LFCS Operation of Running Systems Practice Test 6 — 15 Questions

Question 1

A process (PID 1234) is hung and cannot be killed with SIGTERM. To force termination, which signal should be sent?

Accepted Answer

kill -9 1234 (SIGKILL). SIGKILL (signal 9) is the correct choice because it cannot be caught, blocked, or ignored by the process. Unlike SIGTERM, which allows the process to perform cleanup, SIGKILL immediately terminates the process at the kernel level, making it the only reliable way to force-kill a hung process that ignores other signals.

Answer

kill -15 1234 (SIGTERM)

Answer

kill -2 1234 (SIGINT)

Answer

kill -1 1234 (SIGHUP)

Question 2

Which command shows the default target for systemd?

Accepted Answer

systemctl get-default. The correct command to display the default target (the systemd unit that the system boots into by default) is `systemctl get-default`. This command reads the symlink at `/etc/systemd/system/default.target` and outputs its target, such as `multi-user.target` or `graphical.target`. Option C is correct because it directly queries systemd for the current default boot target.

Answer

systemctl show default

Answer

systemctl list-default

Answer

systemctl default

Question 3

An e-commerce company runs a critical application on a Linux server that occasionally becomes unresponsive. The server has 64GB RAM and runs a Java application. The operations team notices that during peak hours, the system becomes very slow and eventually the application crashes with 'OutOfMemoryError'. After restart, it works fine for a while. They suspect a memory leak but also want to ensure the system does not go down during peak hours. The system uses systemd to manage the Java service. The administrator needs to implement a solution that: (1) automatically restarts the service if it becomes unresponsive, (2) limits the memory usage of the service to prevent OOM kills on the system, and (3) provides early warning of high memory usage. Which of the following approaches best meets these requirements?

Accepted Answer

Configure systemd service with WatchdogSec=30, Restart=on-failure, MemoryMax=32G. Also set up a log watcher that alerts when memory usage exceeds 28G via journalctl and a custom script.. It uses systemd's WatchdogSec to detect unresponsiveness and Restart=on-failure to automatically restart the service, while MemoryMax=32G enforces a hard memory limit via cgroups to prevent OOM kills. The custom log watcher provides early warning by alerting when memory usage exceeds 28G, satisfying all three requirements.

Answer

Set up a cron job to run every minute that checks memory usage with free and if > 90%, restart the service with systemctl restart. Also configure MemoryMax=32G in the systemd unit.

Answer

Configure sysctl vm.overcommit_memory=2 to prevent overcommit, and allocate huge pages for Java. Also set Restart=always in the systemd unit.

Answer

Use ulimit -v 33554432 in the service script to limit virtual memory, and set Restart=always. Also configure a cron job to send alerts when dmesg shows OOM.

Question 4

A system running RHEL 8 experiences intermittent crashes. After reboot, 'journalctl -p err -b -1' outputs: 'PID 1234 (myapp) ended due to signal: KILL'. Which diagnostic step should the administrator perform next?

Accepted Answer

Check journalctl for 'oom-kill' entries or use 'dmesg | grep -i oom'.. The 'PID ended due to signal: KILL' message indicates the process was terminated by a SIGKILL (signal 9), which is commonly sent by the Out-Of-Memory (OOM) killer when the system runs low on memory. Checking journalctl for 'oom-kill' entries or using 'dmesg | grep -i oom' directly confirms whether the OOM killer was responsible, making D the correct next diagnostic step.

Answer

Review logrotate configuration for myapp logs.

Answer

Run strace to capture system calls of myapp before restarting.

Answer

Enable core dumps and reproduce issue.

Question 5

Which TWO commands can be used to display real-time process resource usage on a Linux system? (Choose two.)

Accepted Answer

htop. Option B (htop) is correct because htop is an interactive process viewer that continuously refreshes and displays real-time CPU, memory, and per-process resource usage, updating by default every second. Option E (top) is correct because top is the classic interactive monitor that dynamically refreshes process and resource statistics in real time, also defaulting to a periodic update interval. Option A (vmstat 1) reports virtual memory, CPU, and I/O statistics every 1 second, but it summarizes system-wide counters rather than per-process resource usage, so it does not display process-level real-time usage. Option C (ps -aux) is a snapshot command that lists processes at the moment of execution and then exits, providing no continuous real-time updates. Option D (free -h) only shows current memory and swap utilization in human-readable units and does not display process resource usage at all.

Answer

vmstat 1

Answer

ps -aux

Answer

free -h

Question 6

A system administrator is troubleshooting a production web server running CentOS 7 that became unresponsive. The server is still pingable, but SSH connections timeout. The admin performs an out-of-band console login. The server appears frozen; typing commands shows no output. The admin is able to trigger a Magic SysRq key sequence (Alt+SysRq+f) to kill the hung processes. After that, the server resumes normal operation. However, the admin wants to understand the root cause. Upon checking 'dmesg', they see repeated messages: 'NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s!' followed by stack traces from a kernel thread. Which action should the admin take to prevent recurrence while maintaining system stability?

Accepted Answer

Update the server's BIOS/firmware and check for kernel updates.. Soft lockup errors on CentOS 7 often indicate kernel bugs or hardware/firmware issues that cause CPUs to stall for extended periods. Updating the BIOS/firmware can resolve underlying hardware timing problems, while kernel updates may include patches for known soft lockup bugs. This approach addresses the root cause without disabling or weakening the watchdog mechanism, preserving system stability.

Answer

Replace the power supply unit to ensure stable power.

Answer

Increase the soft lockup threshold via sysctl to reduce false positives.

Answer

Add 'nosoftlockup' to the kernel boot parameters.

Question 7

A user reports that a background process (PID 3456) is consuming 95% of CPU and causing system slowness. The process name is 'crypto-miner'. The administrator needs to immediately stop this process and ensure it does not restart. Which set of commands should the administrator execute?

Accepted Answer

kill -9 3456, then locate the cron job or systemd service that starts it, and disable/remove it.. It addresses both immediate termination and persistence removal. The SIGKILL signal (kill -9) immediately terminates the process, and disabling the cron job or systemd service prevents automatic restart, which is critical for a malicious or unwanted process like 'crypto-miner'.

Answer

renice -n 19 -p 3456 and let it run with lower priority.

Answer

kill -9 3456 and then notify the user.

Answer

kill -15 3456 and hope it terminates.

Question 8

An administrator is investigating why the 'tomcat' service fails to start on a RHEL 8 server. The output of 'systemctl status tomcat' shows: 'Loaded: loaded (/etc/systemd/system/tomcat.service; enabled; vendor preset: disabled) Active: failed (Result: exit-code) since ... Process: 4567 ExecStart=/opt/tomcat/bin/startup.sh (code=exited, status=1/FAILURE)'. The 'journalctl -u tomcat' shows: 'Error: JAVA_HOME is not defined correctly, cannot execute /usr/lib/jvm/java-11-openjdk/bin/java'. The admin checks /opt/tomcat/bin/startup.sh and sees it references JAVA_HOME. The admin verifies that Java 11 is installed at /usr/lib/jvm/java-11-openjdk. Which action should the admin take to fix the service?

Accepted Answer

Add 'Environment=JAVA_HOME=/usr/lib/jvm/java-11-openjdk' to the [Service] section of /etc/systemd/system/tomcat.service and run 'systemctl daemon-reload && systemctl restart tomcat'.. Systemd services can have environment variables set via the `Environment=` directive in the unit file. Adding `JAVA_HOME=/usr/lib/jvm/java-11-openjdk` to the `[Service]` section ensures the variable is available to the `ExecStart` process. Running `systemctl daemon-reload` reloads the unit definition, and `systemctl restart tomcat` applies the change. This is the proper method for configuring environment variables for systemd-managed services on RHEL 8.

Answer

Edit /opt/tomcat/bin/startup.sh and hardcode JAVA_HOME.

Answer

Run the startup script manually with 'bash /opt/tomcat/bin/startup.sh'.

Answer

Set JAVA_HOME globally using 'export JAVA_HOME=/usr/lib/jvm/java-11-openjdk' in /etc/profile.

Question 9

A developer reports that a custom daemon fails to start after a reboot. The daemon's unit file is located in /etc/systemd/system/custom.service. Which of the following is the most likely cause?

Accepted Answer

The service is not enabled.. The most likely cause is that the service is not enabled (systemctl enable). Even though the unit file exists in /etc/systemd/system, systemd only starts services automatically at boot if they are enabled. Option B is correct. Option A is incorrect because manually starting the service after installation would work but does not affect boot behavior. Option C is unlikely because SELinux policy blocks would produce a different error (e.g., denial messages). Option D is incorrect because firewall rules do not prevent systemd from starting a service; they affect network connectivity.

Answer

The service was not started manually after installation.

Answer

The SELinux policy blocks the service.

Answer

A firewall rule is blocking inbound connections.

Question 10

A Linux server's root filesystem is filling up quickly. You suspect that a user's process is writing a large log file and that the file has been deleted, but the space is still held. Which command will show the deleted file and the process holding it open?

Accepted Answer

lsof +L1. The correct tool is lsof +L1, which specifically finds open files with a link count of zero, meaning they have been deleted but are still held open by a process. This is the standard method to identify the culprit consuming disk space invisibly to df and du.

Answer

df -h

Answer

du -sh /var/log

Answer

fuser -m /

Question 11

A system administrator needs to ensure that the 'nginx' service starts automatically after a reboot on a system using systemd. The service unit file exists and is currently disabled. Which command should be used?

Accepted Answer

systemctl enable nginx. The systemctl enable nginx command is the correct way to configure a service to start automatically at boot. It creates the necessary symlinks without starting the service now, which matches the requirement to ensure automatic startup after a reboot.

Answer

systemctl start nginx

Answer

systemctl daemon-reload

Answer

systemctl enable --now nginx

Question 12

A Linux server's boot process fails after a kernel upgrade. The GRUB2 menu appears, but selecting the newest kernel panics. You need to boot the previous kernel temporarily to recover the system. Which command should you use at the GRUB2 menu to edit the selected entry's boot parameters?

Accepted Answer

Press 'e' to edit the selected menu entry, then modify the 'linux' line and press Ctrl+X to boot.. To temporarily modify boot parameters in GRUB2, you edit the menu entry by pressing 'e', adjust the kernel command line, and boot with Ctrl+X. This allows booting an older kernel or adding options like 'systemd.unit=emergency.target' without permanent changes. The other methods are either for GRUB Legacy or incorrect for this purpose.

Answer

Press 'a' to append parameters to the kernel line, then press Enter to boot.

Answer

Press 'c' to enter the GRUB command line, then run 'boot' with the previous kernel path.

Answer

Press 'Tab' to list available kernels, then type the kernel version and press Enter.

Question 13

A Linux server is configured to boot into a graphical target, but after a recent kernel update, the system hangs at a black screen after the GRUB menu. You can still access a rescue shell via the installation media. Which command should you use to make the system boot into a multi-user text target by default, allowing you to troubleshoot?

Accepted Answer

systemctl set-default multi-user.target. The default systemd target is managed via the default.target symbolic link. To change it persistently, the systemctl set-default command is used. Setting it to multi-user.target ensures the system boots to a text console, allowing the administrator to diagnose the graphical failure. This is the standard method for altering the default runlevel equivalent in systemd-based distributions.

Answer

systemctl isolate multi-user.target

Answer

grub2-mkconfig -o /boot/grub2/grub.cfg

Answer

systemctl enable multi-user.target

Question 14

A junior administrator needs to check the current system time and date on a Linux server. Which command will display this information?

Accepted Answer

date. The date command is the standard utility to display the current system date and time. It is simple, universally available, and directly outputs the information requested without additional details.

Answer

hwclock

Answer

uptime

Answer

timedatectl status

Question 15

An administrator needs to ensure that a custom service, /usr/local/bin/monitor.sh, starts automatically at boot and is restarted if it crashes. The service should run as user 'monitor' and should not depend on network being online. Which systemd unit file configuration is most appropriate?

Accepted Answer

Create a unit file with [Service] Type=simple, ExecStart=/usr/local/bin/monitor.sh, Restart=always, User=monitor, and [Install] WantedBy=multi-user.target.. A systemd service unit must specify the correct Type, ExecStart, Restart policy, User, and installation target. For a script that runs in the foreground, Type=simple is suitable. Restart=always ensures the service is restarted regardless of exit status, providing resilience. User=monitor runs it with least privilege. WantedBy=multi-user.target enables start at boot without network dependency. This combination satisfies all stated requirements.

Answer

Create a unit file with [Service] Type=simple, ExecStart=/usr/local/bin/monitor.sh, Restart=on-abnormal, User=monitor, and [Install] WantedBy=multi-user.target.

Answer

Create a unit file with [Service] Type=simple, ExecStart=/usr/local/bin/monitor.sh, Restart=always, User=monitor, and [Install] WantedBy=graphical.target.

Answer

Create a unit file with [Service] Type=forking, ExecStart=/usr/local/bin/monitor.sh, Restart=on-failure, User=monitor, and [Install] WantedBy=network-online.target.