Files output by uuencode(1) consist of a header line, followed by a number of body lines, and a trailer line. The 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.
The header line is distinguished by having the first 5 characters be begin followed by a space, or else a hyphen and either base64 or encoded or both (also separated with a hyphen). The base64 option says the file has been encoded using base64. The encoded option says the output file name has been base64 encoded. It is never encoded with traditional uuencoding. This is a GNU extension. These are followed by a mode (in octal), and a string which names the remote file. The mode is separated from the begin clause and the file name by a single space character.
The traditional uuencoded file body consists of a number of lines, each at most 62 characters long (including the trailing newline). These consist of a character count letter, followed by the encoded characters, followed by a newline. The character count is a single printing character, and represents an integer, the number of bytes the rest of the line represents. Such integers are always in the range from 0 to 63 and can be determined by subtracting the character space (octal 40) from the character.
Groups of 3 bytes are stored in 4 characters, 6 bits per character. All are offset by a space to make the characters printing. The last line may be shorter than the normal 45 bytes. If the size is not a multiple of 3, this fact can be determined by the value of the count on the last line. Extra garbage will be included to make the character count a multiple of 4. The body is terminated by a line with a count of zero. This line consists of one ASCII space.
The trailer line consists of end on a line by itself.
base64 encoded files follow the specified format for the body, but also include a begin-base64 header and a trailer line of four = characters.
begin-base64-encoded 644 VE9ETw==
This introduces a base64 encoded file named, TODO with that name encoded using base64 encoding.
begin-encoded 644 5$]$3P``
This introduces an encoded file named, TODO with that name encoded using uuencoding. The encoding is a lot less friendly. Please prefer base64 encoding.
IEEE Std 1003.1, plus extensions
The -encoded suffix to the begin header line is a GNU extension. Recipients must have the GNU uudecode program to decode them.
uuencode(1), uudecode(1), base64(1GNU)
The uuencode file format appeared in BSD 4.0 .