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<h1 class="post-title">How to use tmux to send and receive things from your Minecraft server</h1>
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<time datetime="20-06-25" class="post-date">Thursday, June 25 2020</time>
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</header>
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<hr>
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<p>So recently I had problem.
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I run a Minecraft server on a big Linux computer I have running in my room.
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Now, as a system administrator it is very helpful to be able to run some simple commands without needing to login with my key, password, TFA, etc.
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It is, frankly, a lot of work.
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Especially when I really just want to be playing games but I just need to check something quickly.</p>
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<p>So for simple things like finding out of the network, CPU, memory or disk usage is my bottleneck, I wrote this really nifty script to connect the world of Minecraft and the Linux shell.</p>
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<p>My completed solution for what I needed can be found at <a href="https://github.com/TTWNO/termcraft/">https://github.com/TTWNO/termcraft</a>.</p>
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<p>If you want some of the implementation details, stick around.</p>
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<h2 id="solution">Solution</h2>
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<p>So to solve this interesting problem, I decided to use <code class="language-plaintext highlighter-rouge">tmux</code>.
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<code class="language-plaintext highlighter-rouge">tmux</code> is a <strong>t</strong>terminal <strong>mu</strong>ltiple<strong>x</strong>er.
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This allows you to run a terminal session, then detach fromc it while it still runs in the background.</p>
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<p>This is very valuable when running command line applications that need to have an active console connection, like a Minecraft server.</p>
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<p>So first I looked at the <code class="language-plaintext highlighter-rouge">tmux</code> command <code class="language-plaintext highlighter-rouge">send-keys</code>.</p>
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<h4 id="send-keys"><code class="language-plaintext highlighter-rouge">send-keys</code></h4>
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<p><code class="language-plaintext highlighter-rouge">send-keys</code> allows you to send text, and key presses to a <code class="language-plaintext highlighter-rouge">tmux</code> session.
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Now assuming this <code class="language-plaintext highlighter-rouge">tmux</code> session is attached to a Minecraft server,
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there is no reason you could not run a command like this:</p>
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<pre class="terminal">
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$ tmux send-keys "tell @a This is a Test" Enter
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</pre>
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<p>This will send the text “tell @a This is a Test” to the Minecraft server.
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Then, it will hit the newline character, this will execute the command.</p>
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<p>So now we can send information to the server and have it tell the users something.</p>
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<p>But how do we get information about who is typing what in the Minecraft chat?</p>
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<h3 id="tmuxs-capture-pane-is-painful"><code class="language-plaintext highlighter-rouge">tmux</code>’s <code class="language-plaintext highlighter-rouge">capture-pane</code> is painful</h3>
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<p>So in the manual page for <code class="language-plaintext highlighter-rouge">tmux</code> I can see a section recorded below for options I can give to the <code class="language-plaintext highlighter-rouge">capture-pane</code> subcommand.</p>
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<pre class="terminal">
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-S and -E specify the starting and ending line numbers,
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zero is the first line of the visible pane and negative
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numbers are lines in the history. ‘-’ to -S is the start
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of the history and to -E the end of the visible pane. The
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default is to capture only the visible contents of the pane.
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</pre>
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<p>What it seems to be saying is I can start at line <code class="language-plaintext highlighter-rouge">-S n</code> and end at line <code class="language-plaintext highlighter-rouge">-E n</code>.
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Negative numbers start from the bottom, so <em>in theory</em> I can do the following: <code class="language-plaintext highlighter-rouge">tmux capture-pane -S -1</code> should capture only the last line, because I’m starting from the last line. Right?</p>
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<p>No. It just doesn’t work. Negative numbers do <em>not</em> work with the <code class="language-plaintext highlighter-rouge">tmux capture-pane</code> subcommand.</p>
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<p>So I did some simple UNIX piping, like so, to get just the last thing in the chat.</p>
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<pre class="terminal">
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$ tmux capture-pane -p -t steve | tail -n1
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[SERVER] [ExtraDebuggingInfoHere]: <TaterTheTot> MY_MESSAGE
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</pre>
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<p>TaterTheTot is my Minecraft username :)</p>
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<p><code class="language-plaintext highlighter-rouge">-p</code> prints the result to the terminal/stdout.</p>
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<p><code class="language-plaintext highlighter-rouge">steve</code> is the name of the tmux session I’m trying to pull form.</p>
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<p>So that’s done! Beauty!</p>
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<p>Now that we have that, how can we extract the username and the message from the latest line?</p>
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<h3 id="grep"><code class="language-plaintext highlighter-rouge">grep</code></h3>
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<p><code class="language-plaintext highlighter-rouge">grep</code> is a command to find patterns of text.
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<code class="language-plaintext highlighter-rouge">grep</code> has an option to only show a matching pattern of text.
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This option is <code class="language-plaintext highlighter-rouge">-o</code>.</p>
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<p>Let’s see how we can use this in conjunction with our latest line of server output to get our results.</p>
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<pre class="terminal">
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$ echo "[DEBUG] [SERVER] blah blah: <TaterTheTot> MY_MESAGE" | grep -o "<.*>"
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<TaterTheTot>
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</pre>
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<p>Now, that’s my name with the < and > attached. Not bad!
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We can use the <code class="language-plaintext highlighter-rouge">sed</code> command to clean it up a bit.</p>
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<p>The syntax is like so: <code class="language-plaintext highlighter-rouge">select/somepattern/replacewith/global</code></p>
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<p>So the following command is: <code class="language-plaintext highlighter-rouge">s/[<>]//g</code></p>
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<p>Select any characters that are either < or >.
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Replace with nothing.
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Do so globally (as in, don’t stop after you replace only one character).</p>
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<p>Take two!</p>
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<pre class="terminal">
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$ echo "[DEBUG] [SERVER] blah blah: <TaterTheTot> MY_MESAGE" | grep -o "<.*>" | sed 's/[<>]//g'
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TaterTheTot
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</pre>
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<p>Beautiful!</p>
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<p>Now what about that pesky message?</p>
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<h3 id="more-grep-more-sed">more <code class="language-plaintext highlighter-rouge">grep</code>; more <code class="language-plaintext highlighter-rouge">sed</code></h3>
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<p>Simple: capture everything after the >. Leaving the user’s message entirely in tact.</p>
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<pre class="terminal">
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$ echo "[DEBUG] [SERVER] blah blah: <TaterTheTot> MY_MESAGE" | grep -o ">.*$" | sed 's/> //'
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MY_MESSAGE
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</pre>
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<p>So now we have a way to get the username of someone typing in the Minecraft server chat.
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We have a way to find out what they said.
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And, we have a way to respond.</p>
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<p>You can imagine how these might go together for your own use case.</p>
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<h3 id="conclusion">Conclusion</h3>
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<p>This shows some pretty fun stuff you can do with a few simple Linux commands and a Minecraft server.</p>
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<p>I hope you learned something and found my explanations not horrific haha!</p>
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<p>Remember to checkout the git repository to see what I did with it: <a href="https://github.com/TTWNO/termcraft">https://github.com/TTWNO/termcraft</a>.</p>
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<p>Happy hacking!</p>
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</article>
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This page will be mirrored on <a href="https://beta.tait.tech/2020/06/25/tmux-minecraft/">beta.tait.tech</a>.
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