The effects of adding users to a Linux system

This article covers my journey of learning about the Linux filesystem architecture and using my knowledge to write a shell script to create Linux users. In my second year of college, I learned about the Linux filesystem when the concept of users came to my mind. I tried to create users in Linux-based operating systems.

The script is designed for the Red Hat-based operating systems and can also be used for other Linux distributions with some slight modifications. Note that it uses ksh.

[ You might also like: Linux sysadmin basics: User account management ]

The focus is on files located in the /etc directory, which stores the system configuration information. I demonstrate how to work with system files and to create a new user. Root access is a requirement.

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Here are the files that are changed when creating a new user in Linux:

1. /etc/passwd
2. /etc/shadow
3. /etc/group
4. /etc/gshadow
5. /home/USER
6. /var/spool/mail/USER

The system adds security to the user's (USER) home directory and mailbox by setting ownership to the user and to the user's group.

/etc/passwd

This file stores information about the user, such as the username, User ID, Group ID, full name (Comment), and shell. The data is stored in the following format: UserName:Link:UserID:GroupID:Comment:HomeDirectory:Shell

Each line of the file contains the following information about the users:

• Username: Denotes the username provided to the user for easy access.
• Link: Represents that the user's password stored in the /etc/shadow file. If x is removed, then the password will not be checked, and the user cannot log in.
• UserID: Commonly known as UID and used to uniquely represent a user. It ranges from 0 to 65,535.
  • UID = 0 -> root user account
  • UID = 1,000 to 60,000 -> Non-root/general accounts
  • UID = 1 to 999 and 60,001 to 65,535 -> System users
• GroupID: Same as UserID but used to represent a group uniquely.
• Comment: Additional information about the user. Typically, the user's full name.
• HomeDirectory: Location of the user's home directory.
• Shell: User's default shell.

/etc/shadow

This file stores the information about the user's password and contains the password in an encrypted form.

The data is stored in the following format:

UserName:EncryptedPassword:LastPasswordChange:MinPwdAge:MaxPwdAge:WarningPeriod:InactivityPeriod:ExpirationDate:Unused

• Username: Denotes the username provided to the user.
• Encrypted Password: Password is encrypted using hashing algorithms like sha256, sha512, md5, etc., and stored in the file.
• Last Password Change: Stores the epoch date since the password was last changed.
• Min Password Age: Stores the number of days until the next password change.
  • By default, 0 -> No minimum age
• Max Password Age: Stores the maximum number of days after which the user has to change the password.
  • By default, 99,999 -> No maximum age
• Warning Period: Number of days after which the warning is displayed to change the password.
  • By default, NULL
• Inactivity Period: It specifies the number of days after the user password is expired before the account becomes disabled.
• Expiration Date: Epoch date until the account is disabled.
• Unused: Reserved for future use.

Other files

• /etc/group and /etc/gshadow files are for groups in Linux and store the data in the format GroupName:Link:GroupID:MemberList and GroupName:Password:Reserved:MemberList respectively.
• The user's home directory is to be created in the /home directory with the data from the /etc/skel folder.
• Mailbox created in the /var/spool/mail directory.

The script

The following is the shell script I wrote to create new users broken down into parts:

uname=$1
 
id -u $uname&>/dev/null

First, it checks if the user already exists:

if [ $? -ne 0 ]
then
for i in {1000..60000}
do 
id -u $i&>/dev/null & id -g $i&>/dev/null
if [ $? -ne 0 ]
then
break
fi
done
uid=$i
echo "$uname:x:$uid:$uid::/home/$uname:/bin/bash">>/etc/passwd
lastpwdchange=$(datediff 1970-01-01 `date +"%Y-%m-%d"`)
echo "$uname::$lastpwdchange:0:99999:7:::">>/etc/shadow
echo "$uname:x:$uid:">>/etc/group
echo "$uname:!::">>/etc/gshadow
mkdir /home/$uname
cp -a /etc/skel/. /home/$uname/
chown $uname:$uname /home/$uname/ -R
chmod 0700 /home/$uname/ -R
touch /var/spool/mail/$uname
chown $uname:$uname /var/spool/mail/$uname
 
echo "User successfully created"
else
echo "User already exists : try using a unique username">&2
fi

If the user already exists, exit the program, or create the user.

The full code is available publicly on GitHub.

Run the shell script

[ Editor's note: While this script may seem fairly straightforward,  always be sure you understand and trust any scripts you find before installing them or escalating their permissions on your system. ]

1) Place the shell script in the directory /usr/bin/ to make it accessible as a command.

2) Add execute permission for root user:

# chmod u+x /usr/bin/ksh_useradd

3) Run the command with the username as the argument:

ksh_useradd USERNAME

4) The script creates the user account. You must set a password for the new user before first login.

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[ Free online course: Red Hat Enterprise Linux technical overview. ] 

Wrap up

This script can make the process of creating users a little easier than remembering switches for the useradd command. Sure, you can use the useradd command to create users, set permissions, and create the user's mailbox, but this script performs the same actions as useradd and helps you to learn what goes on behind the scenes with your filesystem. I hope that the explanations above help you to understand the user creation-related configuration files and their fields a little more clearly.