2.5 Sources of random data

The shuf, shred, and sort commands sometimes need random data to do their work. For example, ‘sort -R’ must choose a hash function at random, and it needs random data to make this selection.

Normally these commands use the device file /dev/urandom 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.

/dev/urandom suffices for most practical uses, but applications requiring high-value or long-term protection of private data may require an alternate data source like /dev/random or /dev/arandom. The set of available sources depends on your operating system.

To use such a source, specify the --random-source=file option, e.g., ‘shuf --random-source=/dev/random’. The contents of file should be as random as possible. An error is reported if file does not contain enough bytes to randomize the input adequately.

To reproduce the results of an earlier invocation of a command, you can save some random data into a file and then use that file as the random source in earlier and later invocations of the command.

Some old-fashioned or stripped-down operating systems lack support for /dev/urandom. On these systems commands like shuf by default fall back on an internal pseudorandom generator initialized by a small amount of entropy.