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UUENCODE(1P) POSIX Programmer's Manual UUENCODE(1P)

PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.

NAME
uuencode - encode a binary file

SYNOPSIS
uuencode [-m][file] decode_pathname

DESCRIPTION
The uuencode utility shall write an encoded version of the named input file, or standard input if no file is specified,
to standard output. The output shall be encoded using one of the algorithms described in the STDOUT section and shall
include the file access permission bits (in chmod octal or symbolic notation) of the input file and the decode_pathname,
for re-creation of the file on another system that conforms to this volume of IEEE Std 1003.1-2001.

OPTIONS
The uuencode utility shall conform to the Base Definitions volume of IEEE Std 1003.1-2001, Section 12.2, Utility Syntax
Guidelines.

The following option shall be supported by the implementation:

-m Encode the output using the MIME Base64 algorithm described in STDOUT. If -m is not specified, the historical
algorithm described in STDOUT shall be used.

OPERANDS
The following operands shall be supported:

decode_pathname

The pathname of the file into which the uudecode utility shall place the decoded file. Specifying a decode_path-
name operand of /dev/stdout shall indicate that uudecode is to use standard output. If there are characters in
decode_pathname that are not in the portable filename character set the results are unspecified.

file A pathname of the file to be encoded.

STDIN
See the INPUT FILES section.

INPUT FILES
Input files can be files of any type.

ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of uuencode:

LANG Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions
volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of international-
ization variables used to determine the values of locale categories.)

LC_ALL If set to a non-empty string value, override the values of all the other internationalization variables.

LC_CTYPE
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-
byte as opposed to multi-byte characters in arguments and input files).

LC_MESSAGES
Determine the locale that should be used to affect the format and contents of diagnostic messages written to stan-
dard error.

NLSPATH
Determine the location of message catalogs for the processing of LC_MESSAGES .

ASYNCHRONOUS EVENTS
Default.

STDOUT
uuencode Base64 Algorithm
The standard output shall be a text file (encoded in the character set of the current locale) that begins with the line:

"begin-base64 %s %s\n", <mode>, <decode_pathname>

and ends with the line:

"====\n"

In both cases, the lines shall have no preceding or trailing <blank>s.

The encoding process represents 24-bit groups of input bits as output strings of four encoded characters. Proceeding from
left to right, a 24-bit input group shall be formed by concatenating three 8-bit input groups. Each 24-bit input group
then shall be treated as four concatenated 6-bit groups, each of which shall be translated into a single digit in the
Base64 alphabet. When encoding a bit stream via the Base64 encoding, the bit stream shall be presumed to be ordered with
the most-significant bit first. That is, the first bit in the stream shall be the high-order bit in the first byte, and
the eighth bit shall be the low-order bit in the first byte, and so on. Each 6-bit group is used as an index into an
array of 64 printable characters, as shown in uuencode Base64 Values .

Table: uuencode Base64 Values

Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad)----------
15 P 32 g 49 x
16 Q 33 h 50 y

The character referenced by the index shall be placed in the output string.

The output stream (encoded bytes) shall be represented in lines of no more than 76 characters each. All line breaks or
other characters not found in the table shall be ignored by decoding software (see uudecode ).

Special processing shall be performed if fewer than 24 bits are available at the end of a message or encapsulated part of
a message. A full encoding quantum shall always be completed at the end of a message. When fewer than 24 input bits are
available in an input group, zero bits shall be added (on the right) to form an integral number of 6-bit groups. Output
character positions that are not required to represent actual input data shall be set to the character '=' . Since all
Base64 input is an integral number of octets, only the following cases can arise:

1. The final quantum of encoding input is an integral multiple of 24 bits; here, the final unit of encoded output shall
be an integral multiple of 4 characters with no '=' padding.

2. The final quantum of encoding input is exactly 16 bits; here, the final unit of encoded output shall be three charac-
ters followed by one '=' padding character.

3. The final quantum of encoding input is exactly 8 bits; here, the final unit of encoded output shall be two characters
followed by two '=' padding characters.

A terminating "====" evaluates to nothing and denotes the end of the encoded data.

uuencode Historical Algorithm
The standard output shall be a text file (encoded in the character set of the current locale) that begins with the line:

"begin %s %s\n" <mode>, <decode_pathname>

and ends with the line:

"end\n"

In both cases, the lines shall have no preceding or trailing <blank>s.

The algorithm that shall be used for lines in between begin and end takes three octets as input and writes four charac-
ters of output by splitting the input at six-bit intervals into four octets, containing data in the lower six bits only.
These octets shall be converted to characters by adding a value of 0x20 to each octet, so that each octet is in the range
[0x20,0x5f], and then it shall be assumed to represent a printable character in the ISO/IEC 646:1991 standard encoded
character set. It then shall be translated into the corresponding character codes for the codeset in use in the current
locale. (For example, the octet 0x41, representing 'A', would be translated to 'A' in the current codeset, such as 0xc1
if it were EBCDIC.)

Where the bits of two octets are combined, the least significant bits of the first octet shall be shifted left and com-
bined with the most significant bits of the second octet shifted right. Thus the three octets A, B, C shall be converted
into the four octets:

0x20 + (( A >> 2 ) & 0x3F)
0x20 + (((A << 4) | ((B >> 4) & 0xF)) & 0x3F)
0x20 + (((B << 2) | ((C >> 6) & 0x3)) & 0x3F)
0x20 + (( C ) & 0x3F)

These octets then shall be translated into the local character set.

Each encoded line contains a length character, equal to the number of characters to be decoded plus 0x20 translated to
the local character set as described above, followed by the encoded characters. The maximum number of octets to be
encoded on each line shall be 45.

STDERR
The standard error shall be used only for diagnostic messages.

OUTPUT FILES
None.

EXTENDED DESCRIPTION
None.

EXIT STATUS
The following exit values shall be returned:

0 Successful completion.

>0 An error occurred.

CONSEQUENCES OF ERRORS
Default.

The following sections are informative.

APPLICATION USAGE
The file is expanded by 35 percent (each three octets become four, plus control information) causing it to take longer to
transmit.

Since this utility is intended to create files to be used for data interchange between systems with possibly different
codesets, and to represent binary data as a text file, the ISO/IEC 646:1991 standard was chosen for a midpoint in the
algorithm as a known reference point. The output from uuencode is a text file on the local system. If the output were in
the ISO/IEC 646:1991 standard codeset, it might not be a text file (at least because the <newline>s might not match), and
the goal of creating a text file would be defeated. If this text file was then carried to another machine with the same
codeset, it would be perfectly compatible with that system's uudecode. If it was transmitted over a mail system or sent
to a machine with a different codeset, it is assumed that, as for every other text file, some translation mechanism would
convert it (by the time it reached a user on the other system) into an appropriate codeset. This translation only makes
sense from the local codeset, not if the file has been put into a ISO/IEC 646:1991 standard representation first. Simi-
larly, files processed by uuencode can be placed in pax archives, intermixed with other text files in the same codeset.

EXAMPLES
None.

RATIONALE
A new algorithm was added at the request of the international community to parallel work in RFC 2045 (MIME). As with the
historical uuencode format, the Base64 Content-Transfer-Encoding is designed to represent arbitrary sequences of octets
in a form that is not humanly readable. A 65-character subset of the ISO/IEC 646:1991 standard is used, enabling 6 bits
to be represented per printable character. (The extra 65th character, '=', is used to signify a special processing func-
tion.)

This subset has the important property that it is represented identically in all versions of the ISO/IEC 646:1991 stan-
dard, including US ASCII, and all characters in the subset are also represented identically in all versions of EBCDIC.
The historical uuencode algorithm does not share this property, which is the reason that a second algorithm was added to
the ISO POSIX-2 standard.

The string "====" was used for the termination instead of the end used in the original format because the latter is a
string that could be valid encoded input.

In an early draft, the -m option was named -b (for Base64), but it was renamed to reflect its relationship to the
RFC 2045. A -u was also present to invoke the default algorithm, but since this was not historical practice, it was omit-
ted as being unnecessary.

See the RATIONALE section in uudecode for the derivation of the /dev/stdout symbol.

FUTURE DIRECTIONS
None.