defaults: preferences as data

Every app's settings are a plist keyed by a domain (usually the bundle id). defaults talks to cfprefsd, the daemon that owns them. Always go through defaults; never edit the .plist file directly. cfprefsd caches, and a file edit is overwritten or ignored.

Read

defaults read com.apple.dock autohide          # one key  -> 1
defaults read-type com.apple.dock autohide     # -> Type is boolean
defaults read -g AppleInterfaceStyle           # -g = global domain (NSGlobalDomain) -> Dark
defaults read com.apple.finder                 # whole domain
defaults domains | tr ',' '\n'                 # every domain you have
defaults find ShowPathbar                      # search all domains for a key or value
defaults export com.apple.dock - | plutil -p - # whole domain as a plist, readable

The domain/default pair of (X, Y) does not exist is not an error about your command: it means the key was never set and the app is using its built-in default. Writing the key creates it.

Write — always state the type

defaults write com.apple.finder AppleShowAllFiles -bool true
defaults write com.apple.dock tilesize -int 48
defaults write com.apple.screencapture location -string "$HOME/Pictures/Screenshots"
defaults write local.example list -array a b c
defaults write local.example map -dict-add key value
defaults delete com.apple.dock tilesize        # back to the app's default

Without -bool/-int/-float, the value is written as a string. Some APIs coerce "true", many don't, and you can't tell which from outside; write the real type. Check with read-type after writing.

Make it take effect

Most apps read preferences at launch. After a write:

killall Dock          # Dock, Mission Control, Launchpad
killall Finder        # Finder
killall SystemUIServer # menu bar extras

For any other app: quit and reopen it. Some system settings only apply at next login. Killing an app the user has open loses their unsaved state in it; say so before you do it.

Find the key behind a setting (the diff method)

There is no index of defaults keys. Find one by watching it change:

defaults read > /tmp/before.txt            # every domain, ~700 KB
# flip the setting once in System Settings or the app's Settings window
defaults read > /tmp/after.txt
diff /tmp/before.txt /tmp/after.txt

Narrow it once you know the domain: defaults read com.apple.finder > before. Also diff defaults -currentHost read for per-machine settings (stored under ByHost/), which a plain read does not show.

Traps

• Sandboxed apps keep preferences in a container. For many App Store and Apple apps the file lives at ~/Library/Containers/<bundle-id>/Data/Library/Preferences/<bundle-id>.plist, and reading it may need Full Disk Access for your terminal (mac-permissions skill). defaults read <bundle-id> resolves the container for you when access allows.
• defaults delete <domain> leaves the file behind. Verified: after deleting a whole domain, defaults read says it doesn't exist but the .plist file is still in ~/Library/Preferences/. Harmless; remove the file only if you created it.
• Managed (MDM) preferences win. On a work Mac a profile can force a value; your write succeeds and is ignored. defaults read shows the forced value.
• Some settings are not in defaults at all: anything gated by TCC, most of System Settings' privacy panes, and per-user settings stored in databases. Those are mac-permissions and mac-automation territory.

Script a whole setup

Put the writes in one script, then one killall per affected app at the end. Back up first: defaults export com.apple.dock ~/dock-backup.plist, and restore with defaults import com.apple.dock ~/dock-backup.plist.