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.