Programmatically send an iMessage from Terminal

sendMessage.applescript

on run {targetBuddyPhone, targetMessage}
tell application "Messages"
    set targetService to 1st service whose service type = iMessage
    set targetBuddy to buddy targetBuddyPhone of targetService
    send targetMessage to targetBuddy
end tell
end run

This sends an iMessage to a buddy. Note: If the number you specify isn't in your buddy list, then the command will fail. For security reasons, Apple does not allow you to programmatically start a conversation with a new buddy. However, there is a workaround for this - see https://gist.github.com/alekhinen/b3a112cfbb5cad70e6e2. It's a bit kludgy as it opens up the Messages app and goes through the UI to send a new message, but it works.