CompTIA Linux+ XK0-006 (V8) Practice Question

usermod -aG docker -s /bin/zsh sam works because:

• The -G docker option names the supplementary group to add.
• The -a (append) flag keeps sam's existing supplementary groups; leaving it out would replace them.
• The -s /bin/zsh option sets the new login shell.
• Finally, sam is the account being modified, so everything is done in one invocation.

The other choices fail at least one requirement:

• A command that omits -a drops all other group memberships.
• Placing sam after the shell path (or after docker) makes the command operate on the wrong account.
• Using two commands joined with "&&" is not a single step, and tools like gpasswd cannot set the login shell at all.