosen to maximize the damage they do to the old data. While this will work on floppies, the patterns are designed for best effect on hard drives. For more details, see the source code and Peter Gutmann's paper Secure Deletion of Data from Magnetic and Solid-State Memory, from the proceedings of the Sixth USENIX Security Symposium (San Jose, California, 22--25 July, 1996). The paper is also available online http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html.

Please 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. Exceptions include:

• Log-structured or journaled filesystems, such as those supplied with AIX and Solaris.

• 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.

If you are not sure how your filesystem operates, then you should assume that it does not overwrite data in place, which means that shred cannot reliably operate on regular files in your filesystem.

Generally speaking, it is more reliable to shred a device than a file, since this bypasses the problem of filesystem design mentioned above. However, even shredding devices is not always completely reliable. For example, most disks map out bad sectors invisibly to the application; if the bad sectors contain sensitive data, shred won't be able to destroy it.

shred makes no attempt to detect or report these problem, just as it makes no attempt to do anything about backups. However, since it is more reliable to shred devices than files, shred by default does not truncate or remove the output file. This default is more suitable for devices, which typically cannot be truncated and should not be removed.

shred [option]... file[...]

The program accepts the following options. Also see 2. Common options.

`-f'

`--force'

Override file permissions if necessary to allow overwriting.

`-NUMBER'

`-n NUMBER'

`--iterations=NUMBER'

By default, shred uses 25 passes of overwrite. This is enough for all of the useful overwrite patterns to be used at least once. You can reduce this to save time, or increase it if you have a lot of time to waste.

`-s BYTES'

`--size=BYTES'

Shred the first BYTES bytes of the file. The default is to shred the whole file. BYTES can be followed by a size specification like `k', `M', or `G' to specify a multiple. See section 2.2 Block size.

`-u'

`--remove'

After shredding a file, truncate it (if possible) and then remove it. If a file has multiple links, only the named links will be removed.

`-v'

`--verbose'

Display status updates as sterilization proceeds.

`-x'

`--exact'

Normally, shred rounds the file size up to the next multiple of the filesystem block size to fully erase the last block of the file. This option suppresses that behavior. Thus, by default if you shred a 10-byte file on a system with 512-byte blocks, the resulting file will be 512 bytes long. With this option, shred does not increase the size of the file.

`-z'

`--zero'

Normally, the last pass that shred writes is made up of random data. If this would be conspicuous on your hard drive (for example, because it looks like encrypted data), or you just think it's tidier, the `--zero' option adds an additional overwrite pass with all zero bits. This is in addition to the number of passes specified by the `--iterations' option.

`-'

Shred standard output.

This argument is considered an option. If the common `--' option has been used to indicate the end of options on the command line, then `-' will be interpreted as an ordinary file name.

The intended use of this is to shred a removed temporary file. For example

i=`tempfile -m 0600` exec 3<>"$i" rm -- "$i" echo "Hello, world" >&3 shred - >&3 exec 3>-

Note that the shell command `shred - >file' does not shred the contents of file, since it truncates file before invoking shred. Use the command `shred file' or (if using a Bourne-compatible shell) the command `shred - 1<>file' instead.

You might use the following command to erase all trace of the file system you'd created on the floppy disk in your first drive. That command takes about 20 minutes to erase a 1.44MB floppy.

shred --verbose /dev/fd0

Similarly, to erase all data on a selected partition of your hard disk, you could give a command like this:

shred --verbose /dev/sda5