Linux: User and Group Management – Key Commands and Utilities

Users and Groups

Managing Users

Adding Users

useradd - this cmd is used to add new user to the system

-d           specify home dir

-e           expiry date

-s            shell

-G           group

-u           UID

/etc/login.defs  - this file stores all the parameters for users

passwd [username]  - creates a password for the user

usermod

This modifies user's account

-m          used with -d, this moves contents of the current home dir to the new one specified

-U           unlocks password

-L            locks account

-l             changes username (usermod -l new_username old_username)

chage

This command deals with account expiration

-m          sets the minimum time before a user can change password. Usage: -m 1 (1 day)

-M          sets the maximum time after which password needs to be changed

-d           sets the last day the password was changed (can be used to manipulate account info)

-I            sets number of inactive days allowed before password expires

-E           sets expiration date

-W          sets warndays – warn the user before the password expires

Deleting Accounts

userdel                              

This cmd is used to delete users from a system.

Options:

-r            remove all the files from home dir and mail spool

-f            force deletion of the account (when the user is logged in)

-h           get help

Managing groups

/etc/group - this file controls the group membership (also /etc/gshadow)

/etc/passwd - this file contains info on the user's primary group (also /etc/shadow)

newgrp - this cmd is used for changing current group to a different one

chgrp - this cmd is used to change the group of a file or dir

First 100 GIDs and UIDs (0-99) are reserved for system use (generally)

0 UID/GID belongs to root - always

 

Adding groups

groupadd

This cmd adds a new group to the system.

Options:

-g           specifies a GID

-r            creates a system group

-f            forces creation

 

Modifying groups

groupmod

This cmd modifies a group.

Options:

-g           specify a new group ID for an existing group

-n           specify a new group name for an existing group

gpasswd

This cmd sets a password for the group

Options:

-a           add a user to the group                  usage:   -a user group

-d           delete a user from the group        usage:   -d user group

-R           configures a group to not allow the cmd newgrp

-r            removes the password from a group

-A           specify group admin

 

Deleting groups

groupdel

This cmd deletes a group

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