What is the IO file format?
The .io extension is an uncommon alias for a cpio archive - a Unix archive format dating to 1977 that bundles files and directories into a single stream. Standard cpio archives use the .cpio extension; .io is a rare non-standard variant produced by some tools or scripts.
cpio (copy in/out) stores files as a sequence of records: each record consists of a fixed header, a filename and the raw file contents. The archive ends with a TRAILER!!! sentinel entry. Several header formats exist - the SVR4 newc format (ASCII magic 070701/070702) is the most common today, while the old binary format uses the 2-byte magic 0xC771. The crc variant of newc adds a per-file CRC32 checksum; other formats carry none.
cpio does not compress on its own; archives are typically piped through gzip, bzip2 or xz (producing .cpio.gz or .cpio.xz). To extract, use cpio -idv < archive.cpio, or tools such as bsdtar and 7-Zip that auto-detect the format.
Despite being largely replaced by tar for general archiving, cpio remains essential in Linux: the initramfs early-boot root filesystem that the kernel loads at startup is a newc cpio archive, and the payload inside RPM packages is a cpio stream. It also ships on BSD and macOS as bsdcpio.
Security & safety
RISK: MEDIUMA cpio archive is data, not executable, but two cautions apply. First, cpio preserves file paths and permissions verbatim - a maliciously crafted archive could contain absolute or '../' paths that write outside the target directory ("path traversal"), so extract into a fresh empty folder and inspect the listing ('cpio -itv') before extracting. Second, an archive can carry executable scripts/binaries - never run extracted content you don't trust. The format itself has no encryption or signature (except the newc-crc checksum, which guards against corruption, not tampering).
Format details
in a nutshell- Generic / domain-suffix usage - .io is primarily an internet country-code domain (British Indian Ocean Territory), widely used by tech brands - not a file format at all.
- Neuratron PhotoScore (.io) - Sheet-music scan/recognition file produced by PhotoScore (music OCR), unrelated to cpio archives.
- Application data files - Some games and apps save their own data with a .io extension; identify by the creating program.
Programs that open IO files
Windows1 app
7-Zip
Open-source Right-click > 7-Zip > Open archive; 7-Zip detects cpio and extracts the files. Decompress .gz first if double-packed. 
macOS1 app
cpio / bsdtar (built-in)
Built-in macOS ships bsdcpio and bsdtar - 'cpio -idv < file' or 'tar -xf file' extracts it from Terminal. 
Linux3 apps
cpio (GNU coreutils-adjacent)
Open-source Run 'cpio -idv < archive.io' to extract (the -d makes directories). For .cpio.gz, 'gunzip -c file | cpio -idv'.
bsdtar / libarchive
Open-source Run 'tar -xf archive.io' - libarchive auto-detects and extracts cpio (and handles compression transparently).
7-Zip / p7zip
Open-source Open the archive in 7-Zip; it recognizes cpio and lets you browse/extract entries. Technical details
deep spec| Archive format | cpio (copy in/out) stream archive; .io is a non-standard alias - the standard extension is .cpio |
| Header formats | bin (old binary), odc (POSIX portable ASCII), newc/SVR4 (070701), crc/newc-crc (070702) |
| ASCII magic bytes | 070701 or 070702 (SVR4 newc); 070707 (odc); old binary: 0xC771 (LE) or 0x71C7 (BE) |
| Compression | None built-in; archives piped through gzip/bzip2/xz to produce .cpio.gz or .cpio.xz |
| Encryption | None; cpio has no built-in encryption support |
| Record structure | Per-entry: header + filename + file data; archive terminates with a TRAILER!!! sentinel entry |
| Checksum | Only the crc (newc-crc) format stores a per-file CRC32 checksum; bin, odc and plain newc have none |
| MIME type | application/x-cpio |
| Entry count limit | No practical limit in SVR4 newc format; Linux initramfs images contain thousands of entries |
| Platform | Linux, Unix, BSD, macOS (bsdcpio); Windows via 7-Zip or WSL |
| Key use cases | Linux initramfs early-boot root filesystem, RPM package payloads, Unix system backups |
| Extraction commands | cpio -idv < archive.cpio; or bsdtar -xf archive.cpio; 7-Zip handles cpio natively |
| Typical size | Highly variable - KB for small archives to many GB for system backups or initramfs images |
| Released | cpio: PWB/Unix, 1977; standardized in POSIX.1 (pax/cpio formats) |
| Latest version | GNU cpio 2.x (current); formats: bin, odc, newc (SVR4), crc, hpodc |
| Open standard | Yes · royalty-free |
| Specification | www.gnu.org |
IO conversions
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