LFCS Essential Commands — 10 Questions

Question 1

An administrator wants to ensure that a background process continues running after logout. Which command should be used to start the process?

Accepted Answer

nohup sleep 100 &. `nohup` allows a process to ignore the SIGHUP signal that is sent to background processes when the parent shell exits, ensuring it continues running after logout. The `&` places the command in the background, and `nohup` redirects output to `nohup.out` by default, making it the standard way to run a process immune to hangups.

Answer

runproc sleep 100 &

Answer

sleep 100 &

Answer

sleep 100 & disown

Question 2

A technician needs to display the contents of a compressed file named archive.tar.gz without extracting it. Which command should be used?

Accepted Answer

zcat archive.tar.gz | tar -t. It uses `zcat` to decompress the `.gz` layer on the fly and pipes the resulting uncompressed tar archive into `tar -t`, which lists the contents without extracting. This allows viewing the file listing without first decompressing to disk, satisfying the requirement to display contents without extraction.

Answer

tar -tf archive.tar

Answer

tar -xzf archive.tar.gz

Answer

zcat archive.tar.gz

Question 3

Which TWO commands can be used to create a new empty file?

Accepted Answer

touch file. The `touch` command is specifically designed to create an empty file if it does not already exist, or update its timestamps if it does. Option D is correct because using the shell redirection operator `>` with no command before it (or with a null command) creates an empty file by truncating any existing content or creating a new zero-byte file.

Answer

mkdir file

Answer

cat file

Answer

echo 'text' > file

Question 4

A developer accidentally deleted a critical file /var/log/app.log. The system administrator knows that the file was recently backed up using a cron job that runs 'tar -czf /backup/logs.tar.gz /var/log/'. Which command should the administrator use to restore the file from the backup without extracting the entire archive?

Accepted Answer

tar -xzf /backup/logs.tar.gz --to-stdout var/log/app.log > /var/log/app.log. `tar -xzf` extracts the specified file (`var/log/app.log`) from the compressed archive and `--to-stdout` sends its content to standard output, which is then redirected to recreate the file at `/var/log/app.log`. This restores the single file without extracting the entire archive, matching the requirement precisely.

Answer

tar -xf /backup/logs.tar.gz var/log/app.log

Answer

tar -tf /backup/logs.tar.gz | grep app.log

Answer

cp /backup/logs.tar.gz:/var/log/app.log /var/log/app.log

Question 5

A system administrator needs to find all files in /home that are owned by user 'alice' and have been modified in the last 7 days. The administrator then wants to compress those files into a single archive named alice_recent.tar.gz. Which of the following commands accomplishes this?

Accepted Answer

tar -czf alice_recent.tar.gz -T <(find /home -user alice -mtime -7). It uses `tar -czf` with process substitution to compress the files found by `find` into a gzipped tar archive named alice_recent.tar.gz. The `-z` flag enables gzip compression, and `-T <(...)` reads the list of files from the process substitution, which runs `find` to locate files owned by alice modified within the last 7 days. Options A, B, and D either do not compress or use incompatible formats.

Answer

find /home -user alice -mtime -7 -exec tar -rf alice_recent.tar.gz {} +

Answer

find /home -user alice -mtime -7 -print | cpio -o > alice_recent.tar.gz

Answer

find /home -user alice -mtime -7 | tar -cvf alice_recent.tar.gz

Question 6

A system administrator is troubleshooting a user's report that a command 'myapp' is not found. The administrator checks the PATH variable and sees it includes /usr/local/bin. The administrator verifies that the binary 'myapp' exists in /usr/local/bin with permissions 755. However, running 'myapp' still fails with 'command not found'. What is the most likely cause?

Accepted Answer

The shell's hash table is stale; run 'hash -r'. The shell caches the locations of executables in a hash table to avoid searching PATH repeatedly. When a new binary is added to a directory already in PATH, the shell may still have a stale entry (or no entry) for that command, causing it to report 'command not found' even though the binary exists. Running 'hash -r' clears the entire hash table, forcing the shell to re-scan PATH on the next invocation.

Answer

The user's PATH does not include /usr/local/bin

Answer

The binary is a shell script missing a shebang

Answer

The binary does not have execute permission for the user

Question 7

A user with uid 1000 tries to read /etc/shadow and gets 'Permission denied'. The user is not in the shadow group. Which of the following actions would allow the user to read the file without changing the file's group or permissions?

Accepted Answer

Add user1 to the shadow group using usermod -aG shadow user1. Adding user1 to the shadow group grants group-level read access to /etc/shadow without altering the file's permissions or group ownership. The file is typically owned by root:shadow with permissions 640 (rw-r-----), so members of the shadow group can read it. The usermod -aG command appends the user to the supplementary group, preserving existing group memberships.

Answer

Set an ACL using setfacl to grant user1 read access

Answer

Use chmod o+r /etc/shadow as root

Answer

Change the file owner to user1 using chown

Question 8

A system administrator runs 'grep -r 'error' /var/log' and gets many false positives. They want to search only for the exact word 'error' as a whole word, case-insensitively, and display line numbers. Which command should they use?

Accepted Answer

grep -rwin 'error' /var/log. It combines all required flags: `-r` for recursive search, `-w` for whole-word matching (using word boundaries), `-i` for case-insensitive search, and `-n` for displaying line numbers. The `-w` flag ensures that only the exact word 'error' is matched, not substrings like 'error404' or 'error-prone', which eliminates false positives.

Answer

grep -rwi 'error' /var/log

Answer

grep -rin 'error' /var/log

Answer

grep -rwn 'error' /var/log

Question 9

Order the steps to mount an ext4 filesystem from an external USB drive automatically at boot.

Accepted Answer

Identify the UUID using blkid, create the mount point directory, add an entry to /etc/fstab using the UUID with options defaults and dump/pass values, then run mount -a to test.. Using UUID in fstab ensures persistent mounting; testing with mount -a verifies no errors.

Answer

Plug in the USB drive, create the mount point, add an entry to /etc/fstab using the device name (e.g., /dev/sdb1), then reboot without testing.

Answer

Run mount -a first, then edit /etc/fstab to add the entry using UUID, and finally create the mount point.

Answer

Add an entry to /etc/fstab using the device name, manually mount with mount command, then create the mount point after mounting.

Question 10

A system administrator needs to find all files under /var/log that have been modified within the last 7 days. Which command accomplishes this task?

Accepted Answer

find /var/log -mmin -10080. Options B and C are both correct because the `find` command can search for files modified within the last 7 days using either `-mtime -7` (modification time in days) or `-mmin -10080` (modification time in minutes, where 10080 minutes equals 7 days). Option A checks access time (`-atime`), not modification time. Option D checks change time (`-ctime`), which includes metadata changes. Therefore, both B and C achieve the task.

Answer

find /var/log -atime -7

Answer

find /var/log -ctime -7