linux - 为啥ps -a 与ps a 两个命令打印出的结果不一样？加横杠与不加横杠有啥区别呢？

monini@monini-ThinkPad-T410:~$ ps -a

  PID TTY          TIME CMD
16837 pts/1    00:00:00 sudo
16856 pts/1    00:00:00 su
16857 pts/1    00:00:00 bash
16891 pts/1    00:00:00 eclipse
16892 pts/1    00:03:13 java
16937 pts/1    00:00:00 dbus-launch
17733 pts/1    00:00:10 java
31743 pts/5    00:00:00 ps

下面是 ps a 打出的结果：

monini@monini-ThinkPad-T410:~$ ps a
  PID TTY      STAT   TIME COMMAND
  803 tty7     Ssl+  31:39 /usr/bin/X -core :0 -seat seat0 -auth /var/run/lightdm/root/:0 -nolisten tcp vt7 -novtswitch
  811 tty1     Ss+    0:00 /sbin/agetty --noclear tty1 linux
16707 pts/1    Ss     0:00 bash
16837 pts/1    S      0:00 sudo su
16856 pts/1    S      0:00 su
16857 pts/1    S      0:00 bash
16891 pts/1    S+     0:00 ./eclipse
16892 pts/1    Sl+    3:13 /usr/bin/java -Dosgi.requiredJavaVersion=1.6 -XX:MaxPermSize=256m -Xms40m -Xmx512m -jar /home/xujianxing/eclipse/eclipse//plugins/org.eclipse.equinox.launcher_1.3.0.v20140415-20
16937 pts/1    S+     0:00 dbus-launch --autolaunch=89513375dda449418c27b0cd9ece4e44 --binary-syntax --close-stderr
17733 pts/1    Sl+    0:10 /usr/lib/jvm/java-7-openjdk-amd64/bin/java -Dfile.encoding=UTF-8 -classpath /home/monini/workspace/shiyu_Web/CommonRestService/target/classes:/home/monini/workspace/shiy
21439 pts/5    Ss     0:00 bash
31771 pts/5    R+     0:00 ps a