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.TH uuencode 5 "May, 2001" "Apple Computer, Inc."
.SH NAME
uuencode file format
.NM uuencode
.ND description of the uuencode file format
.SH DESCRIPTION
The
.XR uuencode 1
command generates files in a format that allows them to be successfully
transferred by systems which strip the high bit from an 8-bit byte.
.XR uudecode 1
decodes uuencoded files.
.PP
The uuencode file format consists of three sections: header, body, and trailer.
The header is a line is of the form:
.PP
begin 644 "filename.ext"
.PP
where "644" is a
.XR chmod 1
-format permissions byte for the file and "filename.ext" is the name of
the encoded file.
.PP
The body section is the encoded representation of the source file. Three
bytes of input file data are encoded into four bytes of output data.
.PP
The 24 input bits are divided up into four pieces of six bits
each. The integer value 32 (the ASCII value for the space character) is
added to each of these pieces to move them outside of the range of control
characters. To avoid using the space character in the encoding, pieces with
value zero are encoded using backquote (ASCII value 96) instead of zero. The
resulting character is one of the this set (ASCII values 96,33-95):
.DQ `!"#$%&'()*+,-./012356789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_
.PP
A line itself contains three segments: a length character (encoded using
the "add a space" algorithm described above), the body of the line,
typically (although not required to be) 60 output characters long,
representing 45 input bytes, and (of course) a linefeed. The length
character specifies the number of valid input bytes on the line (so, for
a line which is 60 encoded bytes, the length value would be 45).
Decoding programs should decode no further than the specified length on
a single line.
.PP
The trailer, which must exist, consists of a single backquote
("`", ASCII 96) character on a line by itself, directly followed by
.DQ end
on a line by itself.
.PP
.DQ .uue
is the canonical filename extension for uuencoded files.
.SH BUGS
uudecode does not read all permutations of the file format described in
this man page.
.PP
Ancient versions of uuencode used a space character (ASCII 32) in the
encoding to represent zero. Many (arguably broken) mailers and transport
agents stripped, rewrapped, or otherwise mangled this format, so the space
was later changed to the backquote, ASCII 96. Decoders may attempt to
read the older format if they wish, though it's unlikely to be encountered
in practice at this point in time.
.PP
The uuencode encoding method is highly ASCII-centric. In particular, the
character set used doesn't work well on EBCDIC-based systems. (EBCDIC,
generally used by IBM mainframes, is an old alternative character encoding;
most computers use ASCII instead).
.PP
Many variants of uuencode on various platforms generate different forms
of line checksums, using to represent the checksum one or more encoded
characters after the last counted character in a line. Because these
formats are different and impossible to distinguish (with certainty),
such characters should be ignored by decoding implementations.
.PP
The uuencode encoding format has no provisions for segmented files.
Writers of segmenting utilities should be careful to avoid using
character sequences that may naturally occur in the encoding (such
as sequences of dashes ("---")) to divide sections.
.SH SEE ALSO
The MIME Base64 encoding (documented in RFC 2045) is a consistent,
cross-platform-savvy message encoding which should be used in place of
UUEncode wherever possible.
.PP
The Unix-Hater's Handbook (IDG, 1994) identifies the folly of the
older zero-encoded-as-space versions of uuencode.
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