# Debian Users and Groups

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.

## Adding Users

Create a user with a home directory:

```bash
useradd -m username
```

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

```bash
adduser username
```

Set password:

```bash
passwd username
```

## Removing Users

Delete user but keep home directory:

```bash
deluser username
```

Delete user and home:

```bash
deluser --remove-home username
```

## Groups

Create a group:

```bash
groupadd groupname
```

Add user to group:

```bash
usermod -aG groupname username
```

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

List groups for user:

```bash
groups username
```

## Common Groups

- `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

## Listing Users

```bash
cut -d: -f1 /etc/passwd
```

View full user info:

```bash
getent passwd username
```

## User Configuration Files

- `/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.