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11.6 shred: Remove files more securely

shred overwrites devices or files, to help prevent even extensive forensics from recovering the data.

Ordinarily when you remove a file (see rm invocation), its data and metadata are not actually destroyed. Only the file’s directory entry is removed, and the file’s storage is reclaimed only when no process has the file open and no other directory entry links to the file. And even if file’s data and metadata’s storage space is freed for further reuse, there are undelete utilities that will attempt to reconstruct the file from the data in freed storage, and that can bring the file back if the storage was not rewritten.

On a busy system with a nearly-full device, space can get reused in a few seconds. But there is no way to know for sure. And although the undelete utilities and already-existing processes require insider or superuser access, you may be wary of the superuser, of processes running on your behalf, or of attackers that can physically access the storage device. So if you have sensitive data, you may want to be sure that recovery is not possible by plausible attacks like these.

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 this is often the preferred method. However, some storage devices are expensive or are harder to destroy, so the shred utility tries to achieve a similar effect non-destructively, by overwriting the file with non-sensitive data.

Please note that shred relies on a crucial assumption: that the file system and hardware overwrite data in place. Although this is common and is the traditional way to do things, but many modern file system designs do not satisfy this assumption. Exceptions include:

For ext3 and ext4 file systems, shred is less effective when the file system is 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. The ext3/ext4 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’). Alternatively, if you know how large the journal is, you can shred the journal by shredding enough file data so that the journal cycles around and fills up with shredded data.

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

Generally speaking, it is more reliable to shred a device than a file, since this bypasses file system design issues mentioned above. However, devices are also problematic for shredding, for reasons such as the following:

shred makes no attempt to detect or report these problems, 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 deallocate or remove the output file. This default is more suitable for devices, which typically cannot be deallocated and should not be removed.

Finally, consider the risk of backups and mirrors. 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. So if you keep any data you may later want to destroy using shred, be sure that it is not backed up or mirrored.

shred [option]… file[…]

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

-f
--force

Override file permissions if necessary to allow overwriting.

-n number
--iterations=number

By default, shred uses 3 passes of overwrite. You can reduce this to save time, or increase it if you think it’s appropriate. After 25 passes all of the internal overwrite patterns will have been used at least once.

--random-source=file

Use file as a source of random data used to overwrite and to choose pass ordering. See Random sources.

-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 Block size.

-u
--remove[=how]

After shredding a file, deallocate it (if possible) and then remove it. If a file has multiple links, only the named links will be removed. Often the file name is less sensitive than the file data, in which case the optional how parameter, supported with the long form option, gives control of how to more efficiently remove each directory entry. The ‘unlink’ parameter will just use a standard unlink call, ‘wipe’ will also first obfuscate bytes in the name, and ‘wipesync’ will also sync each obfuscated byte in the name to disk. Note ‘wipesync’ is the default method, but can be expensive, requiring a sync for every character in every file. This can become significant with many files, or is redundant if your file system provides synchronous metadata updates.

-v
--verbose

Display to standard error all status updates as sterilization proceeds.

-x
--exact

By default, shred rounds the size of a regular file up to the next multiple of the file system block size to fully erase the slack space in the last block of the file. This space may contain portions of the current system memory on some systems for example. Use --exact to suppress that behavior. Thus, by default if you shred a 10-byte regular file on a system with 512-byte blocks, the resulting file will be 512 bytes long. With this option, shred does not increase the apparent 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 storage device (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.

You might use the following command to erase the file system you created on a USB flash drive. This command typically takes several minutes, depending on the drive’s size and write speed. On modern storage devices a single pass should be adequate, and will take one third the time of the default three-pass approach.

shred -v -n 1 /dev/sdd1

Similarly, to erase all data on a selected partition of your hard disk, you could give a command like the following.

# 1 pass, write pseudo-random data; 3x faster than the default
shred -v -n1 /dev/sda5

To be on the safe side, use at least one pass that overwrites using pseudo-random data. I.e., don’t be tempted to use ‘-n0 --zero’, in case some disk controller optimizes the process of writing blocks of all zeros, and thereby does not clear all bytes in a block. Some SSDs may do just that.

A file of ‘-’ denotes standard output. The intended use of this is to shred a removed temporary file. For example:

i=$(mktemp)
exec 3<>"$i"
rm -- "$i"
echo "Hello, world" >&3
shred - >&3
exec 3>-

However, the command ‘shred - >file’ does not shred the contents of file, since the shell truncates file before invoking shred. Use the command ‘shred file’ or (if using a Bourne-compatible shell) the command ‘shred - 1<>file’ instead.

An exit status of zero indicates success, and a nonzero value indicates failure.


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