Script security best practices in Bash Scripting

Introduction

Keeping scripts safe stops bad things from happening to your computer or data. It helps protect your work and privacy.

When writing scripts that handle passwords or secret keys

When sharing scripts with others or running scripts from the internet

When automating tasks that change important files or system settings

When running scripts on a server or multi-user computer

When your script uses input from users or other programs

Syntax

Bash Scripting

# Example of safe script practices
# 1. Use strict mode
set -euo pipefail

# 2. Check inputs
if [[ $# -eq 0 ]]; then
  echo "Error: Missing argument" >&2
  exit 1
fi

# 3. Avoid running commands as root unless needed

# 4. Use full paths for commands
/usr/bin/grep "pattern" file.txt

# 5. Do not store passwords in plain text

set -euo pipefail helps catch errors early and avoid unexpected behavior.

Always check inputs to avoid running dangerous commands by mistake.

Examples

This is called 'strict mode' and makes your script safer by stopping on errors.

Bash Scripting

set -euo pipefail

# This stops the script if any command fails, if you use an unset variable, or if a pipeline fails.

This checks if no argument is provided and stops the script with an error message.

Bash Scripting

if [[ $# -eq 0 ]]; then
  echo "Error: Missing argument" >&2
  exit 1
fi

Using full paths avoids running the wrong program if someone changes your PATH.

Bash Scripting

/usr/bin/grep "pattern" file.txt

Storing passwords in scripts can expose them to others.

Bash Scripting

# Never store passwords like this:
PASSWORD="mypassword"
# Instead, read from secure input or environment variables.

Sample Program

This script safely checks for a filename argument, verifies the file can be read, and searches for the word 'root' using the full path to grep.

Bash Scripting

#!/bin/bash
set -euo pipefail

# Check if user provided a filename
if [[ $# -eq 0 ]]; then
  echo "Usage: $0 filename" >&2
  exit 1
fi

FILE="$1"

# Check if file exists and is readable
if [[ ! -r "$FILE" ]]; then
  echo "Error: File '$FILE' does not exist or is not readable." >&2
  exit 1
fi

# Use full path for grep
/usr/bin/grep "root" "$FILE" || echo "No 'root' found in $FILE"

Important Notes

Always run scripts with the least privileges needed.

Never trust user input without checking it first.

Keep sensitive data out of scripts or use secure storage methods.

Summary

Use strict mode (set -euo pipefail) to catch errors early.

Check all inputs before using them in your script.

Use full paths for commands and avoid storing secrets in plain text.

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