(coreutils.info)Random sources

---

Next: Target directory Prev: Disambiguating names and IDs Up: Common options

---

Enter node , (file) or (file)node

2.7 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.

   By default these commands use an internal pseudo-random generator
initialized by a small amount of entropy, but can be directed to use an
external source with the ‘--random-source=FILE’ option.  An error is
reported if FILE does not contain enough bytes.

   For example, the device file ‘/dev/urandom’ could be used 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 pseudo-random number generator.  But be aware
that this device is not designed for bulk random data generation and is
relatively slow.

   ‘/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 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.  Rather
than depending on a file, one can generate a reproducible arbitrary
amount of pseudo-random data given a seed value, using for example:

     get_seeded_random()
     {
       seed="$1"
       openssl enc -aes-256-ctr -pass pass:"$seed" -nosalt \
         </dev/zero 2>/dev/null
     }

     shuf -i1-100 --random-source=<(get_seeded_random 42)