LFCS Practice Test 6 — 10 Questions

Question 1

A system administrator monitors a server that periodically becomes unresponsive for a few seconds. Investigation reveals that the kernel OOM killer is being invoked, but standard memory usage monitoring tools like 'free' and 'top' show less than 50% memory usage. The server runs a web server and a database. Which of the following is the most likely cause and diagnostic step?

Accepted Answer

The kernel is using a large amount of memory for slab caches. Use 'slabtop' to examine kernel memory allocations.. Option D is correct because the kernel OOM killer can be triggered by high slab cache usage, which is memory allocated for kernel data structures like inode and dentry caches. Standard tools like 'free' and 'top' may show low overall memory usage because they report only user-space memory, while slab caches are accounted separately. Using 'slabtop' allows the administrator to identify which kernel slab caches are consuming excessive memory, often due to a large number of small files or heavy filesystem metadata operations.

Answer

The server is experiencing a fork bomb. Use 'ps -ef | wc -l' to count processes.

Answer

The database is using swap space excessively. Use 'swapon -s' to check swap usage.

Answer

The web server is leaking file descriptors. Use 'lsof -n' to check open files.

Question 2

Which THREE of the following are valid Linux filesystem types that can be used for root partitions on a modern Linux system?

Accepted Answer

XFS. XFS is a high-performance 64-bit journaling filesystem created by Silicon Graphics, now widely supported in the Linux kernel. It is a valid choice for root partitions on modern Linux systems, especially for large file and high-concurrency workloads, and is the default filesystem in Red Hat Enterprise Linux 7 and later.

Answer

NTFS

Answer

FAT32

Question 3

A system administrator is configuring a custom systemd service that runs a Python script. The script logs output to stdout. The administrator wants to ensure that the service restarts automatically if it crashes, but only after a 10-second delay. Which directive should be added to the [Service] section of the unit file?

Accepted Answer

Restart=on-failure
RestartSec=10. Option B is correct because `Restart=on-failure` ensures the service restarts only when it exits with a non-zero exit code or is terminated by a signal (e.g., SIGKILL), which matches the 'crashes' scenario. Adding `RestartSec=10` introduces a 10-second delay before the restart attempt, as required. The other options either restart on success (irrelevant) or always restart (which would restart even on intentional stops), or omit the restart condition entirely.

Answer

Restart=on-success
RestartSec=10

Answer

Restart=always
RestartSec=10

Answer

RestartSec=10

Question 4

A system administrator notices that a new 500GB SSD (/dev/sdb) is not being recognized by the system after installation. The server uses UEFI and GPT partitioning. Which command should the administrator run first to verify that the disk is detected by the kernel?

Accepted Answer

lsblk. The `lsblk` command lists all block devices recognized by the kernel, including those without a filesystem or partition table. Since the disk is new and not yet partitioned, `lsblk` will show it if the kernel has detected it, making it the correct first diagnostic step.

Answer

fdisk -l /dev/sdb

Answer

cat /proc/cpuinfo

Answer

lsusb

Question 5

A Linux administrator needs to create a RAID 5 array using three disks: /dev/sdb, /dev/sdc, and /dev/sdd, each 2TB. The administrator wants to ensure the array can be reassembled automatically after a reboot. Which command should be used to create the array?

Accepted Answer

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd --metadata=1.2. Option D is correct because it uses `--metadata=1.2`, which stores the superblock at a standard 4 KiB offset from the start of each device, enabling the kernel to automatically discover and assemble the RAID array on reboot without manual intervention. RAID 5 with three disks provides striping with distributed parity, offering a balance of performance and redundancy.

Answer

mdadm --create /dev/md0 --level=5 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd --metadata=0.90

Answer

mdadm --create /dev/md0 --level=1 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd

Answer

mdadm --create /dev/md0 --level=0 --raid-devices=3 /dev/sdb /dev/sdc /dev/sdd

Question 6

A server runs a custom application that listens on TCP port 8080. The administrator wants to ensure the application starts automatically on boot and restarts if it crashes. Which systemd unit file directive should be used to achieve the restart behavior?

Accepted Answer

Restart=on-failure. The `Restart=on-failure` directive in a systemd unit file instructs systemd to automatically restart the service unit when it exits with a non-zero exit code, is terminated by a signal (including SIGKILL), or times out. This directly satisfies the requirement for the application to restart if it crashes, as a crash typically results in an unclean exit that triggers the restart condition.

Answer

RestartSec=5

Answer

Type=notify

Answer

RemainAfterExit=yes

Question 7

An administrator runs 'systemctl status sshd' and sees the output above. The administrator wants sshd to start automatically at boot. Which command should be used?

Accepted Answer

systemctl enable sshd. The `systemctl enable sshd` command creates the necessary symlinks in the systemd unit configuration directories (e.g., `/etc/systemd/system/multi-user.target.wants/`) so that the sshd service is started automatically at boot. This is the correct way to configure a service to start on boot in a systemd-based Linux distribution.

Answer

systemctl reenable sshd

Answer

systemctl mask sshd

Answer

systemctl start sshd

Question 8

A developer reports that a web application's logs are not being written to /var/log/myapp.log. The service runs as user 'myapp' and the log directory /var/log/myapp/ has permissions 755 owned by root. What is the most likely cause?

Accepted Answer

The service user 'myapp' does not have write permission to the log directory.. Option D is correct because the /var/log/myapp/ directory has permissions 755, which grants read and execute access to the 'others' category but not write. Since the service runs as user 'myapp', which is not the owner (root) and not in the root group, it falls under 'others' and thus lacks write permission. Without write permission on the directory, the service cannot create or write to /var/log/myapp.log, even if the file itself might have different permissions.

Answer

AppArmor is denying access.

Answer

SELinux is blocking the write.

Answer

The service is logging to systemd-journald instead of a file.

Question 9

An administrator wants to ensure that a custom service (myapp.service) starts only after the network is available and the PostgreSQL database service is running. Which systemd unit file directive should be used?

Accepted Answer

After=network.target postgresql.service. Option D is correct because the `After=` directive in a systemd unit file specifies ordering constraints, ensuring that `myapp.service` starts only after `network.target` and `postgresql.service` have reached the 'started' state. This does not create a dependency that forces those units to start; it only orders the startup sequence, which is exactly what the administrator needs to guarantee the service starts after the network and PostgreSQL are available.

Answer

Requires=network.target postgresql.service

Answer

Wants=network.target postgresql.service

Answer

BindsTo=network.target postgresql.service

Question 10

Which TWO statements are true about systemd service unit files? (Choose two.)

Accepted Answer

The [Service] section is mandatory for a service unit file.. Option C is correct because the [Service] section is mandatory in a systemd service unit file; without it, systemd cannot determine how to manage the service process. This section defines the service type, execution command, and other runtime parameters essential for the unit to function.

Answer

Environment variables can be loaded using EnvFile= directive.

Answer

The default service type is 'forking'.

Answer

The [Service] section must appear before the [Unit] section.