File system permissions (--allow-read, --allow-write) in Deno - Mini Project: Build & Apply

File System Permissions with Deno (--allow-read, --allow-write)

📖 Scenario: You are building a simple Deno script that reads from and writes to files. To keep your computer safe, Deno requires you to explicitly allow file system access using --allow-read and --allow-write flags.This project will guide you to create a script that reads a text file, appends a message, and writes it back, while understanding how to use these permission flags.

🎯 Goal: Create a Deno script that reads the content of message.txt, appends the text "\nHello from Deno!", and writes it back to the same file. You will also learn how to run the script with the correct file system permissions.

📋 What You'll Learn

Create a variable with the file path message.txt

Create a variable with the message "\nHello from Deno!"

Read the file content using Deno.readTextFile

Append the message to the content

Write the updated content back using Deno.writeTextFile

Run the script with --allow-read and --allow-write flags

💡 Why This Matters

🌍 Real World

Scripts that read and update files are common for logging, configuration updates, or data processing tasks.

💼 Career

Understanding file system permissions and asynchronous file operations is essential for backend development and scripting with Deno.

1

Set up the file path variable

Create a variable called filePath and set it to the string "message.txt".

Deno

# Create a variable called filePath and set it to "message.txt"
# Your code here

Hint

Use const filePath = "message.txt"; to store the file name.

2

Create the message variable

Add a variable called message and set it to the string "\nHello from Deno!".

Deno

const filePath = "message.txt";
# Create a variable called message and set it to "\nHello from Deno!"
# Your code here

Hint

Use const message = "\nHello from Deno!"; to store the text to append.

3

Read the file and append the message

Write an async function called appendMessage that reads the file content using await Deno.readTextFile(filePath), appends the message variable to it, and stores the result in a variable called updatedContent.

Deno

const filePath = "message.txt";
const message = "\nHello from Deno!";

// Write async function appendMessage to read and append message
# Your code here

Hint

Use await Deno.readTextFile(filePath) inside an async function to read the file.

Then add the message to the content.

4

Write the updated content back and call the function

Inside the appendMessage function, write the updatedContent back to the file using await Deno.writeTextFile(filePath, updatedContent). Then call the appendMessage() function at the end of the script.

Deno

const filePath = "message.txt";
const message = "\nHello from Deno!";

async function appendMessage() {
    const content = await Deno.readTextFile(filePath);
    const updatedContent = content + message;
    // Write updatedContent back to file
    # Your code here
}

// Call appendMessage function
# Your code here

Hint

Use await Deno.writeTextFile(filePath, updatedContent) to save the changes.

Don't forget to call appendMessage() to run your code.

Practice

1. What does the --allow-read flag do when running a Deno program?

easy

A. Allows the program to read files or folders specified.

B. Allows the program to write or modify files.

C. Blocks the program from accessing the network.

D. Runs the program without any permissions.

Solution

1. Step 1: Understand the purpose of --allow-read

   This flag grants permission to read files or folders.

2. Step 2: Compare with other flags

   --allow-write is for writing, not reading. Network access is unrelated.

3. Final Answer:

   Allows the program to read files or folders specified. -> Option A

4. Quick Check:

   --allow-read = read permission [OK]

Hint: Read permission means accessing files, not changing them [OK]

Common Mistakes:

• Confusing read permission with write permission
• Thinking it controls network access
• Assuming it grants all permissions

2. Which of the following is the correct way to run a Deno script app.ts with permission to write files?

easy

A. deno run app.ts --allow-write

B. deno run app.ts --allow-read

C. deno --allow-write run app.ts

D. deno run --allow-write app.ts

Solution

1. Step 1: Recall Deno command syntax

   The flags come immediately after deno run and before the script name.

2. Step 2: Identify correct flag placement

   deno run --allow-write app.ts is correct; options A and C have wrong order, D uses wrong flag.

3. Final Answer:

   deno run --allow-write app.ts -> Option D

4. Quick Check:

   Flags go after deno run [OK]

Hint: Flags come right after 'deno run', before script name [OK]

Common Mistakes:

• Placing flags after the script name
• Mixing up --allow-read and --allow-write
• Putting flags before 'run'

3. Consider this Deno script readfile.ts that reads data.txt and prints its content. What happens if you run deno run readfile.ts without any flags?

medium

A. The script creates data.txt if missing.

B. The script runs and prints the file content.

C. The script throws a permission error.

D. The script runs but prints nothing.

Solution

1. Step 1: Understand default permissions

   Without --allow-read, Deno denies file reading for safety.

2. Step 2: Predict script behavior

   Trying to read data.txt without permission causes a runtime error.

3. Final Answer:

   The script throws a permission error. -> Option C

4. Quick Check:

   No --allow-read means no file reading [OK]

Hint: No read flag means reading files causes error [OK]

Common Mistakes:

• Assuming Deno allows reading by default
• Thinking the file is created automatically
• Believing the script prints empty content silently

4. You have a Deno script that writes to log.txt. Running deno run --allow-read write.ts causes a permission error when writing. What is the fix?

medium

A. Add --allow-write flag when running the script.

B. Remove --allow-read flag and run without flags.

C. Change the script to read instead of write.

D. Run the script with --allow-net flag.

Solution

1. Step 1: Identify the permission needed

   Writing files requires --allow-write, not just --allow-read.

2. Step 2: Apply correct flag

   Run the script with --allow-write to allow file writing.

3. Final Answer:

   Add --allow-write flag when running the script. -> Option A

4. Quick Check:

   Write permission needs --allow-write [OK]

Hint: Write needs --allow-write, not --allow-read [OK]

Common Mistakes:

• Using only --allow-read for writing
• Removing flags thinking it grants all permissions
• Confusing network permission with file permission

5. You want to run a Deno script that reads from input.txt and writes to output.txt. Which command correctly grants the minimum permissions needed?

hard

A. deno run --allow-net script.ts

B. deno run --allow-read=./input.txt --allow-write=./output.txt script.ts

C. deno run --allow-write=./input.txt --allow-read=./output.txt script.ts

D. deno run --allow-read --allow-write script.ts

Solution

1. Step 1: Understand fine-grained permissions

   Using --allow-read=path and --allow-write=path limits access to specific files.

2. Step 2: Match permissions to files

   Read permission for input.txt and write permission for output.txt is needed.

3. Step 3: Check options

   deno run --allow-read=./input.txt --allow-write=./output.txt script.ts correctly assigns read to input and write to output. deno run --allow-read --allow-write script.ts grants full access, which is more than needed. deno run --allow-write=./input.txt --allow-read=./output.txt script.ts swaps permissions. deno run --allow-net script.ts is unrelated.

4. Final Answer:

   deno run --allow-read=./input.txt --allow-write=./output.txt script.ts -> Option B

5. Quick Check:

   Grant only needed file permissions [OK]

Hint: Use path-limited flags to grant minimal permissions [OK]

Common Mistakes:

• Granting full read/write when only specific files needed
• Swapping read and write paths
• Using unrelated flags like --allow-net