From our last section you have learned System administration related commands. Whether its exam or real world senior first and top most work for a system administrator is user management. In Linux user and group management is done by these files.
/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
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 system.
#mkdir /backup #cp /etc/passwd /backup #cp /etc/group /backup #cp /etc/shadow /backup
useradd is used to create user. Several options are used with useradd command but you will learn about them in our next assignments. In this assignment you are going to learn what exactly happen in these files when a new user is added. First watch carefully last lines of these files.
#cat /etc/passwd |more #cat /etc/shadow |more #cat /etc/group |more
Add a user.
#useradd vinita #passwd vinita
Now read these files again with cat command or 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
All these files are changed when a user is created in passwd file entries are done in following format 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 file entry is straight forward. Whatever showing beside the user name is the password of user vinita in MD5 encrypt format.
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.
Same as group, users home directory is also created in /home partition and user get the ownership of this directory.
To create a user without password use –d switch .
#useradd nikki #passwd -d nikki
To create group use groupadd commands. Group created by this command is called secondary group.
#groupadd test #cat /etc/group |grep test
To add user in this group use usermod commands
#usermod –G test vinita
This command will make vinita user to member of test 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
userdel command is used to delete user. When a user 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.
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