Every process on Debian runs as a user. Users have numeric identifiers (UIDs), groups have GIDs. Permissions and resource limits are assigned per-user and per-group.

Create a user with a home directory:

useradd -m username

The interactive version (prompts for gecos, shell, etc.):

adduser username

Set password:

passwd username

Delete user but keep home directory:

deluser username

Delete user and home:

deluser --remove-home username

Create a group:

groupadd groupname

Add user to group:

usermod -aG groupname username

The -a appends; without it, existing groups are replaced.

List groups for user:

groups username

• sudo — run commands with sudo
• docker — use Docker without sudo
• audio — access audio devices
• video — access video devices
• www-data — web server process owner
• adm — read log files

cut -d: -f1 /etc/passwd

View full user info:

getent passwd username

• /etc/passwd — user accounts (readable, no passwords)
• /etc/shadow — hashed passwords (root-only)
• /etc/group — group definitions
• /etc/gshadow — group passwords (rarely used)

Changes made with useradd, usermod, groupadd update these files automatically. Manual editing is possible but error-prone; use the commands instead.