Thanks to computernetworkingnotes.com
In Linux user and group management is done by these for important files.
Linux files responsible for User managements
/etc/shadow store all the Linux password in MD5 encryptions format /etc/passwd store all user related information's /etc/group store all group related information's
back-up files responsible for User managements
In this assignment we will modify these files. So it’s better to take back-up before doing this assignment because your little mistake can crash Linux systems.
#mkdir /backup #cp /etc/passwd /backup #cp /etc/group /backup #cp /etc/shadow /backup
Create a simple user
useradd is used to create user. Several options are used with useradd command but you will learn about then in our next assignments. In this assignment your task is to learn what exactly happens in these files when a new user is added. First observe the last line for these files.
#cat /etc/passwd |more #cat /etc/shadow |more #cat /etc/group |more
Now add a simple user.
#useradd vinita #passwd vinita
Now read these files again with cat command alternate you can use |grep switch to filter the output
#cat /etc/passwd |grep vinita #cat /etc/shadow |grep vinita #cat /etc/group |grep vinita # cd /home #ls –ld vinita
User’s entry in passwd
All these files are changed when a user is created In passwd files entries are in following formats separated by :
vinita users login name x password required to login 503 unique user id 504 unique group id /home/vinita users home directory /bin/bash user shell
In shadow files entry is straight forwards. Whatever showing beside the user name is the password of user vinita in MD5 encrypt format.
User’s entry in group
Whenever you create a normal user, users primary group form same name is automatically created. As you can verify by looking in /etc/group. 504 is the unique group id.
User’s home directory
Same as group, users home directory is also created in /home partition and user get the ownership of this directory.
How to create a user without password.
To create a user without password use –d switch .
#useradd nikki #passwd -d nikki
How to create a group.
To create group use groupadd commands. Group created by this command is called secondary group.
#groupadd test #cat /etc/group |grep test
How to add user in groups
To add user in this group use usermod commands
#usermod –G test vinita
This command will make vinita user to member of test group.
How to delete secondary group
You can delete a group by groupdel commands
#groupdel test #cat /etc/group |grep test
You cannot delete users primary group until user exist for example
How to delete User
userdel command is used to delete user. When a users is deleted user’s primary group will automatically be deleted.
#userdel nikki #groupdel nikki groupdel: group nikki does not exist.
Whenever you delete user with userdel command. entry of user will be removed from these files. But users home folder and mail folder will not be deleted. As you can see in image. If you want completely remove user including his home folder and mail folder use –r switch with userdel commands.