How do I find the windowid to pass to screencapture -l?

For some applications you can use AppleScript:

screencapture -l$(osascript -e 'tell app "Safari" to id of window 1') test.png

It doesn't work with Chrome though.

The IDs are also shown in Quartz Debug (available from developer.apple.com/downloads) if you run defaults write com.apple.QuartzDebug QuartzDebugPrivateInterface -bool true.

I wrote a little command line utility to retrieve the Window ID for apps that don't support AppleScript. Get it here: https://github.com/smokris/GetWindowID

You can then capture a specific window by specifying its bundle name and window title:

screencapture -l$(./GetWindowID "Vuo Editor" "untitled composition") VuoEditorWindow.png

I wrote a command line utility that wraps over screencapture and the Quartz windowing library.

Grab it here: https://github.com/alexdelorenzo/screenshot

Use it like this:

screenshot Chrome -t "Stack Overflow"