Hal is a artificial intelligence I wrote for the Minecraft server I run. Here I hope to shed some light onto what he does and how he works. I'm going to run with the anthropomorphized 'he' even though Hal is really just a bunch of shell code. If you want to try him out anytime, you can through my online demo here.

Let's dive right in!

What is Hal?

Hal is a text based artificial intelligence intended to make playing Minecraft on a server easier and more fun.

How does he work?

Whenever a player types something in chat or certain events happen, some text is written to a log file. Basically, Hal monitors this log file for changes, grabs those changes, and then looks for something he can act on. In the context of Minecraft, these are commands for changing the weather, time of day, moving the player around, telling jokes, or just chatting. For example:

      <Steve> Hal tell me a joke
      [Hal] Whats Cobblestones favorite music? Rock music.

Hal saw "<Steve> Hal tell me a joke" in the log file, recognized his name and tried to figure out what to do by looking for additional words of phrases he recognizes. If you have some familiarity with programming, you've probably encountered string comparison and regular expressions before. If you're not, don't worry; they're pretty straight forward. We all have some experience with regular expressions, even if you didn't know it. When you see something like

      Take the child(ren) to the park

You know that this really means

      Take the child OR the children to the park

If you check out Hal's source code, you'll see he's basically a regular expression machine. Did the player say 'hello'? If yes, then I reply 'Hey there player!'. Otherwise, keep trying to find a match. One of Hal's neat features is that he can key off multiple phrases in the same line. For example

      <Steve> Hey hal, take me home. Oh, and make it day
      [Hal] Hello there Steve!
      [Hal] Sorry Steve, either you never told me where home was or I forgot!
      [Hal] Sunshine on the way!
      /time set day

What can he do?

  • Hold a basic conversation
  Hey Hal, how are you?
  Howdy Hal, tell me a joke
  whatever Hal
  • Change weather or time of day
  Hal make it sunny and make it daytime
  • Change player gamemode
  Hal put me in creative mode
  • Apply status effects to players
  Hal make me healthy
  Hal make me invisible
  • Send messages to other players, and save them for the next time they log in if they're not currently in game
  Hal tell Notch to add more features
  • Teleport the player to other players, locations specified in a configuration file or a user specified home
  Hal take me to the castle
  Hal take me to Steve
  Hal set home as 465 132 798
  take me home Hal
  • Remember, recall and forget arbitrary information
  Hal remember my favorite color is blue
  tell me about my favorite color Hal
  Hal forget about my favorite color
  • Answer math problems
  Hal what's 5 * 5 + 36 ^ 1.27
  • Answer questions about various topics
  tell me about math Hal

How is he organized?

Hal's functionality is broken up into independent modules. For instance, Hal is capable of remembering things for the players.

      <Steve> Hal remember that my favourite color is green
      [Hal] Okay Steve, I'll remember!
      <Steve> Hal tell me about my favourite color
      [Hal] Okay Steve, here's what I know about "my favourite color":
      [Hal] "my favourite color is green"

The code that handles this logic is in Likewise, there's seperate modules for chatting, handling future events through intentions, and teleporting the players around the map. When a new line from the player is seen, you can think of Hal handing the line to each of the modules in turn to see if they can do anything with it.

Bash as a programming language

Hal is written entirely in Bash, a Unix (and now Windows?) scripting language. This might seem like an odd choice, and in many ways it is, but it has afforded a couple advantages over other programming languages.

For one, regular expressions are very easy to work with. In Bash, you have direct access to grep, sed and friends. Hal is essentially a regular expression matching engine, so easy access to and simple syntax for expressions help quite a bit.

Other advantages include platform independent code, easy access to file system objects, and small size. Hal requires no libraries, has no compile time, and will run on any Linux system with Bash v4 or higher.

Most of Hal's matching is done in case statements, which aren't true regular expressions, but "glob" matching.

There are a number of common complaints about shell code; hard to read, difficult to test, and error prone. These may be valid points, but there are certainly steps that can be taken to address them.

  • Coding standards exist for shell. Following them makes your code safer and easier to read

  • Unit testing is a breeze if you adopt a functional programming paradigm. Moving related functions into modules and ensuring that you have unit tests for each function makes identifying regressions and edge cases possible.

  • There are a number of security features builtin to Bash that we can utilize. The set command contains a number of restricting options, forcing you to adopt more safe programming practices. Use of the local, readonly and export builtins allow more control over variables.

  • A lot of problems can be avoided simply by correct programming practices. Minimize side effects in functions, have clear control flow, and validate all inputs, whether they come from the user or the system.

Snippet from modules/

  69 player_left(){
  70   # : ' none -> none
  71   # Say goodbye, comment on player count
  72   # '
  73   say "Goodbye ${USER}! See you again soon I hope!"
  74   let NUM_PLAYERS--
  76   if (( NUM_PLAYERS < 0 )); then
  77     say "I seem to have gotten confused..."
  78     NUM_PLAYERS=0
  80   elif (( NUM_PLAYERS == 0 )); then
  81     say "All alone..."
  82     QUIET=0
  84   elif (( NUM_PLAYERS == 1 )); then
  85     say "I guess it's just you and me now!"
  86   fi
  88   ran_command
  89 }

The web demo

The demo demo/ is one of the most difficult parts of Hal to understand. It's an HTTP web server in Bash for one, but also handles the task of allowing clients to communicate with Hal through HTTP POST.

This snippet from demo/ shows the key functionality that allow the web server to work.

  • Line 162 starts Hal in debug mode, Hal is only communicated with through HAL_INPUT_FILE and HAL_OUTPUT_FILE, which represent latest.log and stdout when Hal is working off of a real Minecraft server.

  • Lines 165 to 167 is how the demo communicates with clients. nc really wasn't designed for this kind of set up, and has a terrible habit of closing unexpectedly. We can get around this with while true; do ... done. Another problem is that input redirects, e.g. < may block. We solve this by using process substitution and cat. cat is always trying to read from the server2client fifo and does not block. Lastly, what the client sends the server is redirected to the client2server fifo, which is read from in other places.

  • Getting these 3 lines right took just about as long as the rest of the web_server combined. Tricky!

Snippet from /demo/

 158   mkfifo ${server2client} ${client2server}
 160   # start hal
 161   echo "Starting hal..."
 162   bash ../ ${HAL_INPUT_FILE} ../ ${ROOT_DIR} ${HAL_OUTPUT_FILE} &
 163   readonly HAL_PID=$!
 165   while true; do
 166     nc -l -p ${PORT} < <(cat "${server2client}") > "${client2server}"
 167   done &
 169   # start http server
 170   echo "Starting server..."

The complete flow of information from client to hal looks something like this.

  Client Browser -> HTTP POST -> -> File System -> Hal

And the responses (if Hal decides to respond, look like this

  Hal -> File System -> -> HTTP Reply -> Client Browser

Advanced features

Hal would still be neat if all he did was give preset replies to Minecraft chat, but there's more than that built in.

Function callbacks and intentions

Although not used much currently, Hal supports registering callback functions that will be triggered when a regular expression is matched on the input line.

For example, the following code in modules/, allows Hal to ask for confirmation after the user says something like Hal be quiet. Once the callback is set, Hal will look for yes or sure in the next 3 lines that appear. If a match is made, the intent_be_quiet function will be called. Otherwise, Hal will forget and move on.

  16   case "$CLINE" in
  17     *'be quiet'*)
  18       say 'Oh... Are you sure?'
  19       set_intent 'yes|sure' 'intent_be_quiet'
  20       ran_command
  21       ;;

This same functionality allows richer interaction with the Minecraft server as well. For instance, when teleporting a player to another player, Hal set's an intention on the error message the server returns when the player doesn't exist. This way, Hal can tell the player what happened.

Hal can also recognize and answer math questions, and query Wikipedia for answers if the user asks him about a topic he doesn't know about.

([vV]ery )*Fuzzy matching

Hal is very permissive in how inputs are matched to expressions. Even something simple like what's up can be tricky to match correctly due to the number of possible variations in user input.

  • whats up
  • what's up
  • what is up
  • what up

And Hal has to be in there somewhere too, either before whats up or after it in each of those cases. What if there's more text after whats up, or before? What if there isn't? It turns out Bash case statements and globs do a great job here. The order of the case statements allow us to describe matching priorities, and the globs handle the uncertainty. As an added bonus, glob matching is faster than regular expression matching!

For example, consider the following snippet from modules/

  39     *'tell '*' about everything'*)
  40       recall_everything
  41       ;;
  42     *'tell '*' about '*)
  43       recall_phrase
  44       ;;
  45     *'tell '*)
  46       tell_player
  47       ;;

This produces the following desired outcomes

  • Hal, tell me about everything -> list all memories for current player
  • tell me about math Hal -> look through player memories for 'math'. if it doesn't occur anywhere, search Wikipedia for 'math'
  • Hal tell Steve to come over here -> send message "come over here" to player Steve

And that's the basic gist!