The "systemctl mask nginx" command completely disables the "nginx" service, so that any start operation on that service fails. This makes "systemctl mask nginx" the correct answer. The "systemctl stop nginx" command stops the service if it is currently running. The other commands are invalid.

The "nmap" command is a network exploration tool, security scanner, and port scanner, making "nmap 10.20.30.40" the correct answer. The "ping" command sends ICMP packets and waits for a response. It does not determine which ports are open on the host. The "scan" command is an invalid command. The "lsof" command lists open files. It can be used to identify open ports on the local system, but not on remote hosts.

You can find the currently used scheduler in the virtual file located at "/sys/block/DEV/queue/scheduler", making "cat /sys/block/sdc/queue/scheduler" the correct answer. "/proc/block/sdc/queue/scheduler" does not exist. The "grep" command is used to search for patterns or strings within a file. The "lssched" command does not exist.

The "getenforce" command displays the current mode of SELinux. It does not provide information about previous SELinux states. Also, the "getenforce" command has nothing to do with firewalls or passwords.

Commands that execute successfully, be they binary executables or shell scripts, return an exit status of 0. If an error is encountered, then a non-zero exit status is returned. Valid exit statuses are integers that fall between 0 and 255.

The "su" command allows you to switch to another user on the system. The syntax of "su - ACCOUNT" starts a login shell with an environment similar to a real login, making "su - jason" the correct answer. Running "su jason" retains the existing environment. "sudo jason" attempts to execute a command named "jason" as the root user, making it an invalid answer. "twitch" is not a valid Linux command.

Multiple containers can mount the same volume. Also, a volume can be mounted read-write for some containers and read-only for other containers, at the same time.

There are two forms of command substitution: $(command) and `command`. Even though the newer format of $(command) is considered the current best practice, `command` is still valid syntax. Therefore both options are correct.

True. The hostnamectl command can be used to query and change the system hostname.

The "Permission denied" error most likely occurred because they did not have execute permissions to the file. In short, they forgot to add execute permissions to the script. They could have done so with "chmod +x script" or something like "chmod 755 script". It is not the file name or file extension that determines if it can be executed, it is the permissions assigned to the file. If the user placed the script in another location and attempted to execute a file script that does not exist, they would have received a "No such file or directory" error message instead.