File system permissions (--allow-read, --allow-write) in Deno - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the --allow-read flag do in Deno?

The --allow-read flag lets a Deno program read files or directories from the file system. Without it, the program cannot access files for reading.

beginner

What is the purpose of the --allow-write flag in Deno?

The --allow-write flag allows a Deno program to create, modify, or delete files and directories. Without this flag, the program cannot change the file system.

intermediate

Why does Deno require explicit permission flags like --allow-read and --allow-write?

Deno uses permission flags to protect your computer from unsafe code. It asks for permission before reading or writing files, similar to how apps ask for access on your phone.

intermediate

How can you allow a Deno script to read only a specific file?

Use --allow-read=path/to/file to give permission only to that file. This limits the script from reading other files.

beginner

What happens if you try to write a file in Deno without --allow-write permission?

Deno will stop the program and show an error saying it does not have permission to write files. This prevents accidental or harmful changes.

Which flag allows a Deno program to read files?

A --allow-write

B --allow-run

C --allow-net

D --allow-read

The --allow-read flag grants permission to read files.

What does --allow-write NOT allow?

A Reading files

B Modifying existing files

C Deleting files

D Creating new files

--allow-write allows writing but not reading files.

How do you restrict --allow-read to only one directory?

A --allow-write=./mydir

B --allow-read

C --allow-read=./mydir

D --allow-read=all

You specify the path after --allow-read= to limit access.

What happens if you run a Deno script that reads files without --allow-read?

A It throws a permission error

B It writes files instead

C It reads files normally

D It ignores the read commands

Deno blocks file reading and shows a permission error.

Why does Deno use permission flags like --allow-read?

A To speed up file access

B To protect your computer from unsafe code

C To make code shorter

D To allow network access

Permission flags protect your computer by controlling what code can do.

Explain how --allow-read and --allow-write control file system access in Deno.

Think about what each flag lets the program do with files.

Describe what happens if a Deno script tries to read or write files without the proper permission flags.

Consider Deno's security model and how it protects your system.

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