title: What characterizes a Docker image or container as active?
body: <p>One can use the command <a href="https://docs.docker.com/engine/reference/commandline/system_df/" rel="nofollow noreferrer"><code>docker system df</code></a> (<a href="https://web.archive.org/web/20180602170331/https://docs.docker.com/engine/reference/commandline/system_df/" rel="nofollow noreferrer">mirror</a>) (introduced in Docker 1.13.0) to see docker disk usage, e.g.:</p>
  
  <pre><code>username@server:~$ docker system df
  TYPE                TOTAL               ACTIVE             SIZE                RECLAIMABLE
  Images              44                  28                 114.7GB             84.84GB (73%)
  Containers          86                  7                  62.43GB             41.67GB (66%)
  Local Volumes       2                   1                  0B                  0B
  Build Cache                                                0B                  0B
  </code></pre>
  
  <p>What does "active" mean? I.e., what characterizes an image or a container as active?</p>
  
  <p>The Docker <a href="https://docs.docker.com/engine/reference/commandline/system_df/" rel="nofollow noreferrer">documentation on <code>docker system df</code></a> (<a href="https://web.archive.org/web/20180602170331/https://docs.docker.com/engine/reference/commandline/system_df/" rel="nofollow noreferrer">mirror</a>) doesn't explain it. The <a href="https://docs.docker.com/glossary/" rel="nofollow noreferrer">Docker glossary</a> (<a href="https://web.archive.org/web/20180602171845/https://docs.docker.com/glossary/" rel="nofollow noreferrer">mirror</a>) doesn't contain the term "active".</p>
  
  <p>I understand that:</p>
  
  <ul>
  <li>"active" container means that the container is currently running (active containers can be listed with <a href="https://docs.docker.com/engine/reference/commandline/ps/" rel="nofollow noreferrer"><code>docker ps</code></a> or <a href="https://stackoverflow.com/q/45254677/395857">equivalently</a> <a href="https://docs.docker.com/engine/reference/commandline/container_ls/" rel="nofollow noreferrer"><code>docker container ls</code></a>). </li>
  </ul>