Difference between ps a and ps -a

       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.

       -A     Select all processes.  Identical to -e.

       -a     Select all processes except both session leaders (see getsid(2)) and
              processes not associated with a terminal.

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