uuencode is a UNIX utility from the 1980s that turns binary files into printable ASCII so they can pass through email and Usenet. The encoded data is framed by a begin line with permissions and filename, and an end line.

How does the line format work?

Each data line starts with a length character giving how many bytes that line encodes, followed by groups of four characters. Every group of four characters represents three original bytes using 6-bit values offset by 0x20.

What is the back-tick at the end?

A length of zero is written as the back-tick character (0x60) rather than a space, signalling the final data line. This is the modern convention that avoids trailing-space stripping problems.

Is uuencode the same as Base64?

Both map 3 bytes to 4 characters, but uuencode uses values offset by 0x20 across a contiguous ASCII range and adds per-line length bytes plus begin/end framing. Base64 uses a fixed 64-character alphabet and padding.

Can it decode files made by the real uuencode command?

Yes. The decoder ignores the begin and end lines, reads the leading length byte on each line, and stops at the zero-length terminator, matching standard uudecode behaviour for text payloads.

UUencode Encoder/Decoder

What uuencode does

uuencode (UNIX-to-UNIX encode) was the original way to send binary attachments through plain-text systems like email and Usenet before MIME and Base64 became universal. It converts bytes into printable ASCII and frames them with a begin header that carries file permissions and a name, plus an end trailer.

How it works

The data is processed three bytes at a time. Those 24 bits are split into four 6-bit groups, and each group is turned into a printable character by adding 0x20 (a space). A group of zero is written as the back-tick ` (0x60) to dodge trailing-whitespace issues. Lines hold up to 45 input bytes (60 output characters) and begin with a single length character that also follows the +0x20 rule:

begin 644 message.txt
M1V5R82!Ub V]L<P``
`
end

The leading character of each data line tells the decoder how many bytes that line really represents, so any padding in the last group can be discarded. A final line whose length character decodes to zero ends the data stream.

Tips and notes

This tool encodes the UTF-8 bytes of whatever text you type and produces a complete, copy-pasteable uuencode document. When decoding, it skips the begin/end framing automatically and honours the per-line length byte, so it interoperates with output from the classic uuencode command. Decoded bytes are interpreted as UTF-8 text, so non-text payloads may not render cleanly.