Karls-Mac-mini:~ karl$ ps -Upostfix --libxo=json
ps: illegal option -- -
usage: ps [-AaCcEefhjlMmrSTvwXx] [-O fmt | -o fmt] [-G gid[,gid...]]
          [-g grp[,grp...]] [-u [uid,uid...]]
          [-p pid[,pid...]] [-t tty[,tty...]] [-U user[,user...]]
       ps [-L]