lowerdir

lowerdir is a term commonly used in the context of Linux overlay filesystems, particularly OverlayFS, to denote the read‑only layer that forms the base of a merged filesystem view. OverlayFS enables the composition of multiple directories—called layers—into a single virtual filesystem. The lowerdir contains the original contents of files and directories, while a separate upperdir holds any modifications, additions, or deletions made by the user. When the overlay is mounted, the kernel presents a unified directory tree in which files from the upperdir take precedence; if a file does not exist in the upperdir, the version from the lowerdir is shown.

The lowerdir is specified as part of the mount options, for example: mount -t overlay overlay -o lowerdir=/usr/share,upperdir=/tmp/upper,workdir=/tmp/work /mnt/overlay. Multiple lower directories can be stacked by separating them with a colon, creating a hierarchy where earlier entries have higher priority. The lower layers remain immutable during the lifetime of the overlay, ensuring that changes are isolated in the upper layer and can be discarded simply by removing the upperdir and workdir.

Typical use cases include container image construction, live system upgrades, and development environments where a stable base is overlaid with temporary changes. Because the lowerdir is read‑only, it can be shared safely among multiple overlays, reducing storage consumption and simplifying distribution of common base files.