守护进程 (daemon) 是生存期长的一种进程。在系统引导装入时引导，仅在系统关闭时才终止。

特征

ps查看：

[lyq@ISELAB ~]$ ps -axj
   PPID     PID    PGID     SID TTY        TPGID STAT   UID   TIME COMMAND
      0       1       1       1 ?             -1 Ss       0  52:38 /usr/lib/systemd/systemd --system --deserialize 21
      0       2       0       0 ?             -1 S        0   0:03 [kthreadd]
      2       3       0       0 ?             -1 I<       0   0:00 [rcu_gp]
      2       4       0       0 ?             -1 I<       0   0:00 [rcu_par_gp]
      2       6       0       0 ?             -1 I<       0   0:00 [kworker/0:0H-events_highpri]
      2       9       0       0 ?             -1 I<       0   0:00 [mm_percpu_wq]
      2      10       0       0 ?             -1 S        0   5:08 [ksoftirqd/0]
      2      11       0       0 ?             -1 I        0  45:01 [rcu_sched]
      2      12       0       0 ?             -1 S        0   0:11 [migration/0]
      2      13       0       0 ?             -1 S        0   0:00 [watchdog/0]
      2      14       0       0 ?             -1 S        0   0:00 [cpuhp/0]
      2      15       0       0 ?             -1 S        0   0:00 [cpuhp/1]
      2      16       0       0 ?             -1 S        0   0:01 [watchdog/1]
      2      17       0       0 ?             -1 S        0   0:08 [migration/1]

man手册中查看 头部 输出的名称：

F       (l)     Flags  (octal and additive) associated with
               the process.
S       (l)     The state of the process.
UID     (f,l)   The user ID number of  the  process  owner;
               the  login  name  is  printed  under the -f
               option.
PID     (all)   The process ID of the process; it is possi-
               ble  to  kill  a  process  if this datum is
               known.
PPID    (f,l)   The process ID of the parent process.
C       (f,l)   Processor utilization for scheduling.
PRI     (l)     The priority of the process; higher numbers
               mean lower priority.
NI      (l)     Nice value; used in priority computation.
ADDR    (l)     The address of the process.
SZ      (l)     The size in blocks of the core image of the
               process.
WCHAN   (l)     The event for which the process is  waiting
               or  sleeping; if blank, the process is run-
               ning.
STIME   (f)     Starting time of the process.
TTY     (all)   The controlling terminal for the process.
TIME    (all)   The  cumulative  execution  time  for   the
               process.
CMD     (all)   The command name; the full command name and
               its arguments  are  written  under  the  -f
               option.

  依赖于OS实现，父进程ID为0的各进程通常是 内核进程，在引导系统时启动(init 是个例外，它是由内核在引导装入时启动的用户层次的命令)。

  内核进程通常存在于系统的整个生命期中，以超级用户特权运行，无控制中断，无命令行。

在Linux当中常用的：