Unit 2 · Self-check

Unit 2 Self-Check Quiz

Twenty questions covering Linux architecture, the shell and command line, users and groups, permissions and access control, and the development environment. Each question can be expanded to reveal a model answer.

20 questions Self-paced Ungraded

Questions

Multiple choice and short answer

Answer each question before expanding. The questions span all five lessons of Unit 2.

Question 1

Which of the following is the strictest definition of "Linux"?

The Ubuntu operating system.
The kernel — the software that manages hardware and provides fundamental abstractions.
The GNU user space.
Any UNIX-certified operating system.

Show answer

The second choice. Strictly speaking, Linux is the kernel — started by Linus Torvalds in 1991. The complete operating system is GNU/Linux, the kernel combined with the GNU user space. In common usage "Linux" refers to the whole family of distributions, but the strict definition is the kernel. Linux is not certified as UNIX; it implements the POSIX standard instead.

Question 2

A shell command is not found when you type it. Which of the following is the most likely cause?

The command is not in any of the directories listed in $PATH.
The command does not exist on the system.
You do not have execute permission on the command.
The shell is in the wrong working directory.

Show answer

The most common cause is that the command is not in any directory on $PATH. The command might exist — for example, in the current directory or in a directory that is not on the path — but the shell cannot find it by name alone. To run a program that is not on the path, give an explicit path such as ./myprogram. A lack of execute permission usually produces a different error ("Permission denied"), and being in the wrong directory only matters for relative paths.

Question 3

What is the difference between > and >> in a shell command?

Show answer

> redirects standard output to a file, overwriting whatever the file previously contained. >> also redirects standard output but appends to the file instead of overwriting it. The distinction is important because > can destroy existing data. In error, cmd > log.txt may wipe out the previous log.

Question 4

A file has the mode 640. Which of the following is true?

All users can read the file.
The owner can read and write; the group can read; others have no access.
Everyone can read and write.
The owner can execute the file.

Show answer

The second choice. 640 in octal is rw-r-----. The owner (6 = rw-) can read and write. The group (4 = r--) can read only. Others (0 = ---) have no access. This mode is common for files shared within a group but not exposed to the wider world. Note that execute permission is not granted to any category.

Question 5

With a umask of 027, what mode will a newly created directory have?

Show answer

Newly created directories have a base mode of 777. The umask is applied by AND-ing with its complement: ~027 = 750 in octal. So the mode is 777 & 750 = 750, or rwxr-x---. The owner has full access; the group can read and traverse; others have no access. If the same umask were applied to a new file (base mode 666), the result would be 666 & 750 = 640 — read/write for the owner, read for the group.

Question 6

Why does the setuid bit on a binary represent a potential security risk?

Show answer

A setuid binary runs with the effective UID of its owner — usually root — regardless of who invokes it. Any vulnerability in the program — a buffer overflow, a path-traversal error, a failure to sanitise environment variables, an unsafe temporary file — can allow an attacker to execute arbitrary code with root's privileges. Because setuid binaries are typically written in C and interact with untrusted input, they are a classic privilege-escalation target. Modern practice minimises the number of setuid binaries and replaces them with capabilities when possible.

Question 7

What is the difference between a user's primary group and their supplementary groups?

Show answer

The primary group is recorded in the GID field of /etc/passwd. Every user has exactly one. Files the user creates are initially owned by this group, unless the parent directory has the setgid bit set. The supplementary groups are additional groups the user belongs to, listed in the members field of /etc/group. A user can belong to any number of them. When the kernel checks file access, it considers both the primary group and all supplementary groups.

Question 8

Which of the following is the correct command to add a user to a supplementary group without removing them from their existing groups?

usermod -G group user
usermod -aG group user
groupadd user group
chgrp group user

Show answer

usermod -aG group user. The -G option without -a replaces the user's supplementary groups with the listed ones, potentially removing memberships the user needs. The -a flag appends instead of replacing. This is one of the most common mistakes in user administration.

Question 9

Which of the following commands would create a pipeline that counts the number of files in /etc?

ls /etc > wc -l
ls /etc | wc -l
ls /etc && wc -l
wc -l < ls /etc

Show answer

ls /etc | wc -l. The pipe character connects the output of ls to the input of wc, which counts lines. > would redirect to a file, && runs commands sequentially, and the last option has the redirection backward.

Question 10

What is the difference between the R and S state codes that appear in the STAT column of ps on Linux?

Show answer

R means the process is running or runnable — it is currently executing or on the ready queue, ready to execute. S means the process is in interruptible sleep — it is blocked on an event (typically I/O or a timer), and can be woken by a signal. The distinction corresponds to the Ready and Blocked states of the five-state model. A process in S cannot run until its event occurs; a process in R could run if given the CPU.

Question 11

What does the first line of a shell script #!/bin/bash do?

Show answer

It is the shebang. When the script is executed directly (e.g., ./script.sh), the kernel reads the first two bytes. If they are #!, the rest of the line is interpreted as the path to an interpreter, and the kernel runs that interpreter with the script as its argument. Without a shebang, the kernel does not know how to execute the file directly, and the user must invoke the interpreter explicitly (bash script.sh).

Question 12

Which of the following is the correct way to make a shell script executable?

chown +x script.sh
chmod +x script.sh
chgrp +x script.sh
chmod 000 script.sh

Show answer

chmod +x script.sh. The chmod command changes permissions; +x adds execute permission. With no explicit user specification, it applies to all categories (owner, group, others) as modified by the umask. This is the standard way to make a script runnable.

Question 13

What does the stat command report about a file, and how does this differ from the output of ls -l?

Show answer

stat reports the full metadata of a file: size in bytes, number of blocks allocated, inode number, device, link count, owner and group IDs, permissions in both octal and symbolic form, and three timestamps — access (atime), modification (mtime), and status change (ctime). ls -l shows a subset: permissions, link count, owner, group, size, and mtime. The two commands report the same underlying inode data, but stat is more complete, and it shows the octal mode directly rather than only the symbolic form.

Question 14

Which of the following commands will build a C program called prog from prog.c with the course-standard flags?

gcc prog.c
gcc -Wall -Wextra -g -o prog prog.c
gcc -o prog.c prog
make prog.c

Show answer

The second choice. It enables warnings (-Wall -Wextra), includes debug symbols (-g), and specifies the output filename (-o prog). The first choice compiles but silently produces an executable named a.out and no warnings; the third reverses the source and output; the fourth is not a valid invocation for a simple C file.

Question 15

What is the purpose of strace?

Show answer

It prints every system call a program makes, along with the arguments and return value of each. This lets you observe the interface between user space and the kernel at runtime. It is a diagnostic tool: you use it to understand what a program actually does (as opposed to what the source code appears to do), to find where a program is spending time, and to see which files it opens, which devices it accesses, and which errors it encounters. In this course, strace is the primary tool for connecting the system-call concept from Lesson 1.4 to real program behaviour.

Question 16

A shared project directory is set up with mode 2775 and owned by group projectx. What does the leading 2 do?

Enables the sticky bit, restricting deletion of files.
Sets the setgid bit so that new files inherit the directory's group.
Doubles the owner's permissions.
Marks the directory as immutable.

Show answer

The second choice. The 2 in the leading position is the setgid bit. On a directory, setgid causes new files and subdirectories to inherit the directory's group rather than the creating user's primary group. This is essential for shared project directories: without it, a file created by one project member would be owned by that member's primary group, not projectx, and might be inaccessible to other members.

Question 17

Why are the man pages for section 2 (man 2 ...) particularly important in this course?

Show answer

Section 2 of the manual documents the system-call interface — the exact behaviour of fork, read, write, mmap, and every other kernel entry point a user program can invoke. These are the interfaces this course uses throughout, from the process-management labs to the memory-management and file-system labs. The system-call man pages describe arguments, return values, error codes, and subtle details (such as how fork uses copy-on-write) that no textbook covers. On Debian and Ubuntu, they are in a separate package (manpages-dev) and are not installed by default.

Question 18

Which of the following commands would print the last 20 lines of /var/log/syslog?

head -20 /var/log/syslog
tail -20 /var/log/syslog
less /var/log/syslog
cat /var/log/syslog

Show answer

tail -20 /var/log/syslog. The tail command prints the last lines of a file by default. With -20, it prints the last 20. head prints the first lines; less opens a pager that requires user interaction; cat prints the entire file. tail -f is a variant that keeps watching the file for new lines, useful for live logs.

Question 19

What is the difference between a hard link and a symbolic link?

Show answer

A hard link is a directory entry that refers directly to an inode. Multiple hard links to the same inode are indistinguishable; they are all equally "the file". Deleting one decrements the inode's link count, and the file's data is freed only when the count reaches zero. A symbolic link is a file containing a path string; the kernel resolves the path each time the link is followed. Symbolic links can be dangling (if the target is deleted) or cyclic; both cases are handled by returning errors. Hard links survive the deletion of the "original" name; symbolic links do not.

Question 20

Why does the course standardise on the GCC flags -Wall -Wextra -g?

Show answer

-Wall and -Wextra enable a broad set of compiler warnings. These catch real bugs — uninitialised variables, unintended assignments, format-string mismatches, unused parameters that might indicate incomplete refactoring — that would otherwise compile silently and behave unexpectedly. Warnings should be treated as bugs. -g includes debug symbols so that GDB can map machine state back to source lines, variables, and function names. Together, the three flags make debugging tractable, which matters especially for the concurrent and pointer-heavy code the course will cover in later modules. The standard applies to every laboratory and assignment.

Ready for the graded quiz?

Once you are comfortable with every question above, take the graded Unit 2 quiz in the LMS.

Review Unit 2