What is the UU file format?
A .uu file contains uuencoded binary data converted to printable ASCII text, enabling transfer over channels that handle only 7-bit text - such as early UUCP networks and Usenet newsgroups.
Uuencoding was developed at UC Berkeley around 1980 by Mary Ann Horton and became a standard Unix utility (uuencode / uudecode), later standardized by POSIX. The algorithm converts every 3 bytes of binary into 4 printable ASCII characters (in the decimal range 32-95), adding approximately 33% size overhead. Each data line begins with a length character, followed by the encoded groups.
A .uu file starts with a header line such as begin 644 filename.zip (where 644 is the Unix file permissions in octal), followed by encoded data lines, and terminates with an end line. Decoding with uudecode regenerates the original binary. There is no built-in checksum; multi-part Usenet posts relied on newsreader reassembly. A single .uu file wraps one binary payload.
Uuencoding was largely displaced in the mid-1990s by MIME Base64 (RFC 2045, 1996) for email attachments, and by yEnc (2001) for Usenet binary posts. .uu files persist in legacy Usenet archives and old Unix software distributions. Modern Linux systems provide uuencode / uudecode via the sharutils package.
Uuencoding is not compression - it adds size overhead. It may wrap a pre-compressed file, but the encoding step itself always expands the data by about one third.
Security & safety
RISK: MEDIUMThe .uu file is just a text wrapper; the real risk is the binary payload inside. After decoding, treat the resulting file (exe, zip, etc.) with normal caution. Old Usenet archives may contain malware; scan with antivirus after decoding. No execution risk from the .uu text itself.
Format details
in a nutshell- UUE encoded file - Identical format, alternative extension .uue; both encode binary as ASCII for Usenet/UUCP transfer.
Programs that open UU files
Windows3 apps
macOS2 apps
Apple Archive Utility
Built-in Double-click the .uu file; Archive Utility may decode it depending on macOS version. 
uudecode (built-in)
Built-in Terminal: uudecode file.uu - produces the original file in the current directory. 
Linux2 apps
uuencode (sharutils)
Open-source Package sharutils provides both uuencode and uudecode; install via apt/dnf if missing. Technical details
deep spec| Encoding scheme | Binary-to-ASCII: every 3 bytes encoded as 4 characters in printable ASCII range (decimal 32-95) |
| Size overhead | ~33% larger than the source binary |
| Header line | "begin <octal-perms> <filename>" on the first line - e.g., begin 644 archive.zip |
| Data line structure | First byte encodes line length (ASCII 32 + byte count); followed by 4-character groups per 3 source bytes |
| Footer | A zero-length data line (backtick or space) followed by an "end" line to mark the file boundary |
| Integrity | No built-in checksum; multi-part Usenet posts depend on newsreader reassembly of ordered segments |
| Payload capacity | One binary file per .uu file; multi-file sets are split across multiple numbered .uu files |
| Character set | Printable ASCII only (0x20-0x5F); safe for 7-bit text-only transport and UUCP channels |
| Container | Plain text (ASCII / UTF-8 compatible) |
| POSIX standard | uuencode/uudecode utilities defined in the Open Group Base Specifications Issue 7 |
| Encoding vs. compression | Uuencoding expands data by ~33% and is not compression; commonly wraps pre-compressed .gz or .zip files without further reduction |
| Superseded by | MIME Base64 (RFC 2045, 1996) for email; yEnc (2001) for Usenet binary posts |
| Historical origin | Designed for UUCP (Unix-to-Unix Copy) networks circa 1980; became the standard for early Usenet binary distribution |
| Released | 1980, Unix v7 era |
| Latest version | no versioning; superseded by MIME/Base64 |
| Open standard | Yes · royalty-free |
| Specification | pubs.opengroup.org |
UU conversions
Community Q&A
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