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\"      @(#)uuencode.format.5   8.2 (Berkeley) 1/12/94
\"
Dd March 4, 2019
Dt UUENCODE 5
Os
Sh NAME
Nm uuencode
Nd format of an encoded uuencode file
Sh DESCRIPTION
Files output by
Xr uuencode 1
consist of a header line,
followed by a number of body lines,
and a trailer line.
The
Xr uudecode 1
command
will ignore any lines preceding the header or
following the trailer.
Lines preceding a header must not, of course,
look like a header.
Pp
The header line starts with the word
Dq begin ,
a space,
a file mode (in octal),
a space,
and finally a string which names the file being encoded.
Pp
The central engine of
Xr uuencode 1
is a six-bit encoding function which outputs an
Tn ASCII
character.
The six bits to be encoded are treated as a small integer and added
with the
Tn ASCII
value for the space character (octal 40).
The result is a printable
Tn ASCII
character.
In the case where all six bits to be encoded are zero,
the
Tn ASCII
backquote character \` (octal 140) is emitted instead of what
would normally be a space.
Pp
The body of an encoded file consists of one or more lines,
each of which may be a maximum of 86 characters long (including the trailing
newline).
Each line represents an encoded chunk of data from the input file and begins
with a byte count,
followed by encoded bytes,
followed by a newline.
Pp
The byte count is a six-bit integer encoded with the above function,
representing the number of bytes encoded in the rest of the line.
The method used to encode the data expands its size by
133% (described below).
Therefore it is important to note that the byte count describes the size of
the chunk of data before it is encoded, not afterwards.
The six bit size of this number effectively limits the number of bytes
that can be encoded in each line to a maximum of 63.
While
Xr uuencode 1
will not encode more than 45 bytes per line,
Xr uudecode 1
will tolerate the maximum line size.
Pp
The remaining characters in the line represent the data of the input
file encoded as follows.
Input data are broken into groups of three eight-bit bytes,
which are then interpreted together as a 24-bit block.
The first bit of the block is the highest order bit of the first character,
and the last is the lowest order bit of the third character.
This block is then broken into four six-bit integers which are encoded one by
one starting from the first bit of the block.
The result is a four character
Tn ASCII
string for every three bytes of input data.
Pp
Encoded lines of data continue in this manner until the input file is
exhausted.
The end of the body is signaled by an encoded line with a byte count
of zero (the
Tn ASCII
backquote character \`).
Pp
Obviously, not every input file will be a multiple of three bytes in size.
In these cases,
Xr uuencode 1
will pad the remaining one or two bytes of data with null characters until
a three byte group is created.
The byte count in a line containing
null padding will reflect the actual number of bytes encoded, making
it possible to convey how many bytes are null.
Pp
The trailer line consists of
Dq end
on a line by itself.
Sh SEE ALSO
Xr mail 1 ,
Xr uudecode 1 ,
Xr uuencode 1 ,
Xr ascii 7
Sh HISTORY
The
Nm
file format appeared in
Bx 4.0 .
Sh BUGS
The interpretation of the
Nm
format relies on properties of the
Tn ASCII
character set and may not work correctly on non-ASCII systems.

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