To make it easier to manage users, linux provides some common commands that allow you to add, delete, and modify users in a more standardized way.

Add User

With the useradd command, we can create new users.

The following are the relevant files or directories that will be involved when creating a user:

Name	Type	Purpose	Preview
/etc/default/useradd	File	Default variables’ setting file for the useradd command.
/etc/login.defs	File	Default variables’ setting file for the useradd command options.
/etc/shells	File	Available shells for new users.
/etc/skel/	Directory	Initialization profiles for new users.

Here are some common options for useradd command:

The ID of the new user created by default by the useradd command is the value of the last user ID plus 1, followed by the creation of a group with the same name as the user:

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useradd dong
tail -1 /etc/group
tail -1 /etc/passwd
tail -1 /etc/shadow
ll /home | grep 'dong'

Specify User Comment (Full Name)

With the -c option, we can specify comment for the user:

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useradd -c 'Dong Chen' dong_comment
tail -1 /etc/group
tail -1 /etc/passwd
tail -1 /etc/shadow
ll /home | grep 'dong_comment'

Specify Home Directory

With the -d option, we can specify the user’s home directory:

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useradd -d /home/dong_home_dir dong_home
tail -1 /etc/group
tail -1 /etc/passwd
tail -1 /etc/shadow
ll /home | grep 'dong_home_dir'

Specify Group

With the -g option, we can specify the user’s group:

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useradd -g dongs_group dong_group
tail -1 /etc/group
tail -1 /etc/passwd
tail -1 /etc/shadow
ll /home | grep 'dong_group'

Specify User ID

With the -u option, we can specify the user’s ID:

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useradd -u 1100 dong_id
tail -1 /etc/group
tail -1 /etc/passwd
tail -1 /etc/shadow
ll /home | grep 'dong_id'

Create System User

With the -r option, we can create a system user:

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useradd -r dong_system
tail -1 /etc/group
tail -1 /etc/passwd
tail -1 /etc/shadow
ll /home | grep 'dong_system'

Modify User

We can change the user’s information through the usermod command.

Here are some common options for usermod command:

Append Supplementary Groups

With the combination of -a and -G options, we can append more supplementary groups to the user:

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id dong_group
usermod -a -G dongs_another_group dong_group
id dong_group

Delete User

We can delete users through the userdel command.

Here are some common options for userdel command:

Delete Users Without Deleting Related Directories

By default, the userdel command preserves home and mailbox directories when deleting a user:

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useradd user_surplus
id user_surplus
userdel user_surplus
id user_surplus
ll /home | grep 'user_surplus'

Delete Users With Related Directories

With the -r option, the user’s home directory and mailbox directory are also deleted:

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useradd user_surplus2
id user_surplus2
userdel -r user_surplus2
id user_surplus2
ll /home | grep 'user_surplus2'

Password File Integrity Verification

We can use pwck command to check the integrity of user accounts:

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pwck

User Password

We can use the passwd command to specify or change the user’s login password.

Here are some common options for passwd command:

Set User Password Through IO Input

With the ‑‑stdin option, the passwd command gets the data from the IO input as the password:

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cat /etc/shadow | grep 'dong:'
echo "dongspwd" | passwd --stdin dong
cat /etc/shadow | grep 'dong:'

Users Passwords

We can use the chpasswd command to specify or change the users’ login passwords.

Here are some common options for chpasswd command:

Set Users Passwords Through IO Input

Let’s first remove dong’s password:

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passwd -d dong
cat /etc/shadow | grep 'dong:'

We use the chpasswd command to bulk change the users’ passwords:

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cat /etc/shadow | grep -E 'dong:|dongchen:'
printf '%s\n' 'dong:dong123' 'dongchen:chen123' | chpasswd
cat /etc/shadow | grep -E 'dong:|dongchen:'

Change Shell

We can use the chsh command to change the user’s login shell.

Here are some common options for chsh command:

Check for Available Shells

With the -l option, we can view the available login shells:

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chsh -l    # This is equivalent to chsh --list-shells

Change to New Shell

With the -s option, we can assign a login shell to the user:

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useradd dongchen
finger dongchen
chsh -s /bin/sh dongchen
finger dongchen

Change Finger

With the chfn command, we can change user details.

Here are some common options for chfn command:

Change User Details

With the -f option, we can change the user’s basic information:

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finger dongchen
chfn -f 'Dong Chen' -o 1024 -p 12345678 -h 87654321 dongchen
finger dongchen

Change Age

With the chage command, we can change user password expiration information.

Here are some common options for chage command:

Output User Account Aging Information

With the -l option, we can view the aging information for the user’s account:

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chage -l dongchen

References USERADD(8), USERMOD(8), USERDEL(8), PWCK(8), PASSWD(1), CHPASSWD(8), CHSH(1), CHFN(1), CHAGE(1)

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