is this useradd statements correct?
 

i m still kind of new to linux, i have done man useradd and man groupadd, stil unsure, pls check this for me:

objective:
- add group a, b,
- add user u1 into both group, and root into group b

is the following stmts correct?

groupadd a
groupadd b
useradd -g a u1
useradd -g b -M u1
useradd -g b -M root

thank you.

For the secondary group use -G. The -g sets the primary group, so the fourth command resets the primary group of u1 from a to b (i.e. u1 belongs to group b, not a and b). You could combine lines 3 and 4 into one line: Also, the root account is a default account so it will already exist. The command will give you an error. Use usermod to change the profile of an existing account. Your command becomes Note: The effects do not show until you relogin.

the groupadd commands are correct, but don't use useradd to modify existing accounts. instead, use usermod. so the command would be, for example,