Root user and Regular Users

vIn the /etc/passwd file you shared, let’s break down each of your questions to clarify user identification in detail.

1. Identifying the Root User

• Root User Line:

  • The root user is identified by the line starting with root:

    root:x:0:0:root:/root:/bin/bash

  • The root user always has:

    • Username: root

    • UID: 0

    • GID: 0

    • Home Directory: /root

    • Default Shell: /bin/bash (or sometimes another shell, but it will not be /usr/sbin/nologin or /bin/false)

2. How Many Root Users Can Exist?

• Only One Root User (UID 0):

  • In a standard Linux system, there is only one user with the UID 0, which is reserved for the root user. This user has unrestricted access to all system files and processes.

  • Additional Superuser Accounts: You can create additional users with root-level privileges by assigning them UID 0, but this is rarely done as it can complicate security and user tracking. Instead, it’s more common to give certain users sudo privileges, allowing them temporary root access with the sudo command.

3. Identifying Regular Users

• Characteristics of Regular Users:

  • Regular users typically have:

    • UID: Starting from 1000 and up. On many Linux systems, UIDs less than 1000 are reserved for system and service accounts.

    • Shell: Regular users usually have a login shell like /bin/bash or /bin/sh, which allows them to log into the system interactively.

    • Home Directory: Regular users generally have home directories under /home/, for example, /home/rntbci and /home/sysadmin in your case.

• Example of Regular User Lines:

  • In your file, rntbci and sysadmin appear to be regular users:

    rntbci:x:1000:1000:shreyas-vishwanath.joshi,ci,,:/home/rntbci:/bin/bash
    sysadmin:x:1001:1001:System Admin:/home/sysadmin:/bin/bash

4. How Many Regular Users Can Exist?

• No Hard Limit on Regular Users:

  • There is technically no limit to the number of regular users you can create, aside from system resources. However, most Linux distributions start assigning regular user UIDs from 1000 and count upwards.

  • UID Range for Regular Users: Regular users on Ubuntu typically have UIDs from 1000 upwards, though the starting UID may vary slightly depending on distribution and configuration.

5. Filtering to Show Only Root and Regular Users

• Command to Display Only Root and Regular Users:

  • To filter and display only the root and regular users (skipping system and service users), you can use a combination of awk and grep commands to focus on UIDs 0 and 1000 or higher.

  • Example:

    awk -F: '($3 == 0 || $3 >= 1000) {print $1, $3, $6, $7}' /etc/passwd

    • This command extracts only:

      • Users with UID 0 (root).

      • Users with UIDs 1000 or higher (regular users).

    • The output format will be: username UID home_directory shell.

  • Explanation:

    • -F: sets the field separator to :.

    • ($3 == 0 || $3 >= 1000): Filters lines where the UID ($3) is 0 or 1000 and above.

    • {print $1, $3, $6, $7}: Prints the username, UID, home directory, and shell, focusing on key details.

• Alternative Using grep and awk:

  grep -E ':0:|:[1-9][0-9]{3,}:' /etc/passwd | awk -F: '{print $1, $3, $6, $7}'

  • This command first filters UIDs 0 or 1000+ and then formats the output similarly.

Using these commands, you can easily filter and view only root and regular users, leaving out system and service accounts. Let me know if you’d like further assistance with these commands or other user management tips!