title: what is UID in linux?
body: <p>what is UID in linux?
  Wiki and other online sources say it is user ID. But what exactly is user id?
  The name "user id" gives me a sense that it is the user who is using the OS. Like If I have only one user, my user id should be 1. For root, I understand that it is 0.</p>
  
  <p>But I see that the user id is given to every process(application ?). So, does every process has a different uid? Why is it needed when we have pid already to distinguish them.</p>
  
  <pre><code>Ex :
  root:/ # ps 8533
  USER     PID   PPID  VSIZE  RSS     WCHAN    PC        NAME
  u0_a78    8533  363   1526628 52668 ffffffff b6f80d04 S com.google.android.keep
  
  root:/ # cat /proc/8533/status
  Name:   le.android.keep
  State:  S (sleeping)
  Tgid:   8533
  Pid:    8533
  PPid:   363
  TracerPid:  0
  Uid:    10078   10078   10078   10078
  ....
  </code></pre>
  
  <p>So here for the process google keep, the UID is 10078. So, are there more than 10000 users? If yes, then Who exactly is a user?</p>