How macOS actually works

The file behaviours nobody documents

Updated 2026-07-31 · 4 min read

A handful of invisible mechanisms cause most of macOS's 'why did it do that' file moments.

.DS_Store

Finder's per-folder view memory — icon positions, window size, sort order. One appears in every folder you open. Harmless on a Mac, an eyesore on shared drives and in zip files sent to Windows users.

find . -name .DS_Store -delete

Stop them on network shares:

defaults write com.apple.desktopservices DSDontWriteNetworkStores -bool true

._ dot-underscore files (AppleDouble)

Copy a Mac file to a FAT/exFAT drive and macOS splits its metadata into a companion ._filename. Harmless on a Mac, clutter everywhere else. dot_clean merges them back:

dot_clean /Volumes/USBDrive

Firmlinks

Under APFS your Mac is two volumes — a read-only System and your Data — stitched into one apparent tree by firmlinks. It is why /Users and /System/Volumes/Data/Users are the same place, and why a path can look duplicated in Terminal. Not something to touch; useful to recognise.

Resource forks

The old Mac way of attaching data to a file, mostly gone, occasionally lingering:

ls -l@ file           # @ column shows extended attrs incl. forks
xattr -c file         # strip them

Stationery pads and locked files

A file's Get Info has Stationery Pad (opening it makes a copy — a template) and Locked (a flag, not a permission). Both surprise people who ticked them by accident:

chflags nouchg file       # unlock

The theme

None of this is broken; all of it is invisible. Recognising the mechanism turns a baffling Finder moment into a two-word explanation — which is most of what mastering a Mac's files amounts to.

Common questions

What creates ._ files on my USB drive?

macOS storing a Mac file's metadata separately on filesystems that cannot hold it natively. dot_clean merges them.

Why does /Users appear in two places in Terminal?

APFS firmlinks join the read-only System volume and your Data volume into one tree; the paths reach the same files.