uuencode
(Unix-to-Unix encode) A Unix program for encoding binary data as ASCII. Uuencode was originally used with uucp to transfer binary files over serial lines which did not preserve the top bit of characters, but is now used for sending binary files by e-mail and posting to Usenet newsgroups etc. The program uudecode reverses the effect of uuencode, recreating the original binary file exactly.
Uuencoded data starts with a line of the form
begin
where <mode> is the files 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 less 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 from 32 (space) to 32+63 = 95 (underscore). Each group of sixty output characters (corresponding to 45 input bytes) is output as a separate line preceded by an 'M' (ASCII code 77 = 32+45). At the end of the input, if there are N output characters left after the last group of sixty and N>0 then they will be preceded by the character whose code is 32+N. Finally, a line containing just a single space is output, followed by one containing just "end".
Sometimes each data line has an extra dummy character added to avoid problems which mailers that strip trailing spaces. These characters are ignored by uudecode.
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.
Base 64 encoding is probably now more commonly used than uuencode.