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Switching Users and Groups in Linux

1. su Command (Switch User)

The su (substitute user) command allows a user to switch to another user account or execute commands as another user, commonly root.

Key Features:

  • Requires authentication using the password of the target user.
  • Enables complete user switching or the execution of commands under a different user.
  • su : Switches user without changing the environment variables.
  • su - : Starts a new shell session with the full login environment of the target user.

Syntax:

su [OPTIONS] [USERNAME]

Commonly Used Options:

  • - : Loads the target user’s environment variables.
  • -c "command" : Executes a single command as the target user.
  • -s /bin/bash : Changes the shell when switching users.

Examples:

  1. Switch to the root user:

    su

    This prompts for the root user password.

  2. Switch to a specific user (e.g., john):

    su john

    This switches to john’s account and requires authentication.

  3. Run a command as another user:

    su -c "whoami" username

    Example:

    su -c "ls /root" root

    Runs ls /root as root while staying in the current shell.

  4. Start a full login session for a user:

    su - username

    This loads the user’s full environment as if they had logged in directly.

2. sg Command (Switch Group)

The sg (substitute group) command allows users to execute commands as a member of a specified group. This is useful when working with group-based permissions without changing user ownership.

Key Features:

  • Runs commands under a specific group context.
  • Does not require password authentication.
  • The user must already be a member of the target group.
  • Limited to command execution and does not open a full session.

Syntax:

sg groupname -c "command"

Examples:

  1. Run a command as a member of a group:

    sg developers -c "touch file.txt"

    This creates a file under the developers group context.

  2. Start a new shell as a member of a group:

    sg groupname

    This opens a shell where commands execute under the specified group.

  3. Check your current group membership:

    groups

    This displays all groups you belong to.

4. Comparison: su vs. sg

Feature su (Switch User) sg (Switch Group)
Purpose Switch to another user account. Temporarily switch to another group.
Authentication Requires the target user’s password. Requires membership in the target group (no password needed).
Scope Switch user entirely or run commands as them. Execute commands as a specific group member.
Use Case For administrative tasks or user switching. For group-based permissions and tasks.

5. When to Use sg

  • When you need to work on files or execute commands with specific group-based permissions.
  • Example Scenario:
    • You are part of the developers and admins groups.
    • A file is owned by the admins group.
    • Instead of switching users, you can use sg admins to work on the file without changing ownership.