This is a tool for powerful automation of OS X. At its core, Hammerspoon is just a bridge between the operating system and a Lua scripting engine. What gives Hammerspoon its power is a set of extensions that expose specific pieces of system functionality, to the user.
What can it do for me?
You can write Lua code that interacts with OS X APIs for applications, windows, mouse pointers, filesystem objects, audio devices, batteries, screens, low-level keyboard/mouse events, clipboards, location services, wifi, and more.
If you want to explore the options Hammerspoon offers, check out the Getting Started Guide and the full API documentation as well as the already pre-made plugins called Spoons.
Typically you would write a configuration file in Lua that connects events to actions. You might want to bind a keyboard shortcut to a series of window operations, or an applescript. You might want to run a series of commands when your wifi interface connects to your home network. You might want to display an alert when your battery drops below a certain percentage. You might want to do something crazy like have iTunes automatically start playing when your Mac detects you are in Paris.
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