rsort is a command-line filter implementing in-memory MSD radix sort
from stdin to stdout. rsort was built for speed; rsort, unlike GNU sort,
doesn't sort on subkeys, or sort numerically, or fold case.

Most of those features can be duplicated, using rsort and other
standard tools, while still outperforming GNU sort:

   * ASCII sorting. In one test, rsort ran over 500 times faster than
     GNU sort for smallish files--around 250 lines.

   * Scalability. Subject to memory limits, rsort always outperforms GNU
     sort, but less dramatically as files increase in size. For files
     around 4MB having about 100,000 lines, rsort ran over five times
     faster than GNU sort. (5.3 seconds for rsort; 29.2 for GNU sort.)

   * Sorting on subkeys. By ordering fields from most important to least
     important, ASCII sorting becomes identical to sorting on subkeys.
     In one test, using awk+rsort+awk to permute columns, sort, and
     permute back was still THREE TIMES FASTER than GNU sort.

   * Numeric sorting. ASCII-ordering is identical to numeric ordering
     when number fields are fixed length. For example:

       - UNIX-style dates between March 3, 1973 and September 7, 2001
       - Four-digit years
       - telephone numbers
       - ZIP/postal codes
       - Zero-padded numbers in a fixed range

   * Date sorting. For ISO-formatted dates, ASCII sorting is exactly
     by-date sorting. In general, any date format which orders time
     units by decreasing magnitude will be ASCII-sortable (as long as
     numbers are zero-padded).
