Script security best practices in Bash Scripting - Time & Space Complexity

Time Complexity: Script security best practices

O(n)

Understanding Time Complexity

We want to understand how the time it takes to run a script grows when we add security checks.

How do security steps affect the script's speed as it handles more data?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

#!/bin/bash

for file in "$@"; do
  if [ -f "$file" ] && [ -r "$file" ]; then
    # Check if file is owned by current user
    owner=$(stat -c '%u' "$file")
    if [ "$owner" -eq "$UID" ]; then
      cat "$file"
    else
      echo "Skipping $file: not owned by user"
    fi
  else
    echo "Skipping $file: not a readable file"
  fi
done

This script loops over input files, checks if each is a readable file owned by the user, then prints its content.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping over each input file and performing checks.
• How many times: Once for each file passed as input.

How Execution Grows With Input

Each file causes the script to do a fixed set of checks and possibly print content.

Input Size (n)	Approx. Operations
10	About 10 file checks and reads
100	About 100 file checks and reads
1000	About 1000 file checks and reads

Pattern observation: The work grows directly with the number of files; double the files, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the script's running time grows linearly with the number of input files.

Common Mistake

[X] Wrong: "Adding security checks makes the script run in constant time regardless of input size."

[OK] Correct: Each file still needs to be checked, so the time grows with the number of files, not stays the same.

Interview Connect

Understanding how security checks affect script speed shows you can balance safety and performance, a key skill in real scripting tasks.

Self-Check

"What if we added a nested loop to check file contents line by line? How would the time complexity change?"

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