# 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
```