uuencode

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 mode file

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 ouput as ASCII characters which will

lie in the range 32 (space) to 32+63 = 95 (underscore). Each

group of sixty ouptut 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.