Day 2 - Primary Subject 2 Mock Exam: Weak Points in Linux System Administration

📌 Key Points

• "Linux System Administration" is the most heavily weighted subject on the primary written exam. One letter of a command option or one digit of a permission number decides the answer.
• The six easy-to-miss topics: special permission numbers (4/2/1), umask calculation, signal numbers (9/15), RAID levels, LVM order, rpm/yum differences.
• Special permissions: SetUID 4000 (run with the owner's privileges), SetGID 2000 (group inheritance), Sticky 1000 (delete only your own files).
• umask is a subtracted value — it is subtracted from 666 for files and 777 for directories. 022 → 644 / 755.
• Signals: 9 (KILL) cannot be ignored, and 15 (TERM) is the default for kill. For nice, the smaller the value, the higher the priority.
• Low-level tools (rpm, dpkg, insmod) cannot resolve dependencies automatically, while high-level tools (yum, apt, modprobe) can.

Weak Point 1: The Shell and Environment Variables

• The shell is a command interpreter.
• The types, configuration files, and variable distinctions are the exam focus.

• The difference between environment variables and shell variables is a trap.
• If exported with export, it is an environment variable (inherited by child processes); a plain assignment makes it a shell variable (not inherited).

Configuration files in one line: /etc/profile (global, login), ~/.bash_profile (personal, once at login), ~/.bashrc (personal, each time a shell is opened). Aliases usually go in .bashrc.

Weak Point 2: File Permissions — Special Permissions Are the Key

• The basic rwx permissions (read 4, write 2, execute 1) are the basics, and the three special permissions are the exam focus.

• Typical examples: /usr/bin/passwd is SetUID (so a regular user can modify /etc/shadow), and /tmp has the Sticky Bit (anyone can write, but you can delete only your own).

⚠️ umask trap: umask is a value that subtracts permissions. It is subtracted from the default 666 for files and 777 for directories. umask 022 → files 644, directories 755. Execute permission is never added to files by umask.

Watch for uppercase S/T: If SetUID/Sticky is set but there is no execute permission (x), it is shown as uppercase S/T rather than lowercase (indicating a permission conflict).

Weak Point 3: Disk Management — Mounting, Quotas, RAID, LVM

• Disks are tricky because commands and concepts get mixed together.

• The 6 fields of /etc/fstab in order: device · mount point · filesystem · options · dump · fsck order.
• The entries listed here are the targets for automatic mounting at boot.
• Let's also review the RAID levels.

LVM 3-layer structure: Physical Volume (PV) → Volume Group (VG) → Logical Volume (LV). The order is pvcreate → vgcreate → lvcreate. Its biggest advantage is expanding capacity without downtime.

Weak Point 4: Processes and Signals

• Process states, priorities, and signals are within the scope of the exam.

• You must memorize the signal numbers.

⚠️ nice value range: -20 (highest priority) to 19 (lowest). The smaller the value, the higher the priority. Only root can specify negative values.

kill default: kill PID sends 15 (SIGTERM) by default. If it doesn't die, use kill -9. Signal 9 cannot be ignored or blocked by the process.

Weak Point 5: Package Management — RPM and DEB

• Don't confuse the package tools of each distribution family.

Key difference: rpm/dpkg cannot resolve dependencies automatically (they only report an error). yum/apt automatically fetch and resolve dependent packages from repositories.

The 3 steps of compiling from source: ./configure (check the environment, generate the Makefile) → make (compile) → make install (install). Specify the install path with the configure option --prefix.

Weak Point 6: User and Group Management Files

• Account management asks not only about commands but also about the backend files.

• Memorize the 7 fields of passwd (username:x:UID:GID:description:home:shell) together with the password-expiry fields of shadow.

UID conventions: 0 = root, 1-999 = system accounts, 1000 and up = regular users (starting from 500 on some distributions). An account with UID 0 has the same privileges as root, so be careful for security reasons.

Why shadow is protected: If the password hashes are exposed, offline cracking becomes possible, so permissions are restricted to 000/400. That's why you see only an x in the password field when you look at passwd.

Weak Point 7: Job Scheduling — cron and at

• System administration automation tools also belong to this subject.

• The cron fields (minute, hour, day, month, weekday) and the access-control files (cron.allow/cron.deny) are traps.

allow/deny priority: If cron.allow exists, only the users listed there are allowed (deny is ignored). If it doesn't exist, only users not in cron.deny are allowed. allow is stronger than deny.

Weak Point 8: Backup and Compression Commands

• Let's also review the data protection commands.

• You must memorize the tar options — c (create), x (extract), v (show progress), f (file name), z (gzip), j (bzip2).

tar in one line: To bundle, tar czvf out.tar.gz dir; to extract, tar xzvf out.tar.gz. Change z to j for bzip2. Always put f right before the file name.

Weak Point 9: Disk Quotas (quota)

• Quotas, which limit disk usage in a multi-user environment, also belong to system administration.

• Quotas have a soft limit (warning, grace period) and a hard limit (absolute limit).

soft vs hard: With a soft limit, exceeding it is allowed during the grace period, but a hard limit blocks immediately. Both blocks (capacity) and inodes (number of files) can be limited.

Weak Point 10: Modules and Kernel Parameters

• Kernel module management is another pillar of system administration.

insmod vs modprobe: insmod cannot handle dependent modules automatically, but modprobe resolves dependencies as well (the same pattern as the rpm:yum relationship). Persistent kernel parameters go in /etc/sysctl.conf.

System information command roundup: free (memory), uptime (uptime and load), vmstat (virtual memory statistics), iostat (I/O statistics), lscpu (CPU information). They are often asked together as a group for performance diagnosis.

Use the mock exam below for a final check.

📖 Glossary

• Environment variable vs shell variable : A variable exported with export and passed down to child processes / a variable that stays only in the current shell.
• PATH : The list of directories searched for executables when you type a command.
• SetUID / SetGID / Sticky Bit : Run with the owner's privileges / group privileges and group inheritance / can delete only your own files.
• umask : The value subtracted from the default permissions of newly created files and directories. The starting value is 666 for files and 777 for directories.
• fsck : A command that checks and repairs filesystem errors.
• RAID 0 / 1 / 5 : Striping for speed / mirroring for reliability / distributed parity as a compromise between capacity and reliability.
• LVM 3 layers : PV (physical volume) → VG (volume group) → LV (logical volume). Its biggest advantage is expansion without downtime.
• SIGKILL (9) / SIGTERM (15) : Forced termination that a process cannot ignore / the graceful termination request that is the default for kill.
• nice value : How much a process yields (-20 to 19). The smaller it is, the higher the priority, and only root can specify negative values.
• Automatic dependency resolution : The property of automatically handling other required packages or modules, as yum, apt, and modprobe do. rpm, dpkg, and insmod lack it.
• The 3 steps of compiling from source : ./configure (check the environment, generate the Makefile) → make (compile) → make install (install).
• /etc/skel : A collection of template files copied into the home directory when a new account is created.
• soft limit / hard limit : In quotas, a warning line that may be exceeded during the grace period / an absolute limit that blocks immediately.
• anacron : A tool that runs periodic jobs skipped because the system was off, catching up after boot.

📝 Practice Questions

Click a choice to reveal the answer and explanation.

Question 1

Which is the typical command with the SetUID bit set that lets a regular user change their own password?

• A/bin/ls
• B/usr/bin/passwd
• C/etc/shadow
• D/usr/bin/who

Question 2

When the umask value is 022, what are the default permissions of a newly created regular file?

• A755
• B644
• C666
• D777

Question 3

Which special permission restricts each user to deleting only the files they created in a shared directory?

• ASetUID
• BSetGID
• CSticky Bit
• Dumask

Question 4

Which signal cannot be ignored or blocked by the receiving process?

• ASIGTERM (15)
• BSIGHUP (1)
• CSIGINT (2)
• DSIGKILL (9)

Question 5

Which RAID level requires at least 3 disks and uses one distributed parity?

• ARAID 0
• BRAID 1
• CRAID 5
• DRAID 6

Question 6

Which is the correct order of commands for creating a physical volume, a volume group, and a logical volume when setting up LVM?

• Alvcreate → vgcreate → pvcreate
• Bpvcreate → vgcreate → lvcreate
• Cvgcreate → pvcreate → lvcreate
• Dmkfs → pvcreate → lvcreate

Question 7

Which high-level tool installs an RPM package from a repository while resolving dependencies automatically?

• Arpm -ivh
• Bdpkg -i
• Cyum install
• Dmake install

Question 8

Which statement about the nice priority value is correct?

• AThe range is 0 to 39, and the larger the value, the higher the priority
• BThe range is -20 to 19, and the smaller the value, the higher the priority
• CThe range is -19 to 20, and anyone can specify negative values
• DThe range is 1 to 99, and only root can change it

Question 9

Which is the correct order of the fields in /etc/fstab?

• Amount point - device - filesystem - options - dump - fsck order
• Bdevice - mount point - filesystem - options - dump - fsck order
• Cdevice - filesystem - mount point - options - fsck order - dump
• Dfilesystem - device - mount point - dump - options - fsck order

Question 10

Which statement about the difference between environment variables and shell variables is correct?

• AShell variables are automatically inherited by child processes
• BOnly environment variables exported with export are inherited by child processes
• CBoth are always inherited by children
• DEnvironment variables are valid only in the current shell