uuencode [-m] [file] decode_pathname
The following option shall be supported by the implementation:
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> characters.
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 Table 4-22, uuencode Base64 Values.
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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:
A terminating "====" evaluates to nothing and denotes the end of the encoded data.
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> characters.
The algorithm that shall be used for lines in between begin and end takes three octets as input and writes four characters 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 combined 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) |n(XX' ((B >> 4) & 0xF)) & 0x3F) 0x20 + (((B << 2) |n(XX' ((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.
The following sections are informative.
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> characters 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. Similarly, files processed by uuencode can be placed in pax archives, intermixed with other text files in the same codeset.
This subset has the important property that it is represented identically in all versions of the ISO/IEC 646:1991 standard, 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 omitted as being unnecessary.
See the RATIONALE section in uudecode for the derivation of the /dev/stdout symbol.
The Base Definitions volume of POSIX.1-2017, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines
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