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.\"	@(#)uuencode.format.5	8.2 (Berkeley) 1/12/94
.\"
.Dd June 2, 2016
.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 garbage bytes until
a three byte group is created.
The byte count in a line containing
garbage padding will reflect the actual number of bytes encoded, making
it possible to convey how many bytes are garbage.
.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.