Linux User Adminstration

  

There are two type of users in Linux. They are System users and Normal Users. Each user will have a unique ID called UID.

User Type	UID Range
System Users	0 to 999
Normal User	1,000 to 60,000

The user information is stored in the following files.

/etc/passwd – User information is stored
/etc/shadow – password information of the users is stored in this file

Let us discuss more about each file.

In /etc/passwd file, the user details are present in the below format.

Here UID and GID are auto generated by the system. Every user will have a unique user ID (UID) and group ID (GID).

The default home directory in Linux is /home/<username>.

In /etc/shadow file, the encrypted user password details are present.

There are few tools like DES (64 bits), MD5 (128 bits), SHA (512 bits) which will encrypt the password in linux. In RHEL 7, SHA is the default tool.

The tool used to encrypt the password of a specific user can be found using the command

passwd –S <username>

the output looks like the below.

Below are the basic command for user administration.

Create a user:  useradd <username>

change the user password: passwd <username>

lock the user account: usermod –L <username>

Verification: The “!” mark before the encrypted password represents the account is locked.

Now let’s check the usermod command to change each property of /etc/passwd file.

1)      Change the user name: usermod -l <new_name> <old_name>

2)      Change the user ID: usermod –u <new_uid> -o <user_name> (-o is applied when the same UID is used for two users).

3)      Change the comments of a user: usermod –c “<new_comments>” <user_name>

4)      Change the user home directory: usermod -m -d <new_directory_fullpath> <user_name>
   (here –m to move the current home directory content to new home directory)
the default home directory information can be found in the file /etc/default/useradd

5)      Delete a user including home directory and mail spool

userdel –r <<User_Name>>

6)      Change the user login shell: chsh –s <new_shell_name> <user_name>

The available shells on the linux box can be found at /etc/shells

Password information:

chage –l  <<username>>

Change the maximum days of the passwd expiry.



Groups:

The groups information is found in the fail /etc/group

create a new group
# groupadd <<New_Group_Name>>

Append a secondary group to the user

# usermod –aG <<Secondary_Group>> User_Name

List all groups a user belongs to. Here testacl is the user

Set-GID for directories:

By doing this. The newly created child objects will have the current group of dir1