XK0-006 Practice Test 20 — 15 Questions

Question 1

A Linux administrator needs to check which services are listening on TCP port 22. Which command should be used?

Accepted Answer

ss -tlnp | grep :22. The 'ss' command (socket statistics) with the -tlnp flags lists TCP (-t) listening (-l) sockets with numeric ports (-n) and the owning process (-p). Piping to grep :22 filters for port 22, showing which service is listening on SSH. This is the modern replacement for netstat and is available on all current Linux distributions.

Answer

ping -p 22 localhost

Answer

ip addr show port 22

Answer

traceroute -p 22 localhost

Question 2

An administrator wants to use Ansible to ensure a service is running on a remote host. Which Ansible module should be used in a playbook?

Accepted Answer

service. The Ansible 'service' module is designed to manage services on remote hosts — starting, stopping, restarting, and enabling them. To ensure a service is running, you use 'service' with state: started (and optionally enabled: yes). It abstracts underlying init systems (systemd, SysV, upstart) so the playbook works across distributions.

Answer

command

Answer

copy

Answer

shell

Question 3

A Docker container is running in the background. Which command allows the administrator to execute an interactive bash shell inside the running container named 'webapp'?

Accepted Answer

docker exec -it webapp bash. docker exec -it runs an interactive command in a running container. The other commands are for different purposes.

Answer

docker start -i webapp

Answer

docker run -it webapp bash

Answer

docker attach webapp

Question 4

An administrator wants to schedule a script to run every Monday at 3 AM. Which crontab entry is correct?

Accepted Answer

0 3 * * 1 /path/to/script. The crontab syntax requires five fields: minute, hour, day of month, month, and day of week. '0 3 * * 1' means minute 0, hour 3 (3 AM), any day of month (*), any month (*), and day of week 1 (Monday). This precisely schedules the script to run at 3:00 AM every Monday.

Answer

* 3 * * 1 /path/to/script

Answer

0 3 * * * /path/to/script

Answer

0 3 1 * * /path/to/script

Question 5

A system administrator needs to restrict SSH access to a Linux server to only users in the 'sshusers' group. Which configuration change achieves this?

Accepted Answer

Add 'AllowGroups sshusers' to /etc/ssh/sshd_config. The 'AllowGroups' directive in /etc/ssh/sshd_config restricts SSH access to only users who are members of the specified group. When set to 'AllowGroups sshusers', only users belonging to the 'sshusers' group will be permitted to log in via SSH, effectively blocking all others. This is the standard method for group-based access control in OpenSSH.

Answer

Add 'DenyUsers *' to /etc/ssh/sshd_config

Answer

Set 'PermitRootLogin no' in /etc/ssh/sshd_config

Answer

Add 'AllowUsers sshusers' to /etc/ssh/sshd_config

Question 6

A database server running on Linux is experiencing high load. The administrator runs 'strace -p <pid>' and sees many 'epoll_wait' and 'futex' calls. Which THREE of the following are possible causes of the high load? (Choose THREE.)

Accepted Answer

Disk I/O contention causing processes to wait.. 'epoll_wait' indicates the process is waiting for I/O events, and 'futex' calls are used for synchronization. Disk I/O contention can cause the database process to block on these system calls, leading to high load as the kernel schedules other tasks while waiting for I/O to complete.

Answer

CPU frequency scaling is set to powersave.

Answer

A memory leak in the database process.

Question 7

A bash script uses a loop to iterate over files in a directory. Which TWO of the following loop constructs will correctly iterate over each .txt file in the current directory? (Select TWO).

Accepted Answer

for file in *.txt; do. Option C is correct because `for file in *.txt; do` relies on bash pathname expansion (globbing), which expands the unquoted pattern `*.txt` into the list of matching filenames in the current directory, iterating over each one. Option E is correct because `while IFS= read -r file; do ... done < <(find . -maxdepth 1 -name '*.txt')` uses process substitution to feed find's output into the loop; `IFS=` preserves leading/trailing whitespace and `read -r` prevents backslash interpretation, and `find . -maxdepth 1 -name '*.txt'` lists only .txt files directly in the current directory. Option A is wrong because quoting `'*.txt'` suppresses globbing, so the loop runs once with the literal string `*.txt`. Option B is wrong because parsing `ls` output is fragile and breaks on filenames containing spaces, newlines, or glob characters. Option D is wrong because it iterates over indices of a `files` array that is never populated in the scenario, so it does not iterate over the .txt files.

Answer

for file in '*.txt'; do

Answer

for file in $(ls *.txt); do

Answer

for ((i=0; i<${#files[@]}; i++)); do

Question 8

Based on the exhibit, what is the most likely cause of the repeated connection refused errors?

Accepted Answer

The database service is down.. The 'connection refused' error indicates that the client's TCP SYN packet reached the target host on port 3306, but the host actively rejected the connection because no process is listening on that port. This is the classic symptom of the MySQL/MariaDB database service being stopped or crashed, as the OS TCP stack sends an RST packet when a connection attempt hits a port with no listening socket.

Answer

The DNS resolution for the database host fails.

Answer

A firewall is blocking port 3306.

Answer

The database credentials are incorrect.

Question 9

A DevOps engineer needs to debug a bash script that unexpectedly fails when processing files. Which of the following should be added to the script to print each command before execution?

Accepted Answer

set -x. set -x enables shell tracing, which prints each command (with expanded variables) to stderr prefixed by PS4 (usually '+') before execution. This is exactly what is needed to see what the script is doing as it processes files and where it fails.

Answer

set -v

Answer

set -e

Answer

set -u

Question 10

A junior administrator needs to change the ownership of the file /var/www/html/index.html from user 'www-data' to user 'apache' and group 'apache'. Which command will accomplish this?

Accepted Answer

chown apache:apache /var/www/html/index.html. The chown command with user:group syntax changes both the owner and group of a file. Specifying apache:apache sets the owner and group to apache, fulfilling the requirement. Other commands either modify permissions, change only the group, or modify user account properties rather than file ownership.

Answer

usermod -g apache www-data /var/www/html/index.html

Answer

chmod apache:apache /var/www/html/index.html

Answer

chgrp apache /var/www/html/index.html

Question 11

A junior administrator needs to check the disk usage of the /var filesystem to ensure it does not exceed 80% capacity. Which command should be used?

Accepted Answer

df -h /var. The `df -h /var` command displays disk usage for the `/var` filesystem in human-readable format (e.g., GB, MB). This is the correct tool to check filesystem capacity and usage percentage, which directly answers whether usage exceeds 80%.

Answer

fdisk -l /var

Answer

du -sh /var

Answer

lsblk /var

Question 12

An administrator needs to check the kernel ring buffer for hardware error messages after a system crash. Which command should be used?

Accepted Answer

dmesg. dmesg shows kernel ring buffer messages including hardware errors. journalctl -k shows kernel messages from systemd journal, but dmesg is the direct command for the ring buffer.

Answer

journalctl -k

Answer

tail -f /var/log/messages

Answer

strace -e trace=open

Question 13

A user reports that a service fails to start with the error 'Permission denied'. The service runs under a non-root user. Which command should the administrator use to check if the service has the correct file permissions?

Accepted Answer

namei -l /path/to/service. The error 'Permission denied' when starting a service under a non-root user often involves not just the file's own permissions but also the permissions of each directory in the path leading to the service binary. The `namei -l` command recursively lists the permissions of every component in the path, revealing if any parent directory lacks execute (search) permission for the service user, which would block access even if the binary itself is correctly set. This makes it the most comprehensive tool for diagnosing path-based permission issues.

Answer

ls -l /path/to/service

Answer

getfacl /path/to/service

Answer

stat /path/to/service

Question 14

Match each Linux runlevel to its standard systemd target.

Accepted Answer

runlevel 0: poweroff.target. Systemd targets align with traditional SysV runlevels: runlevel 0 → poweroff.target, runlevel 1 → rescue.target, runlevel 2/3/4 → multi-user.target, runlevel 5 → graphical.target, runlevel 6 → reboot.target. Common confusions involve mixing up GUI and non-GUI targets or confusing runlevels for shutdown/reboot.

Answer

runlevel 3: graphical.target

Answer

runlevel 6: rescue.target

Answer

runlevel 2: poweroff.target

Question 15

A system administrator needs to collect performance data over time to analyze CPU and memory usage trends. Which THREE of the following commands can be used to gather historical performance data? (Choose THREE.)

Accepted Answer

iostat. Option A, iostat, is correct because it reports CPU utilization and disk I/O statistics and can be run repeatedly (e.g., iostat 5) or logged over time to reveal performance trends. Option D, sar, is correct because the System Activity Reporter collects and stores historical performance data via the sadc collector and sysstat service, allowing retrieval of past CPU and memory statistics with commands like sar -u or sar -r. Option E, vmstat, is correct because it samples CPU, memory, paging, and I/O statistics at intervals (e.g., vmstat 5) and its output can be captured over time for trend analysis. Option B, uptime, is not correct because it only shows a single snapshot of load average and uptime, not historical performance data. Option C, free, is not correct because it displays a one-time snapshot of current memory usage rather than collecting data over time.

Answer

uptime

Answer

free