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  <div class="content">    <div class="post">  <h1 class="post-title">    <a href="http://fr1nge.xyz/posts/supercharge-your-bash-scripts-with-multiprocessing/">Supercharge Your Bash Scripts with Multiprocessing</a></h1>  <div class="post-meta">          <span class="post-date">        2021-05-05 [Updated: 2021-05-05]      </span>            <span class="post-author">:: Yigit Colakoglu</span>      </div>
    <span class="post-tags">        #<a href="http://fr1nge.xyz/tags/bash/">bash</a>&nbsp;        #<a href="http://fr1nge.xyz/tags/scripting/">scripting</a>&nbsp;        #<a href="http://fr1nge.xyz/tags/programming/">programming</a>&nbsp;      </span>  
  
  
  <div class="post-content"><div>        <p>Bash is a great tool for automating tasks and improving you work flow. However,it is <em><strong>SLOW</strong></em>.  Adding multiprocessing to the scripts you write can improvethe performance greatly.</p><h2 id="what-is-multiprocessing">What is multiprocessing?<a href="#what-is-multiprocessing" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h2><p>In the simplest terms, multiprocessing is the principle of splitting thecomputations or jobs that a script has to do and running them on differentprocesses. In even simpler terms however, multiprocessing is the computerscience equivalent of hiring more than oneworker when you are constructing a building.</p><h3 id="introducing-">Introducing &ldquo;&amp;&rdquo;<a href="#introducing-" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h3><p>While implementing multiprocessing the sign <code>&amp;</code> is going to be our greatestfriend.  It is an essential sign if you are writing bash scripts and a veryuseful tool in general when you are in the terminal. What <code>&amp;</code> does is that itmakes the command you added it to the end of run in the background and allowsthe rest of the script to continue running as the command runs in thebackground. One thing to keep in mind is that since it creates a fork of theprocess you ran the command on, if you change a variable that the command in thebackground uses while it runs, it will not be affected.  Here is a simpleexample:</p>


  <div class="collapsable-code">    <input id="1" type="checkbox"  />    <label for="1">      <span class="collapsable-code__language">bash</span>            <span class="collapsable-code__toggle" data-label-expand="Show" data-label-collapse="Hide"></span>    </label>    <pre class="language-bash" ><code>foo=&#34;yeet&#34;
function run_in_background(){  sleep 0.5  echo &#34;The value of foo in the function run_in_background is $foo&#34;}
run_in_background &amp; # Spawn the function run_in_background in the backgroundfoo=&#34;YEET&#34;echo &#34;The value of foo changed to $foo.&#34;wait # wait for the background process to finish</code></pre>  </div>

<p>This should output:</p><pre><code>The value of foo changed to YEET.The value of foo in here is yeet</code></pre><p>As you can see, the value of <code>foo</code> did not change in the background process even thoughwe changed it in the main function.</p><h2 id="baby-steps">Baby steps&hellip;<a href="#baby-steps" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h2><p>Just like anything related to computer science, there is more than one way ofachieving our goal. We are going to take the easier, less intimidating but lessefficient route first before moving on to the big boy implementation. Let&rsquo;s open up vim and get to scripting!First of all, let&rsquo;s write a very simple function that allows us to easily testour implementation:</p>


  <div class="collapsable-code">    <input id="1" type="checkbox"  />    <label for="1">      <span class="collapsable-code__language">bash</span>            <span class="collapsable-code__toggle" data-label-expand="Show" data-label-collapse="Hide"></span>    </label>    <pre class="language-bash" ><code>function tester(){  # A function that takes an int as a parameter and sleeps  echo &#34;$1&#34;  sleep &#34;$1&#34;  echo &#34;ENDED $1&#34;}</code></pre>  </div>

<p>Now that we have something to run in our processes, we now need to spawn severalof them in controlled manner. Controlled being the keyword here. That&rsquo;s becauseeach system has a maximum number of processes that can be spawned (You can findthat out with the command <code>ulimit -u</code>). In our case, we want to limit theprocesses being ran to the variable <code>num_processes</code>. Here is the implementation:</p>


  <div class="collapsable-code">    <input id="1" type="checkbox"  />    <label for="1">      <span class="collapsable-code__language">bash</span>            <span class="collapsable-code__toggle" data-label-expand="Show" data-label-collapse="Hide"></span>    </label>    <pre class="language-bash" ><code>num_processes=$1pcount=0for i in {1..10}; do  ((pcount=pcount%num_processes));  ((pcount&#43;&#43;==0)) &amp;&amp; wait  tester $i &amp;done</code></pre>  </div>

<p>What this loop does is that it takes the number of processes you would like tospawn as an argument and runs <code>tester</code> in that many processes. Go ahead and test it out!You might notice however that the processes are run int batches. And the size ofbatches is the <code>num_processes</code> variable. The reason this happens is becauseevery time we spawn <code>num_processes</code> processes, we <code>wait</code> for all the processesto end. This implementation is not a problem in itself, there are many caseswhere you can use this implementation and it works perfectly fine. However, ifyou don&rsquo;t want this to happen, we have to dump this naive approach all togetherand improve our tool belt.</p><h2 id="real-chads-use-job-pools">Real Chads use Job Pools<a href="#real-chads-use-job-pools" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h2><p>The solution to the bottleneck that was introduced in our previous approach liesin using job pools.  Job pools are where jobs created by a main process get sentand wait to get executed. This approach solves our problems because instead ofspawning a new process for every copy and waiting for all the processes tofinish we instead only create a set number of processes(workers) whichcontinuously pick up jobs from the job pool not waiting for any other process to finish.Here is the implementation that uses job pools. Brace yourselves, because it iskind of complicated.</p>


  <div class="collapsable-code">    <input id="1" type="checkbox"  />    <label for="1">      <span class="collapsable-code__language">bash</span>            <span class="collapsable-code__toggle" data-label-expand="Show" data-label-collapse="Hide"></span>    </label>    <pre class="language-bash" ><code>job_pool_end_of_jobs=&#34;NO_JOB_LEFT&#34;job_pool_job_queue=/tmp/job_pool_job_queue_$$job_pool_progress=/tmp/job_pool_progress_$$job_pool_pool_size=-1job_pool_nerrors=0
function job_pool_cleanup(){        rm -f ${job_pool_job_queue}        rm -f ${job_pool_progress}}
function job_pool_exit_handler(){        job_pool_stop_workers        job_pool_cleanup}
function job_pool_worker(){        local id=$1        local job_queue=$2        local cmd=        local args=
        exec 7&lt;&gt; ${job_queue}        while [[ &#34;${cmd}&#34; != &#34;${job_pool_end_of_jobs}&#34; &amp;&amp; -e &#34;${job_queue}&#34; ]]; do                flock --exclusive 7                IFS=$&#39;\v&#39;                read cmd args &lt;${job_queue}                set -- ${args}                unset IFS                flock --unlock 7                if [[ &#34;${cmd}&#34; == &#34;${job_pool_end_of_jobs}&#34; ]]; then                      echo &#34;${cmd}&#34; &gt;&amp;7                else                      { ${cmd} &#34;$@&#34; ; }                fi
        done        exec 7&gt;&amp;-}
function job_pool_stop_workers(){        echo ${job_pool_end_of_jobs} &gt;&gt; ${job_pool_job_queue}        wait}
function job_pool_start_workers(){        local job_queue=$1        for ((i=0; i&lt;${job_pool_pool_size}; i&#43;&#43;)); do        job_pool_worker ${i} ${job_queue} &amp;        done}
function job_pool_init(){        local pool_size=$1        job_pool_pool_size=${pool_size:=1}        rm -rf ${job_pool_job_queue}        rm -rf ${job_pool_progress}        touch ${job_pool_progress}        mkfifo ${job_pool_job_queue}        echo 0 &gt;${job_pool_progress} &amp;        job_pool_start_workers ${job_pool_job_queue}}
function job_pool_shutdown(){        job_pool_stop_workers        job_pool_cleanup}
function job_pool_run(){        if [[ &#34;${job_pool_pool_size}&#34; == &#34;-1&#34; ]]; then        job_pool_init        fi        printf &#34;%s\v&#34; &#34;$@&#34; &gt;&gt; ${job_pool_job_queue}        echo &gt;&gt; ${job_pool_job_queue}}
function job_pool_wait(){        job_pool_stop_workers        job_pool_start_workers ${job_pool_job_queue}}</code></pre>  </div>

<p>Ok&hellip; But that the actual fuck is going in here???</p><h3 id="fifo-and-flock">fifo and flock<a href="#fifo-and-flock" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h3><p>In order to understand what this code is doing, you first need to understand twokey commands that we are using, <code>fifo</code> and <code>flock</code>. Despite their complicatednames, they are actually quite simple. Let&rsquo;s check their man pages to figure outtheir purposes, shall we?</p><h4 id="man-fifo">man fifo<a href="#man-fifo" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h4><p>fifo&rsquo;s man page tells us that:</p><pre><code>NAME       fifo - first-in first-out special file, named pipe
DESCRIPTION       A  FIFO  special  file (a named pipe) is similar to a pipe, except that       it is accessed as part of the filesystem.  It can be opened by multiple       processes for reading or writing.  When processes are exchanging data       via the FIFO, the kernel passes all data internally without writing it       to the filesystem.  Thus, the FIFO special file has no contents on the       filesystem; the filesystem entry merely serves as a reference point so       that processes can access the pipe using a name in the filesystem.</code></pre><p>So put in <strong>very</strong> simple terms, a fifo is a named pipe that can allowscommunication between processes. Using a fifo allows us to loop through the jobsin the pool without having to delete them manually, because once we read themwith <code>read cmd args &lt; ${job_queue}</code>, the job is out of the pipe and the nextread outputs the next job in the pool. However the fact that we have multipleprocesses introduces one caveat, what if two processes access the pipe at thesame time? They would run the same command and we don&rsquo;t want that. So we resortto using <code>flock</code>.</p><h4 id="man-flock">man flock<a href="#man-flock" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h4><p>flock&rsquo;s man page defines it as:</p><pre><code> SYNOPSIS        flock [options] file|directory command [arguments]        flock [options] file|directory -c command        flock [options] number
 DESCRIPTION        This utility manages flock(2) locks from within shell scripts or from        the command line.
        The  first  and  second of the above forms wrap the lock around the        execution of a command, in a manner similar to su(1) or newgrp(1).        They lock a specified file or directory, which is created (assuming        appropriate permissions) if it does not already exist.  By default, if        the lock cannot be immediately acquired, flock waits until the lock is        available.
        The third form uses an open file by its file descriptor number.  See        the examples below for how that can be used.</code></pre><p>Cool, translated to modern English that us regular folks use, <code>flock</code> is a thinwrapper around the C standard function <code>flock</code> (see <code>man 2 flock</code> if you areinterested). It is used to manage locks and has several forms. The one we areinterested in is the third one. According to the man page, it uses and open fileby its <strong>file descriptor number</strong>. Aha! so that was the purpose of the <code>exec 7&lt;&gt; ${job_queue}</code> calls in the <code>job_pool_worker</code> function. It would essentiallyassign the file descriptor 7 to the fifo <code>job_queue</code> and afterwards lock it with<code>flock --exclusive 7</code>. Cool. This way only one process at a time can read fromthe fifo <code>job_queue</code></p><h2 id="great-but-how-do-i-use-this">Great! But how do I use this?<a href="#great-but-how-do-i-use-this" class="hanchor" ariaLabel="Anchor">&#8983;</a> </h2><p>It depends on your preference, you can either save this in a file(e.g.job_pool.sh) and source it in your bash script.  Or you can simply paste itinside an existing bash script. Whatever tickles your fancy. I have alsoprovided an example that replicates our first implementation. Just paste thebelow code under our &ldquo;chad&rdquo; job pool script.</p>


  <div class="collapsable-code">    <input id="1" type="checkbox"  />    <label for="1">      <span class="collapsable-code__language">bash</span>            <span class="collapsable-code__toggle" data-label-expand="Show" data-label-collapse="Hide"></span>    </label>    <pre class="language-bash" ><code>function tester(){  # A function that takes an int as a parameter and sleeps  echo &#34;$1&#34;  sleep &#34;$1&#34;  echo &#34;ENDED $1&#34;}
num_workers=$1job_pool_init $num_workerspcount=0for i in {1..10}; do  job_pool_run tester &#34;$i&#34;done
job_pool_waitjob_pool_shutdown</code></pre>  </div>

<p>Hopefully this article was(or will be) helpful to you. From now on, you don&rsquo;tever have to write single threaded bash scripts like normies :)</p>
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