title: why use "use -elf" the result return username with "systemd+"?
body: <p>when I use docker run -itd mysql,then to use ps -elf check the process infomation with &quot;4 S systemd+  257584  257561  1  80   0 - 712611 poll_s Jul17 ?       00:40:16 mysqld&quot;.</p>
  <pre><code>root@xx:/proc/257584/ns# ps -elf | grep mysqld
  4 S systemd+  257584  257561  1  80   0 - 712611 poll_s Jul17 ?       00:40:20 mysqld
  root@xx:/proc/257584/ns# ps -el | grep mysqld
  4 S   999  257584  257561  1  80   0 - 712611 poll_s ?       00:40:21 mysqld
  </code></pre>
  <p>But I use &quot;cat /cat/passwd&quot; can't find username equal to &quot;systemd+&quot;.</p>
  <p>docker Version: 20.10.12</p>
  <p>os  ubuntu20.04</p>