docs: add answer key

answer_key/ANSWER_KEY.md

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+# Operating Systems Lab — Module 1 Answer Key
+
+> **For Teaching Assistants only. Do not share with students.**
+
+---
+
+### 1. Navigation
+
+List all files in `config/`, including hidden ones:
+
+```bash
+cd sisop_module1_playground
+ls -la config/
+```
+
+---
+
+### 2. Permissions
+
+Change `archive/locked.txt` so only the owner can read it:
+
+```bash
+chmod 400 archive/locked.txt
+```
+
+Verify:
+
+```bash
+ls -l archive/locked.txt
+# Expected: -r-------- 1 <user> ...
+```
+
+---
+
+### 3. Search
+
+Find the `"ERROR"` line in `logs/app.log` and save it:
+
+```bash
+grep "ERROR" logs/app.log > logs/error_only.txt
+```
+
+---
+
+### 4. Count
+
+Count files ending in `.tmp` in `data/`:
+
+```bash
+find data/ -name "*.tmp" | wc -l
+```
+
+---
+
+### 5. Chaining
+
+List all files in `data/` sorted in reverse alphabetical order:
+
+```bash
+ls data/ | sort -r
+```
+
+---
+
+### 6. I/O Redirection
+
+Create `info.txt` with the current date and username:
+
+```bash
+echo "$(date) $(whoami)" > info.txt
+```
+
+---
+
+### 7. AWK — Print names (Column 2)
+
+```bash
+awk -F',' 'NR>1 {print $2}' data/users.csv
+```
+
+**Expected output:**
+```
+Budi
+Siti
+Agus
+Dewi
+Eko
+Sari
+```
+
+---
+
+### 8. AWK — Total sum of salaries (Column 4)
+
+```bash
+awk -F',' 'NR>1 {sum += $4} END {print sum}' data/users.csv
+```
+
+**Expected output:** `26200`
+
+---
+
+### 9. AWK — Name and Role of Active users
+
+```bash
+awk -F',' 'NR>1 && $5=="Active" {print $2, $3}' data/users.csv
+```
+
+**Expected output:**
+```
+Budi Admin
+Agus User
+Dewi Manager
+Sari Admin
+```
+
+---
+
+### 10. Bash — `scripts/check_dir.sh`
+
+```bash
+#!/bin/bash
+if [ -d "$1" ]; then
+    echo "Directory '$1' exists."
+else
+    echo "Directory '$1' does not exist."
+fi
+```
+
+Make it executable and test:
+
+```bash
+chmod +x scripts/check_dir.sh
+./scripts/check_dir.sh data/
+./scripts/check_dir.sh fakedir/
+```
+
+---
+
+### 11. Bash — Fix `scripts/hello.sh`
+
+**Bug:** `date_now=date` assigns the string `"date"` instead of executing the command.
+
+**Fix:** Change line 3 to use command substitution:
+
+```bash
+date_now=$(date)
+```
+
+**Corrected `hello.sh`:**
+
+```bash
+#!/bin/bash
+echo "Hello eggboi"
+date_now=$(date)
+echo "Today is $date_now"
+```
+
+---
+
+### 12. Bash — Loop to create 5 files
+
+```bash
+#!/bin/bash
+for i in $(seq 1 5); do
+    touch test_${i}.txt
+done
+```
+
+---
+
+### 13. Cron — Run `hello.sh` every day at 3:30 AM
+
+```
+30 3 * * * /path/to/sisop_module1_playground/scripts/hello.sh
+```
+
+---
+
+### 14. Cron — List all active cron jobs
+
+```bash
+crontab -l
+```
+
+---
+
+### 15. Wildcards — Delete all `.bak` files in `data/`
+
+```bash
+rm data/*.bak
+```
+
+---
+
+### 16. Disk Usage — Total size of the lab directory
+
+```bash
+du -sh sisop_module1_playground/
+```
+
+---
+
+### 17. AWK — Name of the person with the highest salary
+
+```bash
+awk -F',' 'NR>1 {if ($4 > max) {max=$4; name=$2}} END {print name}' data/users.csv
+```
+
+**Expected output:** `Dewi` (salary: 7000)
+
+---
+
+### 18. Sed — Replace `"INFO"` with `"LOG"`
+
+```bash
+sed 's/INFO/LOG/g' logs/app.log > logs/app_log_replaced.log
+```
+
+---
+
+### 19. Processes — Find PID of the current bash session
+
+```bash
+echo $$
+```
+
+Or alternatively:
+
+```bash
+ps -p $$
+```
+
+---
+
+### 20. Archiving — Create a `.tar.gz` of `logs/`
+
+```bash
+tar -czvf logs.tar.gz logs/
+```
+
+---
+
+### 21. Bash — Update, upgrade, and download image
+
+```bash
+#!/bin/bash
+
+# Update and upgrade the system
+sudo apt update && sudo apt upgrade -y
+
+# Create the target directory if it doesn't exist
+mkdir -p new_image
+
+# Download an image from a URL into new_image/
+# Replace <URL> with the actual image URL
+wget -P new_image/ <URL>
+
+#example:
+wget -P new_image/ https://upload.wikimedia.org/wikipedia/commons/thumb/7/71/2010-kodiak-bear-1.jpg/1280px-2010-kodiak-bear-1.jpg
+
+# Alternative using curl:
+# curl -o new_image/image.jpg <URL>
+```
+
+---
+
+### 22. Search — Find `secret.txt` in `maze/`
+
+```bash
+find maze/ -name "secret.txt"
+```
+
+**Expected output:**
+```
+maze/level1_b/level2_d/level3_d/level4_a/secret.txt
+```
+
+Display its contents:
+
+```bash
+cat maze/level1_b/level2_d/level3_d/level4_a/secret.txt
+```
+
+**Expected output:**
+```
+You found it! The secret message is: sisop2026
+```