chevron-thin-right chevron-thin-left brand cancel-circle search youtube-icon google-plus-icon linkedin-icon facebook-icon twitter-icon toolbox download check linkedin phone twitter-old google-plus facebook profile-male chat calendar profile-male

Testing Linux Code

Just as there are a variety of Linux distributions, there is also an abundance of development tools. While the amount of customization and flexibility is one of Linux’s core strengths, it also imposes almost infinite permutations to test, creating a natural obstacle for testing Linux code.

This article appeared as the cover story in the December 2011 issue of Dr. Dobbs

By Gil Zilberfeld and Avi Hein

As Linux attracts more developers, careful thinking and best practices can reduce the unique testing challenges it presents

Software testing — whether performed by developers, by QA, or by other post-developer testers — is essential to ensure final code quality. It checks that the application does what the stakeholders intended it to do and that it meets certain levels of quality. However, when developing in Linux environment, there are unique challenges and problems that are confronted in testing Linux code. This article examines the principal ones and suggests some workarounds.

Different Methodologies and Ways to Test

Two of the most commonly used types of software tests run by developers are unit tests and integration tests. A unit test, which operates on a single unit of code — such as a method or function, should be:

  • Repeatable: You can rerun the same test as many times as you want
  • Consistent: Every time you run it, you get the same result
  • In memory: It has no “hard” dependencies on an external resource (such as the file system, a databases, or network)
  • Fast: It should take a few milliseconds to run a unit test
  • Validate one single concern or “use case” in the system: More than one can make it harder to understand where the problem is, and in turn hard to fix and check.

By comparison, integration tests try to verify the interfaces between components. They are meant to make sure the entire application works correctly together. Compared with unit tests, integration tests are slower than unit tests, harder or more time-consuming to create and run fully, and they do not pinpoint specifically where a problem is located.

Integration tests make a lot of sense in most systems in two particular configurations: as an additional layer of regression testing beyond unit tests to provide greater validation of the software, and in legacy code situations, where unit tests are considered hard to write. In the latter case, legacy code was often not written for testability, so that unit codes take deep an approach to testing. Integration tests can begin to tease out and verify the interactions between larger scale components in the legacy codebase.

While each form of testing has its benefits and pitfalls, they complement each other. Due to the multiple Linux distros and different toolchains, integration tests are necessary to make sure that not just pure logic is tested, but that the code works in different environments.


Linux is one of the most customizable operating systems, with a variety of toolkits and components. This is both a strength and a challenge when developing clean code. Many Linux developers prefer to hand-code, instead of using comprehensive IDEs that provide a development ecosystem. Linux has some of the best text editors available — far better than what’s commonly used in Windows.

Even with the best IDEs (like Eclipse or Netbeans), programming is not as streamlined and integrated as in the Windows world. Because of this lack of integration, developer testing is also not as streamlined, and often loses the benefit of immediate feedback.

Many Distributions and Tools

One of Linux’s major strengths is that there are many distributions and everything is customizable. There are at least 10 major distributions, making up the majority of market share, and over 300 active Linux distributions, according to Distrowatch.

Each distribution has different preset configurations and pre-installed items that tests need. While multiple targeting is achievable, unit testing, which should just test code logic, may require a separate setup in different configurations.

Just as there are a variety of Linux distributions, there is also an abundance of development tools. Compilers in different versions require the test tools to comply with those versions and be set up properly. For example, in C++, you may need specific versions of the GCC compilers, and sometimes different Eclipse versions.

While the amount of customization and flexibility is one of Linux’s core strengths, it also imposes almost infinite permutations to test.

Legacy Code

Because Linux is a popular and generally ideal platform for software development, software developers have used Linux since the kernel’s inception. They’ve also migrated older Windows code to Linux as they move away from Microsoft Windows for their software development platform.

There’s simply a lot of legacy code running on Linux. But testing legacy code can be very difficult, especially without the proper tools. New developers, new language standards, updates and new versions, and new project requirements quickly make manual testing very difficult. And without proper automated tests, if you change one thing, you’ll probably break something else, and not even know it.

Writing unit tests for legacy code is considered hard. Code written without testability in mind gets either low coverage or invites tests which are not really unit tests but are closer to integration tests. They don’t run quickly, can be harder to debug when an error occurs, and they lack the immediate feedback that real unit tests give.

With few exceptions, the only way to test legacy code is via integration testing. It’s usually hard or impossible to write unit tests for legacy code. Various commercial tools can help relieve that problem by being able to test components in isolation from others, without modifying the code. Integration tests can then serve as an additional smoke test to make sure you didn’t break the integration between system components, one level above unit tests.

Multiple Targeting / Multiple Cycles

When doing multiple targeting, the code requires multiple cycles of testing. Some targets do not have unit testing capabilities and require a workaround of continuous integration, such as cross-compiling for specific configurations to run the tests. We see more and more multiple targeting when at least one of the targets allow for unit testing that tests logic. Since logic does not change on different platforms (2 is always bigger than 1), having a target that can run the unit tests is a great option to have in order to make sure the code works.

Regulatory Requirements

Code integrity is not only essential to quality software but also to meet regulatory requirements. Your software must do what it’s specified to do, and the development organization needs to be able to prove this to regulators, as part of its risk mitigation program. Processes, such as developer testing, should be in place to ensure that requirements are met. They help ensure that your software was designed with these concerns in mind and provide verification that your code follows the logic that it was designed for.

There is a multitude of use cases that must be tested to meet regulatory requirements. Because of Linux’s configurability, and extensibility, and hundreds of distributions, when testing Linux code it can be difficult to prove to regulators that your code can uniformly meet regulatory requirements and that the conditions tested can be replicated under alternate configurations.

Many regulations mandate security and data privacy. As “open source” is frequently found on Linux, there are unique security issues to consider when your project uses open source code. On one hand, because of open source code and a community of developers, holes tend to be identified and patched quickly. At the same time, open vulnerabilities do not remain unexploited for long. There is also a tendency of some within the Linux community to ignore these security threats, viewing only Microsoft software as vulnerable to threats. However, according to Security Strategies in Linux Platforms and Applications, “While crackers attack Microsoft software more frequently, they also attack open source software.” Testing for potential security breaches is essential, even in Linux.

As part of reporting code quality metrics, testing can measure “code coverage” — how much of your code is tested. Coverage tools can also verify that every instruction was executed and document the coverage. Because coverage alone is not a metric for quality, unit testing forces review of the program logic, and can detect coding, compiler, and design errors. Collecting multiple metrics helps ensure regulatory compliance; still, you may need a set of different tools to report compliance.

Without the ability to unit test existing code, organizations find it hard to prove they have covered the entire code base. As a result, they throw a lot of money on QA and post-development testing. Other organizations generate reports that do not specify coverage metrics because their tools do not provide it. Regulated industries understand that to move to better, safer products, they should move to better tooling that can offer more exact coverage reporting.

To ensure standardization and the fulfillment of regulatory requirement, it’s ideal to use a consistent toolkit that ensures that you can design a system that can meet regulatory compliance across all the different Linux distributions you intend to ship on.

Testing Best Practices

Despite the challenges discussed previously, there are best practices that can provide confidence and assurance to developers testing Linux code. Some tips include:

  • Use consistent tooling, which can provide streamlined development and verifiable results across platforms and infrastructure.
  • Use productivity tools: Not everything needs to be hand coded. By using appropriate tooling, such as mocking frameworks, you can test even difficult-to-test code.
  • Pick the right test: There is no single type of test that is always appropriate. There are times in which unit tests should be performed while sometimes integration tests are the appropriate path to take.
  • Automate, automate, and automate — automation gives a way to repeat operations and releases us from trying to remember all scenarios. Automation also means that new developers or developers who are returning to their legacy code can work on existing code without worry that they forgot something.
  • Use isolated tests wisely—Isolation testing is good, but you don’t need to isolate everything outside the method. Isolate where things slow tests down (such as databases, file systems, or Web servers). Also, the user needs to interact when third-party components are involved. Isolation may not be necessary if the tests run quickly.


Linux’s role as a platform for software development is well established, and it is likely to only increase. Linux presents unique challenges to test code due to the fluid nature of many community development projects, scattered toolsets, and extensive legacy code. However, these challenges are not insurmountable. By using best practices, such as integration and unit testing, and proper tools, even the most difficult code can be tested.

Try Isolator++ for Linux

Gil Zilberfeld is Product Manager of Typemock, The Unit Testing Company. Avi Hein is Typemock’s Community Manager.