man ps

• 加横线是 standard syntax
• 不加横线是 BSD syntax

To see every process on the system using standard syntax:
ps -e
ps -ef
ps -eF
ps -ely

   To see every process on the system using BSD syntax:
      ps ax
      ps axu

以 a 开头的作用

a Lift the BSD-style "only yourself" restriction, which is imposed upon the set of all processes when some BSD-style (without "-") options are used or when the ps personality setting is BSD-like. The set of processes selected in this manner is in addition to the set of processes selected by other means. An alternate description is that this option causes ps to list all processes with a terminal (tty), or to list all processes when used together with the x option.