Linux 显示所有进程

ps -u

shuai@ubuntu:~$ ps -u 
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
shuai    16912  0.0  0.5  24316  5412 pts/0    Ss   11:29   0:00 bash
shuai    20095  0.0  0.3  39352  3420 pts/0    R+   11:47   0:00 ps -u
shuai@ubuntu:~$ ps -u root
  PID TTY          TIME CMD
    1 ?        00:00:12 systemd
    2 ?        00:00:00 kthreadd
    3 ?        00:00:00 ksoftirqd/0
    5 ?        00:00:00 kworker/0:0H
    7 ?        00:00:32 rcu_sched
    8 ?        00:00:00 rcu_bh
    9 ?        00:00:01 migration/0
   10 ?        00:00:00 lru-add-drain
   11 ?        00:00:00 watchdog/0
   12 ?        00:00:00 cpuhp/0
   13 ?        00:00:00 cpuhp/1
   14 ?        00:00:00 watchdog/1
   15 ?        00:00:01 migration/1
   16 ?        00:00:00 ksoftirqd/1
   18 ?        00:00:00 kworker/1:0H
   19 ?        00:00:00 kdevtmpfs
   20 ?        00:00:00 netns
   21 ?        00:00:00 khungtaskd
   22 ?        00:00:00 oom_reaper
   23 ?        00:00:00 writeback
   24 ?        00:00:00 kcompactd0
   25 ?        00:00:00 ksmd
   26 ?        00:00:02 khugepaged
   27 ?        00:00:00 crypto
   28 ?        00:00:00 kintegrityd
   29 ?        00:00:00 bioset
   30 ?        00:00:00 kblockd
   31 ?        00:00:00 ata_sff
   32 ?        00:00:00 md
   33 ?        00:00:00 devfreq_wq

root 下的进程中有一个 PID = 1 的进程：init 进程
一切用户态进程都是它的后代进程。