Script security best practices in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to safely check if a file exists before reading it.

Bash Scripting

if [ -f "[1]" ]; then
  cat "[1]"
fi

Drag options to blanks, or click blank then click option'

A$filename

B/etc/passwd

Cfilename

D$file

Attempts:

3 left

💡 Hint

Common Mistakes

Using the variable name without $ so it is treated as a string.

Not quoting the variable which can cause errors if it contains spaces.

Using "$filename" ensures the variable is expanded safely and correctly in the test and command.

2fill in blank

medium

Complete the code to prevent running commands from untrusted input.

Bash Scripting

safe_var=$(printf '%q' [1])
echo $safe_var

Drag options to blanks, or click blank then click option'

A'$user_input'

B$user_input

Cuser_input

D"$user_input"

Attempts:

3 left

💡 Hint

Common Mistakes

Not using $ so the variable is treated as a literal string.

Adding extra quotes inside the command causing double quoting.

Using "$user_input" with $ expands the variable safely. printf '%q' escapes special characters to prevent code injection.

3fill in blank

hard

Fix the error in the code to safely read a filename from user input.

Bash Scripting

read -r filename
if [[ -n [1] ]]; then
  cat "$filename"
fi

Drag options to blanks, or click blank then click option'

A$filename

Bfilename

C"$filename"

D'$filename'

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the variable name as a string instead of its value.

Using quotes incorrectly inside the test.

Using "$filename" with $ inside [[ ]] checks if the variable is not empty safely.

4fill in blank

hard

Fill both blanks to safely loop over filenames with spaces.

Bash Scripting

for file in [1]; do
  echo "Processing: $[2]"
done

Drag options to blanks, or click blank then click option'

A"${files[@]}"

Bfiles

Cfile

D"$files"

Attempts:

3 left

💡 Hint

Common Mistakes

Using unquoted variables causing word splitting.

Using wrong variable names inside the loop.

Using "${files[@]}" expands each element safely preserving spaces. The loop variable is 'file' to access each filename.

5fill in blank

hard

Fill all three blanks to create a secure temporary file and write safely.

Bash Scripting

tmpfile=$(mktemp [1])
echo "Data" > [2]
chmod [3] "$tmpfile"

Drag options to blanks, or click blank then click option'

A-t tmp.XXXXXX

B$tmpfile

C600

D-d /tmp

Attempts:

3 left

💡 Hint

Common Mistakes

Not using mktemp options causing insecure temp files.

Writing to wrong file or not using variable.

Setting wrong permissions making file accessible to others.

mktemp with -t creates a temp file with a template. Writing to $tmpfile saves data there. chmod 600 restricts file access to owner only.

Practice

1. Which of the following is a key reason to use set -euo pipefail at the start of a bash script?

easy

A. It automatically logs all commands to a file for debugging.

B. It speeds up the script execution by running commands in parallel.

C. It stops the script immediately if any command fails or if an unset variable is used.

D. It disables all user inputs to prevent accidental changes.

Solution

1. Step 1: Understand what set -euo pipefail does

   This command combination makes the script exit if any command fails (-e), if an unset variable is used (-u), and if any part of a pipeline fails (pipefail).

2. Step 2: Identify the effect on script safety

   By stopping early on errors or unset variables, it prevents the script from continuing in an unexpected or unsafe state.

3. Final Answer:

   It stops the script immediately if any command fails or if an unset variable is used. -> Option C

4. Quick Check:

   Strict mode = stops on errors [OK]

Hint: Remember: strict mode stops on errors and unset variables [OK]

Common Mistakes:

• Thinking it speeds up script execution
• Assuming it logs commands automatically
• Believing it disables user input

2. Which of the following is the correct way to check if a variable user_input is set and not empty in bash?

easy

A. if [ $user_input = "" ]; then echo "Empty"; fi

B. if [ -z "$user_input" ]; then echo "Empty"; fi

C. if [ -n $user_input ]; then echo "Empty"; fi

D. if [ "$user_input" == null ]; then echo "Empty"; fi

Solution

1. Step 1: Understand the test for empty string

   The test [ -z "$user_input" ] returns true if the variable is unset or empty, which is the correct way to check emptiness safely.

2. Step 2: Check other options for correctness

   if [ "$user_input" == null ]; then echo "Empty"; fi compares to 'null' string which is incorrect. if [ -n $user_input ]; then echo "Empty"; fi lacks quotes and reverses logic. if [ $user_input = "" ]; then echo "Empty"; fi lacks quotes and can cause errors if variable is unset.

3. Final Answer:

   if [ -z "$user_input" ]; then echo "Empty"; fi -> Option B

4. Quick Check:

   Use -z with quotes to check empty safely [OK]

Hint: Use quotes and -z to check empty variables safely [OK]

Common Mistakes:

• Not quoting variables causing errors
• Comparing to 'null' string instead of empty
• Using wrong test operators

3. What will be the output of this script snippet if PATH is not set and set -u is enabled?

#!/bin/bash
set -u
echo "Path is: $PATH"

medium

A. No output, script runs silently

B. Path is:

C. Path is: /usr/bin:/bin

D. bash: PATH: unbound variable

Solution

1. Step 1: Understand set -u effect

   The set -u option causes the script to exit with an error if an unset variable is referenced.

2. Step 2: Analyze the script behavior

   Since PATH is not set, referencing $PATH triggers an unbound variable error and the script stops with a message.

3. Final Answer:

   bash: PATH: unbound variable -> Option D

4. Quick Check:

   Unset variable with set -u causes error [OK]

Hint: set -u errors on unset variables immediately [OK]

Common Mistakes:

• Assuming empty string output instead of error
• Thinking script runs silently
• Assuming default PATH value is used

4. Identify the security issue in this script snippet:

#!/bin/bash
read -p "Enter filename: " file
cat $file

medium

A. Using read without validation allows command injection.

B. No issue, script is safe as is.

C. Missing quotes around $file causes syntax error.

D. Using cat is insecure; should use less instead.

Solution

1. Step 1: Analyze input handling

   The script reads user input directly into file and uses it without validation or sanitization.

2. Step 2: Understand risk of command injection

   If the user inputs something like file.txt; rm -rf /, the shell will execute the dangerous command, causing security risk.

3. Final Answer:

   Using read without validation allows command injection. -> Option A

4. Quick Check:

   Unvalidated input risks command injection [OK]

Hint: Always validate or sanitize user input before use [OK]

Common Mistakes:

• Ignoring input validation
• Thinking missing quotes cause syntax error here
• Believing command choice alone fixes injection

5. You want to securely run a script that uses a password stored in a file. Which practice improves security best?

hard

A. Read the password from a file with restricted permissions and never echo it.

B. Store the password in plain text in the script for easy access.

C. Pass the password as a command line argument to the script.

D. Write the password to a temporary file with world-readable permissions.

Solution

1. Step 1: Evaluate password storage methods

   Storing passwords in plain text inside scripts or passing them as command line arguments exposes them to other users or logs.

2. Step 2: Choose secure handling

   Reading the password from a file with strict permissions (e.g., 600) and avoiding printing it prevents leaks and unauthorized access.

3. Final Answer:

   Read the password from a file with restricted permissions and never echo it. -> Option A

4. Quick Check:

   Restrict file permissions and avoid exposing secrets [OK]

Hint: Never expose passwords; restrict file access and avoid echoing [OK]

Common Mistakes:

• Hardcoding passwords in scripts
• Passing secrets as command line arguments
• Using world-readable temporary files