The Free On-line Dictionary of Computing (30 December 2018):
(Unix-to-Unix encode) A Unix program for
encoding binary data as ASCII. Uuencode was originally
used with uucp to transfer binary files over serial lines
which did not preserve the top bit of characters, but is now
used for sending binary files by e-mail and posting to
Usenet newsgroups etc. The program uudecode reverses the
effect of uuencode, recreating the original binary file
Uuencoded data starts with a line of the form
where is the files read/write/execute permissions as
three octal digits and is the name to be used when
recreating the binary data.
Uuencode repeatedly takes in a group of three bytes, adding
trailing zeros if there are less than three bytes left. These
24 bits are split into four groups of six which are treated as
numbers between 0 and 63. Decimal 32 is added to each number
and they are output as ASCII characters from 32 (space) to
32+63 = 95 (underscore). Each group of sixty output
characters (corresponding to 45 input bytes) is output as a
separate line preceded by an 'M' (ASCII code 77 = 32+45). At
the end of the input, if there are N output characters left
after the last group of sixty and N>0 then they will be
preceded by the character whose code is 32+N. Finally, a line
containing just a single space is output, followed by one
containing just "end".
Sometimes each data line has an extra dummy character added to
avoid problems which mailers that strip trailing spaces.
These characters are ignored by uudecode.
Despite using this limited range of characters, there are
still some problems encountered when uuencoded data passes
through certain old computers. The worst offenders are
computers using non-ASCII character sets such as EBCDIC.
Base 64 encoding is probably now more commonly used than