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CPIO (short for "copy in, out") is one of the oldest archive formats still in active use, dating back to early Unix systems in the late 1970s. It stores files along with their metadata — permissions, ownership, and timestamps — in a simple, sequential format that's easy for other tools to read and generate programmatically.
Unlike more consumer-facing formats such as ZIP, CPIO doesn't apply compression on its own — it's typically paired with a separate compression tool like gzip or xz when smaller file sizes are needed. This separation of archiving and compression is a classic Unix design choice, letting each tool do one job well and be combined with others through pipes.
CPIO remains critical in modern Linux systems as the format used for initramfs images — the temporary root filesystem loaded early in the Linux boot process before the real system disk is mounted. It's also used internally by the RPM package format, which stores its file payload as a compressed CPIO archive.
Because of its close ties to system-level Linux tooling, CPIO is far less common in general end-user file sharing than formats like ZIP or 7Z. Working with it usually happens through the command line using the cpio utility itself, or via a general archive tool with CPIO support.
Overall, CPIO endures not because it's the most convenient format for everyday users, but because it's deeply embedded in how Linux systems boot and how RPM packages are built.