# Basics of Linux ### Users and groups

# Linux is multitask and multiuser
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                                <ul>
                                    <li>Several users can log in and work simultaneously</li>
                                    <li>Isolated operating environments</li>
                                    <li>CSC HPC environments are a great example of this</li>
                                </ul>
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                                <ul>
                                    <li>Users defined in `/etc/passwd`</li>
                                    <li>Groups in `/etc/group`</li>
                                </ul>
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                        </div>
                        ---
                        # User
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- Defined in `/etc/passwd` file
                        - name:password:UID:GID:GECOS:directory:shell
                          - name: `cbrown`
                          - password: `x`
                          - UID: `1000`
                          - GID: `1000`
                          - GECOS: [empty]
                          - directory: `/home/cbrown`
                          - shell: `/bin/bash`

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                        ```txt
                        root:x:0:0:root:/root:/bin/bash
                        bin:x:2:2:bin:/bin:/usr/sbin/nologin
                        sys:x:3:3:sys:/dev:/usr/sbin/nologin
                        sync:x:4:65534:sync:/bin:/bin/sync
                        ...
                        cbrown:x:1000:1000::/home/cbrown:/bin/bash
                        ```

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                        ---
                        # Group
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- Defined in `/etc/group` file
                        - group_name:password:GID:members_list
                          - group name: `cbrown`
                          - password: `x`
                          - GID: `1000`
                          - members list: `woodstock`
                            - primary group membership not listed

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```txt
                        root:x:0:
                        bin:x:2:
                        sys:x:3:
                        adm:x:4:cbrown
                        ...
                        cbrown:x:1000:woodstock
                        ```

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                        ---
                        # root
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- Superuser privilege over the Linux
                          - UID: `0`
                          - GID: `0`
                        - Kernel bypasses some security checks for the user id number zero

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                        ---
                        # User and group details
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whoami

groups

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Print effective user name

Print real and effective user and group IDs

Print the groups a user is in

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Thank you

Joona Tolonen

servicedesk@csc.fi