.cpio File Extension

What kind of file is a .cpio?

A .cpio file is a CPIO archive: a sequential container that can bundle files, directories, links, device nodes, and Unix metadata. It commonly uses the conventional application/x-cpio MIME type, which is not registered with IANA.¹ CPIO does not compress data itself. You may also encounter a CPIO stream inside a compressed file or a Linux initramfs image with another suffix.

CPIO has several incompatible header layouts, so no single signature identifies every archive. Common ASCII variants begin with 070707 (odc), 070701 (newc), or 070702 (crc). Historical binary CPIO stores the octal value 070707 as a 16-bit integer, with byte order varying by system.² Check the header and parse its fields rather than trusting the filename alone.

How do I open a .cpio on my computer?

• On Linux and other Unix-like systems, use GNU cpio, bsdcpio, or libarchive's bsdtar.
• On macOS, bsdcpio or bsdtar can inspect and extract compatible archives.
• On Windows, 7-Zip can open CPIO archives, or you can use GNU or libarchive tools in a Unix-compatible environment.³

List an unfamiliar archive before extracting it. A CPIO member can request an absolute path, contain .. components, or create links and special files. Extract into a new directory as an unprivileged user, and disable absolute filenames when your tool provides that option. GNU cpio documents --no-absolute-filenames for this purpose.⁴

A .cpio.gz, .cpio.xz, or similarly wrapped file must first pass through the matching decompressor. The CPIO reader only sees archive headers after the outer compression layer is removed.

Poly support for .cpio

Poly recognizes .cpio as a CPIO archive. It stores, syncs, shares, versions, downloads, and finds the outer file by name and ordinary properties.

Poly does not list or extract members, generate a thumbnail, preview the archive, or index archived paths and content. The Poly agent cannot read files inside it. If you need content-aware search or agent access, extract the archive with a trusted utility and upload the individual files.

History of .cpio

Dick Haight wrote the original cpio utility for AT&T's Unix Support Group. It appeared with PWB/UNIX 1.0 in 1977, and its name describes its original copy-in and copy-out operations.²

Later versions introduced portable ASCII, binary, newc, and crc layouts. Linux still specifies newc or crc CPIO for initramfs images, optionally combined with kernel-supported compression.⁵ The suffix therefore remains useful in boot tooling, firmware, packaging, and legacy Unix interchange even though TAR is more common for general archives.

Working with .cpio and alternatives

Choose the container that matches the receiving system:

Older CPIO layouts impose practical limits. The original PWB layout is limited to 16 MB per file, the later binary layout to 2 GB, and newc to 4 GB.² The crc variant's check value is only a sum of data bytes, not a cryptographic integrity check. Use a separately authenticated hash or signature when provenance matters.

Converting CPIO means extracting and repacking. That process can lose device nodes, hard-link relationships, permissions, or numeric ownership, so retain the original when it is a build input or boot image.