Difference between adduser and usermod -G -a

adduser and usermod are two different utilities which have in common the fact that both can add a user to a group.

According to man adduser

adduser is friendlier front ends to the low level tools like useradd, groupadd and usermod programs.

More info:

• man adduser : adduser, addgroup - add a user or group to the system
• man usermod : usermod - modify a user account

At first glance, yes.

At second glance, usermod -G -a user_name group_name is not correct.

The -G option should be followed by the group name(s).

$ sudo usermod -G -a nogroup muru
[sudo] password for muru:
usermod: group '-a' does not exist
$ sudo usermod -a -G muru nogroup
usermod: user 'nogroup' does not exist

The -a can come before -G, or after the group name(s), but not between -G and the group name(s).

As a side note, adduser itself uses gpasswd:

$ grep gpasswd $(which adduser)
    my $gpasswd = &which('gpasswd');
    &systemcall($gpasswd, '-a',$existing_user,$existing_group);