Uuencode

uuencode這個名字是衍生自 "Unix-to-Unix encoding"，原先是Unix系統下將二進位的資料藉由uucp郵件系統傳輸的一個編碼程式，是一種二進位到文字的編碼。uudecode 是與uuencode搭配的解碼程式，uuencode/decode常見於電子郵件中的檔案傳送以及usenet新聞群組的貼文等等。近來以被MIME這種編碼程序大量取代.

編碼程序

Uuencoded data starts with a line of the form:

begin <mode> <file>

Where <mode> is the file's read/write/execute permissions as three octal digits, and <file> 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 fewer 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 which will lie in the range 32 (space) to 32+63 = 95 (underscore). ASCII characters greater than 95 may also be used; however, only the six right-most bits are relevant.

Each group of sixty output characters (corresponding to 45 input bytes) is output as a separate line preceded by an encoded character giving the number of encoded bytes on that line. For all lines except the last, this will be the character 'M' (ASCII code 77 = 32+45). If the input is not evenly divisible by 45, the last line will contain the remaining N output characters, preceded by the character whose code is 32 + the number of remaining input bytes. Finally, a line containing just a single space (or grave character) is output, followed by one line containing the string "end".

Sometimes each data line has extra dummy characters (often the grave accent) added to avoid problems with mailers that strip trailing spaces. These characters are ignored by uudecode. The grave accent (ASCII 96) can also be used in place of a space character. When stripped of their high bits they both decode to 100000.

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. To solve this problem, the Xxencode format was created as a more robust version of the encoding, which used only alphanumeric characters and the plus and minus symbols.

uuencode編碼範例

The table shows the uuencoding of the three ASCII encoded characters Cat into its uuencoded representation 0V%T:

原始字元	`C`								`a`								`t`
原始ASCII碼（十進位）	67								97								116
ASCII碼（二進位）	0	1	0	0	0	0	1	1	0	1	1	0	0	0	0	1	0	1	1	1	0	1	0	0
新的十進位數值	16						54						5						52
+32	48						86						37						84
編碼後的Uuencode字元	`0`						`V`						`%`						`T`

If the complete uuencoded output of the three ASCII characters Cat might appear as the following

begin 644 cat.txt
#0V%T
`
end

Uuencode對照表

The following table represents the subset of ASCII characters used by UUEncode and the 6-bit binary string they represent.

Printable Representation	ASCII Decimal	Binary Representation	Printable Representation	ASCII Decimal	Binary Representation
(space)	32	000 000	@	64	100 000
!	33	000 001	A	65	100 001
"	34	000 010	B	66	100 010
#	35	000 011	C	67	100 011
$	36	000 100	D	68	100 100
%	37	000 101	E	69	100 101
&	38	000 110	F	70	100 110
'	39	000 111	G	71	100 111
(	40	001 000	H	72	101 000
)	41	001 001	I	73	101 001
*	42	001 010	J	74	101 010
+	43	001 011	K	75	101 011
,	44	001 100	L	76	101 100
-	45	001 101	M	77	101 101
.	46	001 110	N	78	101 110
/	47	001 111	O	79	101 111
0	48	010 000	P	80	110 000
1	49	010 001	Q	81	110 001
2	50	010 010	R	82	110 010
3	51	010 011	S	83	110 011
4	52	010 100	T	84	110 100
5	53	010 101	U	85	110 101
6	54	010 110	V	86	110 110
7	55	010 111	W	87	110 111
8	56	011 000	X	88	111 000
9	57	011 001	Y	89	111 001
:	58	011 010	Z	90	111 010
;	59	011 011	[	91	111 011
<	60	011 100	\	92	111 100
=	61	011 101	]	93	111 101
>	62	011 110	^	94	111 110
?	63	011 111	_	95	111 111
			`	96	(1) 000 000

參見

參考資料

本條目部分或全部內容出自以GFDL授權發佈的《自由線上電腦詞典》（FOLDOC）。

外部連結

UUDeview - open-source program to encode/decode Base64, BinHex, uuencode, xxencode, etc. for Unix/Windows/DOS