| name | upx-5-1-1 |
| description | A skill for using UPX (Ultimate Packer for eXecutables) v5.1.1 to compress and decompress executable files across multiple platforms including Windows, Linux, macOS, DOS, Atari, and PlayStation. Achieves 50%-70% file size reduction with very fast decompression and no runtime memory penalty for most formats. Use when reducing distribution size of executables and DLLs, optimizing storage or download costs, compressing bootable Linux kernels, or packing executables for embedded and resource-constrained environments. |
UPX 5.1.1
Overview
UPX (the Ultimate Packer for eXecutables) is a free, secure, portable, and
high-performance executable compressor. It typically reduces file sizes of
programs and DLLs by 50%-70%, reducing disk space, network load times, download
times, and other distribution costs.
Programs compressed by UPX are completely self-contained and run exactly as
before, with no runtime or memory penalty for most supported formats due to
in-place decompression. Decompression speed exceeds 500 MB/s on modern
machines.
UPX is written in portable endian-neutral C++, distributed under the GNU
General Public License v2+ with special exceptions granting free usage for all
binaries including commercial programs.
Version 5.1.1 was released on March 5, 2026 and adds support for ELF MIPS r3000
(32-bit) shared libraries among other bug fixes.
When to Use
- Reducing distribution size of compiled executables and DLLs
- Optimizing storage or bandwidth for software delivery
- Compressing bootable Linux kernel images (vmlinuz)
- Packing executables for embedded systems with limited flash memory
- Creating smaller release builds for Windows, Linux, macOS, or DOS targets
- Any scenario where executable file size matters and the target format is
supported by UPX
Core Concepts
-
Compression levels: Levels -1 through -9 plus --best. Default is -8 for
files under 512 KiB, -7 otherwise. Levels 1-3 are fast, 4-6 balanced, 7-9
favor ratio, and --best may take a long time.
-
In-place decompression: For most formats UPX decompresses directly into
memory with no temporary files and no runtime overhead beyond the small stub.
-
Compression algorithms: NRV (default, very fast) and LZMA (--lzma flag,
better ratio but significantly slower decompression).
-
Overlay handling: Auxiliary data attached after an executable's logical
end. UPX can copy (default), strip, or skip files with overlays.
-
Security context: UPX inherits the security context of any files it
handles. Packing, unpacking, testing, or listing a file requires the same
security considerations as executing it. Use only on trusted files.
-
Byte-identical round-trip: For most formats, decompressing a packed file
produces a byte-identical copy of the original.
Usage Examples
Compress an executable (default operation):
upx program.exe
Decompress a packed executable:
upx -d program.exe
Test integrity of a compressed file:
upx -t program.exe
List information about compressed files:
upx -l program.exe
Best compression with brute force method selection:
upx --brute program.exe
Ultra brute force for maximum compression:
upx --ultra-brute program.exe
Compress with LZMA algorithm (better ratio, slower decompression):
upx --lzma program.exe
Strip overlay data from the executable:
upx --overlay=strip program.exe
Keep backup of original file:
upx -k program.exe
Write output to a specific file:
upx -o compressed.bin program.exe
Quiet mode (suppress warnings):
upx -q program.exe
Compress a Linux kernel image:
upx -9 bzImage
Advanced Topics
Supported Executable Formats: Complete list of all supported formats with
architecture details and format-specific options → Supported Formats
Command-Line Reference: All options, compression tuning, overlay handling,
and environment variable configuration → Command-Line Reference
Linux-Specific Details: ELF decompression modes, SELinux compatibility,
kernel support, and shared library considerations → Linux Platform Guide
Windows PE Details: Resource compression, icon handling, relocation
stripping, DLL support, and Wine compatibility → Windows PE Guide
