LFCS Practice Test 3 — 30 Questions

Question 1

A user wants to continuously monitor a log file that is being written to by a running service. Which command achieves this?

Accepted Answer

tail -f /var/log/syslog. The `tail -f` command displays the last 10 lines of a file by default and then continues to output new lines as they are appended, making it ideal for real-time monitoring of a growing log file. The `-f` flag (follow) keeps the file open and polls for changes, typically using inotify on Linux, to output new data immediately.

Answer

head -20 /var/log/syslog

Answer

less /var/log/syslog

Answer

cat /var/log/syslog

Question 2

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 3

A system administrator configures a web server using systemd. After creating a custom service unit file, the administrator runs `systemctl daemon-reload` but the service still fails to start with a 'Unit not found' error. What is the most likely cause?

Accepted Answer

The service name was misspelled in the `systemctl start` command.. The 'Unit not found' error after `systemctl daemon-reload` typically indicates that systemd cannot locate a unit with the specified name. While correct placement of unit files is important, systemd scans both /etc/systemd/system/ and /usr/lib/systemd/system/ after a daemon-reload. Therefore, placing a custom unit in /usr/lib/systemd/system/ would not cause this error. The most likely cause is that the service name was misspelled in the `systemctl start` command. A simple typo would lead to a 'Unit not found' message because systemd looks for an exact match. Other options like forgetting `systemctl enable` or group membership do not affect unit discovery at start time.

Answer

The administrator forgot to run `systemctl enable` before starting the service.

Answer

The unit file is placed in /usr/lib/systemd/system/ instead of /etc/systemd/system/.

Answer

The administrator is not in the 'systemd' group.

Question 4

A custom service requires that the network is fully operational before it starts. Which directive should be added to the [Unit] section of the service's unit file to ensure this dependency?

Accepted Answer

After=network-online.target. `After=network-online.target` ensures the service starts only after the network is fully operational, including IP address assignment and connectivity. This target is reached when network managers like systemd-networkd or NetworkManager confirm the network is online, making it suitable for services that require active network interfaces.

Answer

Requires=network.target

Answer

Wants=network-online.target

Answer

After=network.target

Question 5

A user 'dba' tries to login via SSH and fails. Based on the exhibit, what is the most likely cause?

Accepted Answer

The user 'dba' has an invalid login shell.. The exhibit shows that the user 'dba' has an invalid login shell (e.g., /sbin/nologin or /bin/false). When the login shell is set to a non-interactive shell, SSH authentication succeeds but the session immediately closes, preventing the user from logging in. This is a common configuration for system accounts or users who should not have interactive shell access.

Answer

The file /home/dba/file.txt is corrupt.

Answer

The user 'dba' is not in the 'docker' group.

Answer

The home directory /home/dba does not have correct permissions.

Question 6

A DevOps engineer wants to measure how long a specific command takes to execute. Which command should be used?

Accepted Answer

time. The `time` command is specifically designed to measure the execution duration of a command, reporting real time, user CPU time, and system CPU time. It wraps the target command and tracks the elapsed wall-clock time and resource usage, making it the precise tool for benchmarking command performance.

Answer

date

Answer

uptime

Answer

wall

Question 7

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 8

A system administrator is troubleshooting network connectivity from a server that can reach internal resources but cannot access the internet. The server's /etc/sysconfig/network-scripts/ifcfg-eth0 file contains: BOOTPROTO=static, IPADDR=10.0.0.10, NETMASK=255.255.255.0, GATEWAY=10.0.0.1. The administrator runs 'ip route show' and sees: default via 10.0.0.1 dev eth0. However, 'ping 8.8.8.8' fails. Which is the most likely cause?

Accepted Answer

The gateway 10.0.0.1 is not configured to forward traffic to the internet (no NAT or upstream route).. The routing table shows a default gateway (10.0.0.1) is present, so the issue is not a missing route. Since the server can reach internal resources but not the internet, the most likely cause is that the gateway itself (10.0.0.1) is not configured to perform NAT or does not have an upstream route to forward traffic beyond the local subnet. Without this, packets destined for 8.8.8.8 are sent to the gateway but are then dropped because the gateway has no path to the internet.

Answer

The default gateway is missing from the routing table.

Answer

DNS resolution is not configured.

Answer

A firewall is blocking outbound ICMP traffic.

Question 9

Which TWO are valid methods to configure a network interface on a Linux system?

Accepted Answer

Editing /etc/network/interfaces. Option C is correct because /etc/network/interfaces is the standard configuration file used by the ifupdown toolset on Debian-based Linux distributions to define network interfaces, their addressing (static or DHCP), and related parameters. Option D is correct because nmcli connection add is the NetworkManager command-line method for creating a new connection profile that configures an interface, including its IP addressing, gateway, and DNS settings. Option A is not a valid interface configuration method because sysctl only tunes kernel runtime parameters such as net.ipv4.conf.eth0.forwarding (packet forwarding), not interface addressing or link settings. Option B is not valid because systemctl enable network.service merely sets the legacy network service to start at boot; it does not itself configure an interface. Option E is not valid because /etc/sysconfig/network is a Red Hat-style file that historically held global hostname and gateway settings, not per-interface configuration.

Answer

Using sysctl to set net.ipv4.conf.eth0.forwarding

Answer

Using systemctl enable network.service

Answer

Editing /etc/sysconfig/network

Question 10

Which TWO of the following are correct statements about systemd journald configuration?

Accepted Answer

The 'MaxRetentionSec' directive sets the maximum time to retain journal entries.. The 'MaxRetentionSec' directive in journald.conf specifies the maximum time (in seconds) that journal entries are retained before they are deleted. This is a time-based retention policy, distinct from size-based limits, and is used to automatically prune old log entries to manage disk usage.

Answer

The 'RuntimeMaxUse' directive applies to the journal stored in /var/log/journal.

Answer

The 'Compress' directive is set to 'no' by default.

Answer

The 'ForwardToSyslog' directive is set to 'yes' by default.

Question 11

A server is running out of disk space. Which command will show the disk usage of the root filesystem in a human-readable format?

Accepted Answer

df -h /. The `df -h /` command displays disk usage for the root filesystem (`/`) in a human-readable format (e.g., GB, MB) by using the `-h` flag. This is the standard tool for checking filesystem-level disk space, not directory-level usage.

Answer

ls -lh /

Answer

fdisk -l /

Answer

du -sh /

Question 12

Refer to the exhibit. The administrator receives alerts that the root filesystem is almost full. Which command could free up space by removing old log files?

Accepted Answer

find /var/log -name '*.log' -mtime +30 -delete. The `find` command with `-name '*.log'` targets log files, `-mtime +30` selects files modified more than 30 days ago, and `-delete` removes them. This safely frees space by purging only old logs, preserving recent logs needed for troubleshooting.

Answer

truncate -s 0 /var/log/syslog

Answer

rm -rf /var/log/*

Answer

du -sh /var/log

Question 13

An administrator needs to encrypt a block device (/dev/sdc) using LUKS. Which command creates an encrypted LUKS container on the device?

Accepted Answer

cryptsetup luksFormat /dev/sdc. `cryptsetup luksFormat /dev/sdc` initializes the block device with a LUKS header, setting up an encrypted container that can later be opened with a passphrase or key file. This is the standard command for creating a new LUKS partition, as it writes the LUKS metadata and prepares the device for encryption.

Answer

cryptsetup luksOpen /dev/sdc encrypted_device

Answer

openssl enc -aes-256-cbc -in /dev/sdc -out /dev/sdc.enc

Answer

dm-crypt create encrypted /dev/sdc

Question 14

Refer to the exhibit. The administrator attempted to create a user 'newuser' but received an error. Which command should be used to check if the user already exists?

Accepted Answer

cat /etc/passwd | grep newuser. The /etc/passwd file stores all user account information, and using cat to pipe its contents through grep allows the administrator to search for the specific username 'newuser'. If the user exists, grep will output the matching line; if not, no output is returned. This is a standard, quick method to verify user existence without modifying system state.

Answer

passwd -S newuser

Answer

usermod -c newuser

Answer

userdel -v newuser

Question 15

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 16

You are a senior Linux administrator for a large data center. A junior admin reports that a newly deployed application server (192.168.100.50/24, default gateway 192.168.100.1) cannot communicate with a legacy server (192.168.200.50/24, default gateway 192.168.200.1). The two subnets are connected via a router (192.168.100.1 and 192.168.200.1). From the app server, you can ping the legacy server's IP successfully. However, when you try to establish an SSH session from the app server to the legacy server, it times out. You check the legacy server's firewall (ufw) and find that it allows SSH (port 22) from the entire 192.168.0.0/16 range. You also confirm that the SSH daemon is running and listening on 0.0.0.0:22. What is the most likely cause?

Accepted Answer

The app server's firewall (ufw) is blocking incoming SSH responses.. The app server can ping the legacy server successfully, which confirms that ICMP traffic (Layer 3) passes through the router and that the legacy server's firewall allows ICMP. However, SSH (TCP port 22) fails because the app server's own firewall (ufw) is blocking the incoming SSH response packets (SYN-ACK) from the legacy server. Since the SSH client initiates the connection from the app server, the response packets must be allowed by the app server's firewall; if ufw on the app server blocks established or related incoming traffic, the TCP handshake cannot complete, resulting in a timeout.

Answer

The router is dropping TCP packets due to ACLs.

Answer

The legacy server's firewall is not allowing SSH; the rule might be misconfigured.

Answer

The legacy server's SSH service is not listening on the correct interface.

Question 17

A developer needs to temporarily allow incoming TCP connections on port 8080 for testing. Which iptables command adds a rule to the INPUT chain to accept this traffic?

Accepted Answer

iptables -A INPUT -p tcp --dport 8080 -j ACCEPT. The INPUT chain processes traffic destined for the local system, and the `--dport 8080` flag matches incoming TCP packets with destination port 8080. The `-j ACCEPT` target allows these packets through, which is exactly what is needed to temporarily permit incoming TCP connections on port 8080 for testing.

Answer

iptables -A OUTPUT -p tcp --sport 8080 -j ACCEPT

Answer

iptables -A FORWARD -p tcp --dport 8080 -j ACCEPT

Answer

iptables -I INPUT 1 -p tcp --dport 8080 -j DROP

Question 18

Which two commands can add an existing user to a supplementary group?

Accepted Answer

gpasswd -a. Option B, gpasswd -a, is correct because gpasswd with the -a (add) flag adds an existing user to a named supplementary group, e.g. `gpasswd -a alice developers`, modifying /etc/group without altering the user's primary group. Option E, usermod -aG, is correct because usermod with -G specifies supplementary groups and the -a (append) flag ensures the listed group is added to the user's existing supplementary group set rather than replacing it, e.g. `usermod -aG developers alice`. Option A, useradd -G, is wrong here because useradd creates new accounts and its -G flag sets supplementary groups only at account-creation time, so it cannot add an already-existing user. Option C, addgroup, is wrong because it is a Debian/Ubuntu convenience script for creating a new group (and optionally adding a user at creation), not the standard command for adding an existing user to an existing supplementary group. Option D, groupmod, is wrong because it modifies a group's attributes such as its name (-n) or GID (-g), and does not manage user membership.

Answer

useradd -G

Answer

addgroup

Answer

groupmod

Question 19

Which THREE commands can be used to view the contents of a file?

Accepted Answer

cat. The `cat` command (option C) is a standard Unix utility that reads files sequentially and outputs their contents to the standard output. It is one of the most basic and direct ways to view the entire content of a file in the terminal.

Answer

grep

Answer

find

Question 20

A system administrator needs to create a new ext4 filesystem on /dev/sdb1 with a reserved block percentage of 2% instead of the default 5%. Which command should be used?

Accepted Answer

mkfs.ext4 -m 2 /dev/sdb1. The `-m` flag in `mkfs.ext4` sets the reserved block percentage for the superuser, and specifying `-m 2` overrides the default of 5% to reserve only 2% of the blocks on the new ext4 filesystem on /dev/sdb1.

Answer

mkfs.ext4 -R 2 /dev/sdb1

Answer

mkfs.ext4 -r 2 /dev/sdb1

Answer

tune2fs -m 2 /dev/sdb1