Secure Input: Reading Passwords in Shell Scripts

Shell scripts are powerful tools for automating tasks, but handling sensitive information like passwords requires careful consideration. Directly embedding passwords within a script is a security risk. This tutorial explores methods for securely prompting users for passwords within a shell script without echoing the input to the terminal, protecting sensitive data from being displayed or logged.

The Problem: Avoiding Echoed Passwords

By default, when a shell script reads input using the read command, the typed characters are immediately displayed on the terminal. This is undesirable when dealing with passwords. We need a way to disable echoing while still allowing the user to enter their credentials.

Method 1: Using `read -s`

The simplest and most common approach is to use the -s option with the read command. This option tells read to disable echoing.

#!/bin/bash

echo -n "Password: "  # -n prevents a newline after the prompt
read -s password
echo # Add a newline after reading the password (since -s suppresses it)

# Now you can use the $password variable
# Example:
# some_command --password "$password"

Explanation:

echo -n "Password: " displays the prompt without a newline character.
read -s password reads the input from the user and stores it in the password variable, suppressing the display of typed characters.
echo after the read command adds a newline to the terminal output, as -s prevents the newline from the user’s input (Enter key) from being displayed. This ensures subsequent output appears on a new line.

Bash Specific Enhancement:

More recent versions of Bash offer a convenient way to combine the prompt and the read command using the -p option:

#!/bin/bash

read -s -p "Password: " password

# Now use the $password variable

This achieves the same result as the previous example with a more concise syntax.

Method 2: Using `stty` (POSIX Compliant)

The stty command (standard terminal setting) provides more control over terminal characteristics. While read -s is convenient, it’s not strictly POSIX compliant. The following method uses stty to disable and re-enable echoing, ensuring broader compatibility.

#!/bin/sh

# Function to read a secret string
read_secret() {
    # Store the current terminal settings
    stty -echo

    # Read the secret
    read "$@"

    # Restore the original terminal settings
    stty echo
}

# Example Usage
printf "Password: "
read_secret password

# Now you can use the $password variable

Explanation:

stty -echo disables echoing to the terminal.
read "$@" reads the input (using all arguments passed to the function).
stty echo re-enables echoing.
It’s crucial to restore the original settings after reading the input to avoid leaving the terminal in an unusable state.

Considerations and Best Practices

Security: Even with these methods, shell scripts aren’t the most secure way to handle sensitive information. Consider using dedicated password management tools or more secure alternatives when possible.
Error Handling: Consider adding error handling to your script. For example, if the user doesn’t enter a password, the script should handle that gracefully.
Variable Scoping: Be mindful of variable scoping. Ensure that the password variable is only accessible within the necessary parts of your script.
Storing Passwords: Never store passwords in plain text within your script or in any easily accessible file. Consider using key-based authentication or other more secure methods.
Masking: While the above methods prevent echoing, they don’t provide a masking effect (replacing characters with asterisks). Some systems provide tools like askpass (often used by graphical password prompts) that can achieve masking, but these may not be portable across all systems.