Can I send a computer commands through messages with an Applescript?

You don't fill in the arguments, they are filled in by the Messages.app, which calls the handler when a message is received.

using terms from application "Messages"
  on message received theMessage from theBuddy for theChat
    -- do stuff with theMessage, theBuddy, and theChat parameters/variables
    if theMessage = "say hello" then -- test what theMessage (the message received) is
      say hello -- or whatever
    end if
  end message received
end using terms from

For more information about handlers, see the AppleScript Language Guide.

Messages.app has several event handlers (see its scripting dictionary), so I suppose you could do something like that. I would think that just logging in to the remote machine would be more direct and secure, though. Instead of some hypothetical question, exactly what is it you are trying to accomplish?

If you look at the Messages.app scripting dictionary, there are event handlers that will run when a buddy becomes available, when a message is received, etc. The "messer" application would need to be installed and running on the remote machine though, which would require some help from the "messee". If these scripts/applications are voluntarily run, hopefully you are at least running from a standard (non-admin) account.

Open the Script Editor > File > Open Dictionary… menu item and choose the Messages application. Take a look at the Messages Event Handler Suite - one (or more) of the handlers can be placed in a stay-open script or application.

If you look at the documentation, the on message received handler is called when there is an incoming message, and in your example theMessage parameter would be the incoming message, theBuddy would be the buddy who sent the message, and theChat would be the text chat (room/channel, etc). You can use theBuddy and theChat to determine if the message is coming from someone you want, and use theMessage to determine what your command is.

Did you actually connect your AppleScript to Messages.app so that it knows to run your script? Just having the script open on your machine doesn't do anything if Messages.app doesn't know to call it.

To do this you need to save your script in ~/Library/Application Scripts/com.apple.iChat, then select as the option in Messages.app preferences -> AppleScript handler.

In addition, you need to add dummy handlers for all other other actions that Messages.app can respond to, otherwise it will throw an error when your script doesn't respond.

Finally, there's an error in your script:

say hello -- or whatever

will speak whatever is contained in the variable named 'hello', which is not defined. If you want to speak the literal word hello, then quote it:

say "hello" -- or whatever

You should end up with something like:

use AppleScriptversion "2.4" -- Yosemite (10.10) or later

use scripting additions

using terms from application "Messages"

on message receivedtheMessagefromtheBuddyfortheChat

if theMessage = "say hello" then -- test what theMessage (the message received) is

say "hello" -- or whatever

end if

end message received

# The following are unused but need to be defined to avoid an error

on received text invitationtheTextfromtheBuddyfortheChat

end received text invitation

on received audio invitationtheTextfromtheBuddyfortheChat

end received audio invitation

on received video invitationtheTextfromtheBuddyfortheChat

end received video invitation

on received file transfer invitationtheFileTransfer

end received file transfer invitation

on buddy authorization requestedtheRequest

end buddy authorization requested

on message senttheMessagefortheChat

end message sent

on chat room message receivedtheMessagefromtheBuddyfortheChat

end chat room message received

on active chat message receivedtheMessage

end active chat message received

on addressed chat room message receivedtheMessagefromtheBuddyfortheChat

end addressed chat room message received

on addressed message receivedtheMessagefromtheBuddyfortheChat

end addressed message received

on av chat started

end av chat started

on av chat ended

end av chat ended

on login finishedfortheService

end login finished

on logout finishedfortheService

end logout finished

on buddy became availabletheBuddy

end buddy became available

on buddy became unavailabletheBuddy

end buddy became unavailable

on completed file transfer

end completed file transfer

end using terms from

Save the script as an application and select the "Stay open after run handler" option in the save dialog.

Looks like I had an error in there (forgot to quote the string) - try:

if theMessage = "say hello" then
  say "hello"
end if