Process Identifier (PID)

Whenever a process starts one unique positive integer number is assigned to that called Process Identifier(PID). It is ranged between 2 to 32,768. Number 0 and 1 typically reserved for idle process and special init Process. When a new process is started a next unused in sequence number is assigned to that process.

     All current process status and their PID can be seen by using command

# ps ax  
  PID TTY   STAT  TIME COMMAND  
   1 ?    Ss   0:02 /sbin/init  
   2 ?    S   0:00 [kthreadd]  
   3 ?    S   0:18 [ksoftirqd/0]  
   5 ?    S<   0:00 [kworker/0:0H]  
   7 ?    S   0:29 [rcu_sched]  
   .................  
   ................  
   ................  
  2292 ?    Sl   0:08 /usr/lib/upower/upowerd  
  2367 ?    SNl  0:01 /usr/lib/rtkit/rtkit-daemon  
  2410 ?    Sl   0:00 /usr/lib/colord/colord  
  3987 ?    SLl  0:00 /usr/bin/gnome-keyring-daemon --daemonize --login  
  3989 ?    Ss   0:02 init --user  
  4077 ?    Ss   0:19 dbus-daemon --fork --session --address=unix:abstract=/tmp/dbus-7SdHSkpTgx  
  4088 ?    Ss   0:00 upstart-event-bridge  
  4095 ?    Ss   0:00 /usr/lib/x86_64-linux-gnu/hud/window-stack-bridge  
  4106 ?    Sl   0:17 /usr/lib/x86_64-linux-gnu/bamf/bamfdaemon  
  4112 ?    Sl   0:00 /usr/lib/at-spi2-core/at-spi-bus-launcher  

ps command has many option (-ef, -A..)which gives more resolution. To see more option go to manual page of ps command, type

 #man ps