What is the OUT file format?
A file with the .out extension contains output data generated by a program. .out files are generated by running applications and typically store information in text format. Such a file can be seen as a type of log file that records results or events when an application runs. .out files may also be used for analyzing system processes and their completion.
What are .out files needed for?
Output files are commonly generated by programs used for analyzing or testing system functions or devices. Created .out files can then be used to analyze performance of tested components or system configuration.
.out files also serve a second role on Unix-like systems: C/C++ compilers (GCC, Clang) write their compiled result to a.out by default, making .out the extension for Linux executables too. Such a file is an ELF binary - identified by the magic bytes 7F 45 4C 46 at offset 0 - and is run with ./a.out in a terminal. Use the file command to check whether a .out file is plain text or a compiled program.
Security & safety
RISK: MEDIUMA text .out is harmless. The caution applies to executable .out files: an a.out/ELF is a program that will run code, so only execute one you trust and produced/obtained from a known source - never 'chmod +x && ./' a random .out you were sent. On Windows a Linux a.out won't run natively (it needs WSL/Linux), which limits the risk there, but treat any unknown binary .out with the same caution as any unknown executable.
Format details
in a nutshell- Compiled Unix/Linux executable (a.out / ELF) - gcc/clang name their default output 'a.out'; such a .out is a runnable binary (today in ELF format), not a document.
- LaTeX auxiliary .out file - hyperref writes a .out file holding PDF bookmark/outline data during a LaTeX build - a disposable build artifact.
Programs that open OUT files
Windows3 apps
Notepad / Notepad++
Free Open the .out to read it. If it's text you'll see the output; if it's binary (an executable) you'll see gibberish - that tells you it's not a document. 
Visual Studio Code
Free Open large text .out files comfortably (logs/results); it also hints when a file is binary.
HxD (hex editor)
Free Use to identify a binary .out - check the first bytes for the ELF magic (7F 45 4C 46) or other signatures. 
macOS2 apps
Terminal (./a.out)
Built-in If the .out is a compiled program, run it: chmod +x a.out && ./a.out. Use 'file a.out' to confirm what it is. 
Linux2 apps
Terminal (./a.out, file, less)
Built-in Run an executable with ./a.out; read a text .out with 'less file.out'; identify any .out with the 'file' command. Technical details
deep spec| Format role | Generic output container - no fixed internal structure; content is entirely program-defined |
| MIME type | application/octet-stream |
| Magic bytes (text variant) | None - plain ASCII or UTF-8 text with no file signature |
| Magic bytes (ELF executable) | 7F 45 4C 46 (\x7fELF) at offset 0 |
| Typical text content | Program results, test output, event records, diagnostic logs |
| Text encoding | ASCII or UTF-8, uncompressed; line endings are platform-dependent (LF on Unix, CRLF possible on Windows) |
| Default compiler output name | GCC and Clang write to a.out when compiled without the -o flag on Unix-like systems |
| ELF executable sections | .text (machine code), .data (initialised globals), .bss (zero-filled), .rodata (constants) |
| Execute permission required | ELF .out files need chmod +x before ./a.out can be run on Linux/macOS |
| File identification command | file a.out - distinguishes plain text from ELF binary (or other format) |
| Historical a.out binary format | Pre-ELF object format with OMAGIC/NMAGIC/ZMAGIC magic numbers; support removed from Linux kernel in 5.18-5.19 (2022) |
| Platform (text variant) | Cross-platform - generated by any OS, language runtime, or build tool |
| Platform (ELF variant) | Linux and other Unix-like systems; macOS uses the similar Mach-O format instead |
| Typical file size | Bytes to megabytes for text output; a few kilobytes to tens of megabytes for compiled ELF programs |
| Security consideration | An .out ELF file executes arbitrary machine code - never run one from an untrusted source |
| Released | No single origin; the name comes from programs that write results to an 'output' file (and from a.out, Unix's default executable name since the 1970s) |
OUT conversions
Community Q&A
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