🐧 Module 0.4 ✅ Assessment Answers Linux CLI

Module 0.4 —
Assessment Answers

Complete answers for all three sections — multiple choice, short answer, and scenario-based questions on Linux command line for embedded systems.

Section A10 MCQs

Section B5 Short Answers

Section C4 Scenarios

✅ Section A — Multiple Choice Questions

ASelect the single best answer for each question

A1How do you display the last 50 lines of a log file and follow new output?

✅ (b)tail -n 50 -f log.txt

-n 50 sets the line count; -f follows new lines in real time. Press Ctrl+C to stop.

A2Which tool searches source code recursively and is significantly faster than grep?

✅ (b)rg (Ripgrep)

rg is purpose-built for source code, searches recursively by default, and respects .gitignore.

A3Which signal does Ctrl+C send to the foreground process?

✅ (c)SIGINT

SIGINT (signal 2) — interrupt request. The process can catch and handle it. Ctrl+Z sends SIGTSTP (suspend).

A4What happens when you press Ctrl+Z in the terminal?

✅ (b)Suspends the foreground process (SIGTSTP); resume with fg

The process is paused, not killed. Use fg to bring it back or bg to continue it in the background.

A5Why must your Linux user be in the dialout group?

✅ (b)To use serial devices like /dev/ttyUSB0 without sudo

Serial device files in /dev/ are owned by the dialout group. Without membership, access is denied unless you use sudo.

A6What does set -euo pipefail do in a Bash script?

✅ (b)-e: exit on error, -u: no unset vars, pipefail: pipe errors

All three flags together make scripts safe: they fail loudly instead of continuing with broken state.

A7What is the tmux prefix key used before every shortcut?

✅ (b)Ctrl + B

Press Ctrl+B first, then the command key. e.g., Ctrl+B D to detach, Ctrl+B % to split vertically.

A8What permissions does chmod 644 give a file?

✅ (a)Owner: read+write; Group & Others: read-only

644 = rw-r--r--. Owner (6=4+2): read+write. Group/Others (4): read only. Standard for config files.

A9Which command shows active listening ports with their process names?

✅ (b)ss -tlnp

-t TCP, -l listening only, -n numeric ports, -p process names. Use this to debug which program is using a port.

A10What is the primary use of dmesg | tail in embedded development?

✅ (b)Check kernel messages to confirm USB device was detected

Run immediately after plugging in a board. Linux logs the driver load and device file assignment (/dev/ttyUSB0) here.

📄 Section B — Short Answer Questions

B1Explain why picocom is preferred over the Arduino IDE Serial Monitor for serious embedded work.

picocom is a lightweight serial terminal designed for direct terminal use. Unlike the Arduino IDE's Serial Monitor, it works seamlessly over SSH, inside tmux sessions, and in automation scripts. It allows serial data to be logged, redirected, and processed using standard Linux commands.

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Key Advantages

Works over SSH — log into a remote build server and still monitor your board. Runs inside tmux — stays active even if the terminal is closed. Pipeable output — picocom | grep ERROR filters live serial data.

B2Write a one-line pipe that finds all .c files, counts lines in each, sorts by descending length, and shows the top 10.

bash — answer

find . -name "*.c" -exec wc -l {} \; | sort -rn | head -10

How each part works

Part	Meaning
`find . -name "*.c"`	Search all `.c` files from current directory
`-exec wc -l {} \;`	Run `wc -l` (line count) on every file found
`\| sort -rn`	Sort numerically (`-n`) in reverse/descending (`-r`) order
`\| head -10`	Keep only the top 10 results

Workflow summary

1.Find all .c source files.
2.Count lines in each file.
3.Sort by line count, largest first.
4.Display only the top 10 files.

B3What is the difference between > and >> in shell redirection?

> — Overwrite

Redirects stdout to a file. If the file exists, its content is erased first. If it doesn't exist, it is created.

ls > files.txt

>> — Append

Adds new output to the end of an existing file. Does not erase previous content. Creates the file if it doesn't exist.

date >> log.txt

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When to use each

Use > for fresh output you want to overwrite on each run (e.g., latest build status). Use >> for cumulative logs where you need to keep all history (e.g., firmware run logs).

B4Describe a real-life tmux setup you would use during STM32 driver debugging.

A practical tmux setup uses three panes running simultaneously:

bash — set up the session

tmux new -s stm32-debug     # create a named session
# Ctrl+B %  → split to get Pane 2
# Ctrl+B "  → split to get Pane 3

Pane	Command	Purpose
Pane 1	`make` / `pio run`	Build firmware — see compile errors immediately
Pane 2	`st-flash write` / `west flash`	Flash the latest build to the board
Pane 3	`picocom -b 115200 /dev/ttyACM0`	Monitor UART output from the STM32

✓

SSH Resilience

Since tmux sessions survive SSH disconnects, you can detach (Ctrl+B D) and reconnect later with tmux attach -t stm32-debug — the build and monitor stay running.

B5What does the 2>&1 part of cmd > out.txt 2>&1 do, and why does the order matter?

In Linux, every process has numbered file descriptors: stdout = 1 and stderr = 2. The notation 2>&1 means: "redirect file descriptor 2 (stderr) to wherever file descriptor 1 (stdout) currently points."

bash

make > build.log 2>&1    # stdout goes to file, then stderr follows stdout

Why the order matters

Command	Result
`cmd > out.txt 2>&1`	stdout → file, then stderr → same file ✓ Both captured
`cmd 2>&1 > out.txt`	stderr → terminal (old stdout), stdout → file ✗ Errors on screen

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Embedded Use

west build > build.log 2>&1 — captures everything including compiler errors into one log file. Then grep error build.log to find failures quickly.

🔌 Section C — Scenario-Based Questions

C1Your ESP32 does not appear as a serial port after connecting. Describe your diagnostic steps.

Work through these steps in order — each one narrows down the cause:

bash — step 1: check kernel detection

dmesg | tail

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What to look for

A successful detection shows: usb X-X: new full-speed USB device followed by cp210x converter now attached to ttyUSB0. If nothing appears, Linux didn't detect the hardware at all — try a different USB cable or port.

bash — step 2: list serial device files

ls /dev/ttyUSB* /dev/ttyACM*

bash — step 3: confirm USB is detected

lsusb

bash — step 4: check group membership

groups $USER

bash — step 5: check device file permissions

ls -l /dev/ttyUSB0

✅

Common Fixes

Not in dialout group → sudo usermod -aG dialout $USER then log out and back in. Driver not installed → sudo apt install cp210x-dkms. Cable only provides power (no data lines) → swap for a data USB cable.

C2Find every TODO comment across the entire project, with line numbers.

Primary answer — using Ripgrep:

bash — recommended

rg -n TODO

Flag	Meaning
`rg`	Run Ripgrep (searches recursively by default)
`-n`	Show line numbers alongside each match
`TODO`	The search pattern

Alternative — using grep:

bash — alternative

grep -rn TODO .

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Why rg is preferred here

rg automatically skips build/, .git/, and other directories in .gitignore, so results contain only real source code matches — not false positives from build artifacts.

C3Capture raw UART output from /dev/ttyACM0 at 921600 baud for exactly 30 seconds into a log file.

bash — answer

stty -F /dev/ttyACM0 921600 raw && timeout 30 cat /dev/ttyACM0 > capture.log

Part	Purpose
`stty -F /dev/ttyACM0 921600 raw`	Configure serial port: set baud rate 921600, enable raw mode (no line processing)
`&&`	Run the next command only if stty succeeded (exit status 0)
`timeout 30`	Automatically stop after exactly 30 seconds
`cat /dev/ttyACM0`	Read raw bytes from the serial device file
`> capture.log`	Redirect all output into the log file

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Why raw mode matters

Without raw mode, the terminal driver processes the data — interpreting control characters, adding newlines, etc. raw passes bytes through completely unmodified, which is what you want for binary firmware logs.

C4Run a long automated test over SSH that must survive SSH disconnects and remain accessible when you reconnect.

bash — step 1: create a tmux session

tmux new -s test

Inside the tmux session, start your test script or long-running process normally:

bash — step 2: run the test

./run_long_test.sh

Detach from the session (the process keeps running in the background):

keyboard — step 3: detach

Ctrl+B, then D    # session keeps running after SSH disconnects

Reconnect at any time — even after SSH drops:

bash — step 4: reattach

tmux attach -t test

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Why tmux solves this

When an SSH connection drops, all processes attached to that terminal session die — including your test. tmux runs sessions independently of SSH connections. The session lives on the remote server, not inside the SSH tunnel. Detach → disconnect → reconnect → reattach, and the test has been running the whole time.

Module 0.4 — Linux Command Line Assessment Answers · Embedded Systems Foundations

Module 0.4 —Assessment Answers

How each part works

Workflow summary

Why the order matters

Module 0.4 —
Assessment Answers