Secure shell scripting practices

Mastering Secure Shell Scripting: Best Practices for Linux Admins

In the realm of Linux administration and automation, shell scripting is an indispensable skill. While writing scripts in Bash (the Bourne Again SHell) can streamline your work and make tasks more efficient, it’s crucial to ensure that these scripts are secure, especially when they deal with sensitive data or systems. In this blog, we will explore some of the best practices for writing secure shell scripts and provide operating instructions for various Linux package managers, including apt, dnf, and zypper.

1. Validate Input Properly

One of the fundamental rules of secure scripting is to never trust the input. Input validation is critical to protect your scripts from malicious data that could be used for SQL injection, command injection, or data corruption.

Use Bash internal features when possible, like [[ ]] for testing conditions which is safer than [ ].
Sanitize inputs by stripping out unwanted characters or patterning the input strictly.

2. Use Secure Bash Flags

Bash includes several flags that can enhance the security of your scripts:

Set -e (errexit): This flag causes the script to exit immediately if a command fails, which prevents errors from cascading.
Set -u (nounset): This will exit the script if you try to use an uninitialized variable, which can prevent many types of bugs and potential security issues.
Set `-o pipefail: This option sets the exit code of a pipeline to that of the last command to exit with a non-zero status, or zero if no command exited with a non-zero status, enhancing the robustness of error handling.

3. Limit Permissions

Always run scripts with the minimal permissions necessary to perform the required tasks. Minimizing permissions can limit the damage if your script is exploited.

Use sudo sparingly: Only specific commands that require elevated privileges should be run with sudo, not the entire script.

4. Use Shell Parameter Expansion

Shell parameter expansion can provide a safer way to manipulate variables. It avoids external calls and is less prone to errors and external exploits.

For example, ${variable//substring/replacement} can be used to replace parts of variable content without invoking external commands like sed.

5. Secure Use of SSH within Scripts

When your scripts use SSH to connect to remote servers, make sure to:

Use SSH keys instead of passwords: SSH keys are more secure than passwords and don’t require interactive input, which makes them ideal for scripts.
Set strict SSH options, for example: ssh -o BatchMode=yes -o ConnectTimeout=10 user@host.

6. Encrypt Sensitive Data

If your scripts manage sensitive data, consider using encryption tools like GPG to encrypt this data both at rest and during transmission.

Managing Dependencies with Package Managers

Your script might depend on external tools or libraries, so it's important to manage these dependencies securely using Linux package managers.

apt (Debian, Ubuntu, and derivatives):

sudo apt update        # Update package database
sudo apt install package_name  # Install a package

dnf (Fedora, CentOS, Red Hat):

sudo dnf check-update  # Check for available package updates
sudo dnf install package_name  # Install a package

zypper (openSUSE):

sudo zypper refresh   # Refresh package repository data
sudo zypper install package_name  # Install a package

Conclusion

Writing secure shell scripts involves a variety of best practices, from handling permissions and inputs correctly to taking advantage of Bash-specific features for safety and performance. Using reputable package managers like apt, dnf, and zypper also ensures that the dependencies your scripts rely on are securely managed. Adopt these practices not only to safeguard your systems but also to enhance the reliability and efficiency of your administrative tasks in Linux environments.

By following these guidelines, Linux administrators can craft not just functional but also robust and secure Bash scripts, protecting their system from accidental or malicious disruptions.