dscl . -list /Groups PrimaryGrou

[omnium:~] admin% dscl . -list /Groups PrimaryGroupID| awk '{print $2}' | sort -n
-2
-1
0
1
2
3
4
5
6
7
8
9
10
12
16
20
25
26
27
28
29
30
31
45
50
51
52
53
54
55
56
57
58
59
60
61
62
65
66
67
68
69
70
72
73
74
75
76
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
101
102
103
107
120
213