usermod -a表示在原來所屬組的基礎上追加

groupadd group1

groupadd group2

useradd user1

usermod -G group1 user1

id user1

uid=1006(user1) gid=1014(user1) 組=1014(user1),1013(group1)

usermod -G group2 user1

id user1

uid=1006(user1) gid=1014(user1) 組=1014(user1),1010(group2)

useradd user2

usermod  -G group1 user2

id user2

uid=1007(user2) gid=1015(user2) 組=1015(user2),1013(group1)

usermod -a -G group2 user2

id user2

uid=1007(user2) gid=1015(user2) 組=1015(user2),1010(group2),1013(group1)