uudecode

Uudecode is a Unix utility that decodes data that has been encoded with the uuencode program. It converts an ASCII text representation back into its binary form, enabling the transfer of binary files through channels that are designed for text data, such as email or Usenet posts.

The input format expected by uudecode typically begins with a header line that starts with the word begin, followed by a file mode (in octal) and the original filename. The encoded data then follows, consisting of lines that encode groups of three bytes as four printable ASCII characters. Each data line starts with a length character that indicates how many bytes the line will decode to. The data lines use a simple 6-bit encoding where each 6-bit value is mapped to a printable ASCII character, usually by adding 32 to the value. A line with a length of zero ends the data for that file. The encoded content is terminated by a line containing the word end.

When decoding completes, uudecode writes the resulting binary data to a file named as given in the begin line and applies the file mode (permissions) specified in that line. If a file with the same name already exists, it may be overwritten depending on the system and invocation method.

Uudecode originated with Unix and has historical prominence in UUCP, Usenet, and email workflows, where binary data needed to be transported using plain text. With the rise of MIME and base64, its use has diminished, but it remains a standard tool on many Unix-like systems for backward compatibility.