useradd creates an account, usermod changes one and userdel deletes one. passwd sets a password. They edit /etc/passwd, /etc/shadow and /etc/group for you, so you rarely touch those files by hand. Pass -m to create a home directory and -s to set the login shell. groupadd, groupdel and gpasswd do the same job for groups.
The classic mistake is usermod -G without -a. On its own, -G replaces the user's supplementary groups and removes the user from every group you did not list. usermod -aG appends instead. A new group membership applies from the user's next login.
To check the result, id shows a user's groups, and getent shows what the system actually resolves for a user or group.
# User management
useradd username # Add new user
useradd -m -s /bin/bash username # Create with home directory
userdel username # Delete user
userdel -r username # Delete user and home directory
usermod -aG groupname username # Add user to group
passwd username # Change user password
chage -l username # View password aging info
# Group management
groupadd groupname # Create new group
groupdel groupname # Delete group
gpasswd -a username groupname # Add user to group
gpasswd -d username groupname # Remove user from group
# View user/group information
id username # Show user ID and groups
groups username # Show user's groups
getent passwd username # Get user info from database
getent group groupname # Get group info
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