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New user is not taking system default Group ID. System default group id for new user is 100. Whenever i create a new user, GID as well as UID increments by 1. I can understand UID but GID should be 100 for every new user. What is the possible reason? I am using Amazon linux AMI on EC2.

# adduser -D
GROUP=100
HOME=/home
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
CREATE_MAIL_SPOOL=yes

# useradd -m testuser
# useradd -m testuser2

# cat /etc/passwd
testuser:x:501:503::/home/testuser:/bin/bash
testuser2:x:502:504::/home/testuser2:/bin/bash
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I'm not sure about AMI, but on some OSes, adduser is actually a separate Perl wrapper script for useradd. Try using adduser instead when actually adding the user to see if that has any effect. – Garrett Dec 17 '11 at 7:19
i tried adduser also, but unfortunately same result. – Praveen Dec 17 '11 at 7:25
If your system's adduser is a Perl script and not a symlink to useradd, check /etc/adduser.conf and ensure that FIRST_GID is set appropriately. AMI may use GROUP as an alias of that, but the Ubuntu box I'm looking at now has no such definition. – Garrett Dec 17 '11 at 7:40
i don't see any .conf file related to adduser in /etc – Praveen Dec 17 '11 at 7:51
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1 Answer

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Your system is configured to use user groups – to create a new group for each user, with the same name as the username. For example, your user testuser2(502) will have the primary group testuser2(504).

The configuration file for useradd is /etc/login.defs, and this setting is controlled by the USERGROUPS_ENAB statement. You can also temporarily change this behavior by using the options -U/--user-group and -N/--no-user-group.

Remember that useradd and adduser are separate programs; the defaults set in adduser do not necessarily affect the simpler useradd.

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But USERGROUPS_ENAB says "This enables userdel to remove user groups if no members exist". Is related to addition of user? – Praveen Dec 18 '11 at 14:17
@Praveen: Yes, see manual page of useradd, under -N and -U options. – grawity Dec 18 '11 at 18:45
thanks.......... – Praveen Dec 18 '11 at 18:48
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