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uuencode(5)                                                                                      uuencode(5)



NAME
       uuencode file format

DESCRIPTION
       The  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.  decodes uuencoded files.


       The uuencode file format consists of three sections: header, body, and trailer.  The header is a line
       is of the form:


       begin 644 "filename.ext"


       where  "644" is a -format permissions byte for the file and "filename.ext" is the name of the encoded
       file.


       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.

       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):



       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.


       The trailer, which must exist, consists of a single backquote ("`", ASCII 96) character on a line  by
       itself, directly followed by on a line by itself.


       is the canonical filename extension for uuencoded files.


BUGS
       uudecode does not read all permutations of the file format described in this man page.


       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.


       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).


       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.


       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.


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.


       The  Unix-Hater's  Handbook  (IDG, 1994) identifies the folly of the older zero-encoded-as-space ver-sions versions
       sions of uuencode.




Apple Computer, Inc.                              May, 2001                                      uuencode(5)