user18155: In the FFmpeg Filters Documentation some filters have a subsection "Commands" which begins with the sentence

This filter supports the following commands:

e. g. in the section 6.68 volume.
Filters have parameters; it is understandable. But what is the meaning of filter commands and how and when are they used?

user1871: Commands allow you to change the parameters during execution of the command. This is primarily useful when working with streaming media e.g. if the volume gets too loud during a broadcast, send a command to the volume filter to lower it.
To issue commands to filters which support it from outside, ffmpeg has to be compiled with libzmq support. See the docs for the zmq filter for details.
To issue commands from the terminal, press c
The 'command prompt' will be shown :
Enter command: <target>|all <time>|-1 <command>[ <argument>]

Enter a command as per the given format and press Enter.
e.g. overlay@logo -1 x 100

18155: <p>In the <a href="http://ffmpeg.org/ffmpeg-filters.html" rel="nofollow noreferrer">FFmpeg Filters Documentation</a> some filters have a subsection "Commands" which begins with the sentence</p>

<blockquote>
  <p>This filter supports the following commands:</p>
</blockquote>

<p>e. g. in the section <a href="http://ffmpeg.org/ffmpeg-filters.html#volume" rel="nofollow noreferrer">6.68 volume</a>.</p>

<p>Filters have <em>parameters</em>; it is understandable. But what is the meaning of filter <em>commands</em> and how and when are they used?</p>
 1871: <p>Commands allow you to change the parameters <em>during</em> execution of the command. This is primarily useful when working with streaming media e.g. if the volume gets too loud during a broadcast, send a command to the volume filter to lower it.</p>

<p>To issue commands to filters which support it from outside, ffmpeg has to be compiled with libzmq support. See the <a href="https://ffmpeg.org/ffmpeg-filters.html#zmq_002c-azmq" rel="nofollow noreferrer">docs</a> for the zmq filter for details.</p>

<p>To issue commands from the terminal, press <kbd>c</kbd></p>

<p>The 'command prompt' will be shown :</p>

<pre><code>Enter command: &lt;target&gt;|all &lt;time&gt;|-1 &lt;command&gt;[ &lt;argument&gt;]
</code></pre>

<p>Enter a command as per the given format and press <kbd>Enter</kbd>.</p>

<p>e.g. <code>overlay@logo -1 x 100</code></p>
