Users, permissions, and processes / Stage 8 of 13

Delegating commands with sudo

Not running Starting usually ~20 s

Step 7 of 8 · lab · 20 min

Suspend a user account

An employee is handing over their work. Block password and key logins to their account while keeping its data and running process.

Situation

An employee who worked under the labpaused account is handing over their duties. Until the handover is done, the account has to be suspended: no one may log in to it, either with the password or with an SSH key. Deleting it is premature — the home directory holds handover notes, and a background process of labpaused has been agreed to keep running until the handover ends.

Task

The labpaused password is locked, and the password itself is neither deleted nor replaced: access must be restorable.
The account has expired — its end date is in the past.
The login shell is /usr/sbin/nologin, and the home directory in the record is unchanged.
The key from /home/labpaused/.ssh/authorized_keys no longer works: there's no active key file, and an exact copy of it sits in /srv/account-review/authorized_keys.saved, readable by root only (owner root:root, mode 600).
The home directory, .ssh and notes.txt are in place with their original owner and mode, and notes.txt is unchanged; other accounts, sudo and SSH settings are untouched.
The background process of labpaused keeps running — the same one that was started.

Constraints

Don't delete the account and don't kill labpaused's processes.
Account changes need sudo.

The tools you need are already installed. There's no internet on this machine.

Lab · on your own

No commands are given in this lab: type them in the terminal yourself. Goals are checked on the machine after every command.

Goals

0 of 6

labpaused's password is locked and the original one is kept — not done yet
The account has expired — its end date is in the past — not done yet
The login shell is /usr/sbin/nologin and the home directory is unchanged — not done yet
The key from authorized_keys no longer works and its copy is readable by root only — not done yet
The home directory, .ssh and notes.txt are kept; other accounts and settings are untouched — not done yet
The original background process of labpaused keeps running — not done yet

Ctrl+Enter

Starts the machine node1 · Ubuntu 24.04

The machine is starting

Hints

0 of 3

Report a problem

Lab goals

0 of 6

labpaused's password is locked and the original one is kept — not done yet
The account has expired — its end date is in the past — not done yet
The login shell is /usr/sbin/nologin and the home directory is unchanged — not done yet
The key from authorized_keys no longer works and its copy is readable by root only — not done yet
The home directory, .ssh and notes.txt are kept; other accounts and settings are untouched — not done yet
The original background process of labpaused keeps running — not done yet

node1 Ubuntu 24.04

Lab goals

0 of 6

labpaused's password is locked and the original one is kept — not done yet
The account has expired — its end date is in the past — not done yet
The login shell is /usr/sbin/nologin and the home directory is unchanged — not done yet
The key from authorized_keys no longer works and its copy is readable by root only — not done yet
The home directory, .ssh and notes.txt are kept; other accounts and settings are untouched — not done yet
The original background process of labpaused keeps running — not done yet

Users and permissions · picked for this lab

Users and groups

Cheat sheet

$_ — insert into the terminal

id alice uid/gid/groups
sudo useradd -m -s /bin/bash bob create
sudo usermod -aG sudo bob add to a group (with -a, or it replaces all)
sudo passwd bob set a password
sudo passwd -l bob lock password login
sudo chage -M 90 bob maximum password age — 90 days
getent group sudo who's in the group

Management

bash

sudo useradd -m -s /bin/bash alice    # create (with a home directory -m and shell)
sudo passwd alice                      # set a password
sudo usermod -aG sudo alice            # ADD to a group (the -a matters, or it replaces!)
sudo usermod -aG docker,www-data alice
sudo usermod -s /usr/sbin/nologin svc  # forbid interactive login (a service account)
sudo gpasswd -d alice docker           # remove from a group
sudo userdel -r alice                  # delete along with the home directory
sudo groupadd devs                     # create a group
sudo groupdel devs                     # delete a group
adduser bob                            # the interactive wrapper (Debian/Ubuntu)

A common mistake. usermod -G grp without -a replaces all of a user's supplementary groups — use usermod -aG, not -G. The template for a new home directory is /etc/skel.

Passwords, locking, su

bash

sudo passwd -l alice              # lock the password (password login forbidden)
sudo passwd -u alice              # unlock it
sudo usermod -L / -U alice        # the same (lock/unlock)
sudo chage -l alice               # the password aging policy
sudo chage -M 90 -W 7 alice       # max age 90 days, warn 7 days ahead
sudo chage -E 2026-12-31 alice    # the account's expiration date
sudo passwd -e alice              # require a password change at next login

su - bob                          # become bob (a login shell, its own environment)
su -                              # become root (if you know root's password)
sudo -u postgres -i               # better: through sudo, no root password needed (see sudo)

System users (daemons) usually have uid < 1000, nologin, no password (useradd -r).

Added to a group — it has no effect

Group membership applies on the next login. Log back in (or use newgrp <group> for the current shell). Check with id.

Accidentally replaced the groups (`-G` without `-a`)

Restore the needed ones: usermod -aG sudo,docker alice. You can verify against /etc/group (getent group).

Can't delete a user — "currently used by process"

End its processes (pkill -u alice, see processes) or use userdel -f.

Login denied, even though the password is correct

The account/password is locked (passwd -l/chage -E in the past) or the shell is nologin. Check chage -l, getent passwd (the last field is the shell).

Files show the "wrong" group

An SGID directory only assigns its group to NEW files; recursively chgrp -R the old ones (see permissions, chmod-chown).

userdel -r deletes the home directory and mail spool irreversibly — without a backup, the data cannot be recovered. Before deleting, check whether any processes or cron jobs still belong to the user (pkill -u, crontab -l -u): userdel without -f refuses to delete an account with active processes, while -f deletes it anyway, even if processes keep running under the now-nonexistent UID. After deletion, the freed UID can be reassigned to a new user — files left on disk with that UID become "theirs".

Delegating commands with sudo

Situation

Task

Constraints

Signing in is needed to start the machine

Management

Passwords, locking, su

Added to a group — it has no effect

Accidentally replaced the groups (-G without -a)

Can't delete a user — "currently used by process"

Login denied, even though the password is correct

Files show the "wrong" group

Accidentally replaced the groups (`-G` without `-a`)