Unix/Linux System Commands Quiz

The tac command (cat spelled backwards) displays file contents in reverse order, line by line. Unlike cat (normal order), head (first lines), or tail -f (live monitoring), tac is specifically designed for reverse-order output.

In ls -l output, the first character indicates file type: 'd' for directory, '-' for regular file. grep "^d" filters lines starting with 'd', showing only directories. Option A's ls -d lists directory entries but doesn't filter contents by type; Option B uses a variable pattern that won't match the directory marker.

The tee command reads from standard input and writes to both a file and standard output simultaneously, achieving both goals in one command. The > operator only redirects to a file without displaying; combining it with other output requires separate commands.

The du command estimates file space usage, sort -nr sorts this output numerically in reverse order so that the largest entries appear at the top, and head -5 limits the final output to the top five space-occupying files or directories in the system.

In Unix and Linux shells, the special shell parameter $$ expands to the process ID (PID) of the current shell session. It does not output literal characters, nor does it represent the parent process ID, which is retrieved using the $PPID variable.

The df (disk free) command shows disk space usage and available space on mounted filesystems. du (disk usage) shows directory space consumption, not filesystem-level usage. The question specifically asks for filesystem usage, not directory usage.

ls -ltr *.zip directly lists files ending in .zip. ls -ltr | grep "zip$" lists all files, then filters for lines ending in 'zip'. Both achieve the same result for .zip files - the shell glob matches the pattern, and the regex anchors to the end.

Both grep argon and grep "argon" will match lines containing "argon" - the quotes are optional when the pattern has no spaces or special characters. They're functionally equivalent for checking if the argon agent process is running.

kill -9 PID sends SIGKILL to forcefully terminate the process with the specified PID. Option A only lists processes. Option B sends SIGTERM (not -9). Option C has incorrect syntax - ps doesn't output a PID directly for kill. Only D correctly uses kill -9 with a PID placeholder.

The default umask value on most Unix/Linux systems is 022, which sets default permissions to 755 for directories and 644 for files (subtracting 022 from 777/666). Option B (032) would produce unusual permissions, 644 represents file permissions not umask, and 000 would give full permissions to all users.

The vmstat -f command displays the number of fork system calls since boot, directly answering the question about counting forks. Option A shows memory information, plain vmstat shows virtual memory statistics without fork count, and ls lists directory contents.

The ls -a command lists all files in a directory including hidden files (those starting with .). While the question phrasing 'only hidden file' is awkward, ls -a is the standard command for viewing hidden files. Plain ls shows only visible files, ls -s shows sizes, and ls -a .* would only list hidden items starting with dot in the current directory.

The netstat -r command displays the kernel IP routing table, showing network routes and their properties. ifconfig shows network interface configuration but not routing tables, ping tests network connectivity, and 'none' is incorrect.

The netstat -t command shows TCP connections, which are the active internet connections. Plain netstat shows all network connections, netstat -s shows summary statistics, and 'all the above' is incorrect since only the -t option specifically targets active internet connections.

The default nice value in Unix/Linux is 0, which represents normal scheduling priority. Lower values (negative) indicate higher priority, while higher values indicate lower priority. Options 3, 1, and 11 are non-default values.