Usermod-a indicates to append an object to the original group.

Usermod-a indicates to append the groupaddgroup1groupaddgroup2useradduser1usermod-Ggroup1user1iduser1uid1006 (user1) gid1014 (user1) group 1014 (user1), 1013 (group1) on the basis of the original group) usermod-a indicates adding groupadd group1 groupadd group2 useradd user1 usermod-G group1 user1 id user1 uid = 1006 (user1) gid = 1014 (user1) group = 1014 (user1), 1013 (group1) usermod-G group2 user1 id user1 uid = 1006 (user1) gid = 1014 (user1) group = 1014 (user1 ), 1010 (group2) useradd user2 usermod-G group1 user2 id user2 uid = 1007 (user2) gid = 1015 (user2) group = 1015 (user2), 1013 (group1) usermod-a-G group2 user2 id user2 uid = 1007 (user2) gid = 1015 (user2) group = 1015 (user2), 1010 (group2), 1013 (group1)