Documented in Volume 1 of the UNIX Programmers Manual.
% split -4000 filename
will split the contents of the 7000 line file 'filename' into two smaller
files: xaa will contain 4000 lines, xab will contain the rest (3000 lines).
The files xaa, xab (...et cetera) will be in the present working directory.
The original file will be unchanged.
This command is typically used prior to copying across the net, where
files are restricted to some maximum size.
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