mount and findmnt: Inspect Filesystems in CI
mount and findmnt list every mounted filesystem with its device, type, and mount options, revealing read-only, full, or tmpfs mounts that break a build.
A step that fails to write often hits a read-only or full mount. findmnt and mount show the mount table so you can see exactly where a path lives and how it is mounted.
What it does
mount with no arguments prints the current mount table. findmnt renders it as a searchable tree and can target a single path. The options column shows ro (read-only) vs rw, and the FSTYPE column shows tmpfs (RAM-backed), overlay (container layers), nfs, and so on.
Common usage
findmnt
findmnt /tmp # how is /tmp mounted
mount | grep -E ' ro,| ro ' # read-only mounts
findmnt -t tmpfs # all tmpfs (RAM) mountsOptions
| Command / flag | What it does |
|---|---|
| mount | List all mounts (device on path type opts) |
| findmnt <path> | Show the mount backing a specific path |
| findmnt -t <type> | Filter by filesystem type (tmpfs, overlay) |
| findmnt -D | Show free space per mount (df-like) |
| findmnt -o TARGET,SOURCE,FSTYPE,OPTIONS | Pick columns |
In CI
A "Read-only file system" (EROFS) error means the target path is on an ro mount; findmnt <path> confirms it and write to a rw location like /tmp instead. If /tmp is a small tmpfs, large temp files fill RAM and fail; findmnt -D /tmp shows its size. Overlay mounts indicate you are inside a container, where the writable layer may be size-capped.
Common errors in CI
"touch: cannot touch ‘/x’: Read-only file system" is an ro mount; pick a writable path. "No space left on device" (ENOSPC) on a tmpfs means the RAM-backed mount filled, not the disk; check findmnt -D. A bind-mounted source from the host that does not exist shows up as a surprising path in the mount table.