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Mastering Python Subprocess: Tips for ‘check_output’ Arg with Shell=True

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Python is an incredibly versatile and powerful language for a wide range of applications. One of the areas where it really shines is in its ability to control other programs and interact with system processes. The subprocess module is one of the key tools that developers use in this regard, allowing you to control, launch, and monitor other programs from within your Python code.

While there are many aspects of subprocess that can be challenging to master, one particular area that often causes confusion and frustration is the use of the check_output function with the shell=True argument. This powerful combination allows you to run complex shell commands with just a few lines of Python code, but it can also introduce a number of potential security and performance issues if not used properly.

If you’re looking to get the most out of Python’s subprocess module and want to avoid some common pitfalls, then this article is for you. We’ll walk you through the basics of executing shell commands with subprocess, explain how check_output works and why it can be tricky to use, and provide some tips and best practices for working with this powerful function.

Whether you’re a Python beginner or an experienced developer, mastering subprocess is an essential skill for any programmer who needs to interact with external processes in their code. So if you want to learn more about how to use check_output effectively and safely, be sure to read on!

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“Understanding Python Subprocess.Check_output’S First Argument And Shell=True [Duplicate]” ~ bbaz


Python’s subprocess module enables you to spawn new processes, connect to their input/output/error pipes, and obtain their return codes. In this article, we will explore how to utilize the ‘check_output’ argument with shell=True for optimal output experience.

Understanding subprocess.check_output()

The ‘subprocess.check_output’ function is used in Python for getting the output from a subprocess command. It has several arguments that can be passed but we will focus on how to use the ‘check_output’ argument with shell=True.

Using check_output with Shell=True

When the shell=True parameter is provided, the shell is invoked as an interactive command-line interface. The command is sent to the shell for interpretation before being executed. Let’s take a closer look at how this works.

Comparison Table:

Shell=True Shell=False
The shell is invoked as an interactive command-line interface. A shell is not invoked.
The command string passed is interpreted by shell. The command is run directly without being interpreted by shell.
Can cause a Security risk. Less secure as it executes command directly without any interpretation.
Better suited for complex commands. Better suited for simple commands.

Security Implications of Shell=True

Shell=True indicates that the subprocess should be invoked through a shell. Invoking code with shell=True leaves your system open to CWE-78, which is an OS command injection vulnerability. It allows an attacker to execute OS commands by exposing program functions that accept filenames or any other form of input that could contain hostile data.

Handling Exceptions while Using Shell=True

There are various exceptions that can occur when using check_output with the shell=True parameter. Some of these errors include OSError, CalledProcessError, and ValueError. Therefore, it is important to handle exceptions properly to prevent the program from breaking.

Overcoming Security Risks with Shell=False

If you are concerned about security risks associated with using shell=True but still need to pass complex commands, use the ‘shell=False’ parameter. For instance, instead of using ‘ls –la | grep pdf’ which exposes your system to an OS command injection attack, write the equivalent command in a non-interpreted way as follows:

[‘ls’, ‘-la’, ‘|’, ‘grep’, ‘pdf’]

Common Pitfalls to Avoid

To avoid common pitfalls when using check_output with shell=True, consider the following tips:

Tip 1: Be Cautious with Input from External Sources

Make sure you don’t receive any unexpected input from external sources when using shell=True. Otherwise, it might lead to arbitrary code execution.

Tip 2: Always Sanitize User Input

Before passing user input to shell=True, always ensure the input is sanitized. Having unclean data might potentially result in command injection and might compromise the entire application or operating system.


The ‘check_output’ function is widely used with the subprocess module in Python. Therefore, it is vital to have a good understanding of how to use it appropriately with the shell=True parameter for optimal output results. While the usage of Shell=True can lead to security risks, they can be mitigated by being cautious with inputs from external sources and properly sanitizing user input.

Thank you for taking the time to read this article on Mastering Python Subprocess. We hope that you found the tips for ‘check_output’ Arg with Shell=True helpful in your journey to become a better Python programmer.

As you continue to learn and master Python, it is important to remember the value of using subprocess, especially when running commands or scripts in a separate process. With the use of subprocess, you can easily control and manipulate input and output streams, making it a powerful tool for any Python developer to have in their toolkit.

In closing, we encourage you to continue exploring the possibilities of subprocess in Python. It may seem daunting at first, but with practice and persistence, you will soon be able to harness the full power of this versatile library. Thank you again for reading, and we wish you all the best on your Python programming journey!

People Also Ask About Mastering Python Subprocess: Tips for ‘check_output’ Arg with Shell=True

Python is a programming language that has become increasingly popular over the years. One of its strengths is its ability to interact with other programs through subprocesses. The ‘check_output’ function is a useful tool in Python’s subprocess module, but it can be tricky to get right. Here are some common questions people ask about mastering Python subprocess and ‘check_output’ with the shell=True argument:

1. What is the ‘check_output’ function?

The ‘check_output’ function is a convenience function in Python’s subprocess module that allows you to run a command and capture its output as a byte string. It raises an error if the command returns a non-zero exit code. This is useful for running external commands and capturing their output in your Python program.

2. What does the shell=True argument do?

The shell=True argument tells Python to run the command in a shell environment. This means that any special characters, such as pipes or redirections, will be interpreted by the shell before being passed to the command. This can be useful for running complex commands or for running commands that require shell-specific features.

3. What are some tips for using ‘check_output’ with shell=True?

  • Always use a list to pass arguments to ‘check_output’, even when using shell=True. This helps protect against shell injection attacks.
  • Use the shlex.split() function to split a command into a list of arguments. This function takes care of quoting and escaping the arguments properly.
  • Avoid passing user input directly to ‘check_output’ with shell=True, as this can lead to security vulnerabilities.
  • Consider using the function instead of ‘check_output’ when you need more control over the subprocess, such as setting environment variables or redirecting input/output streams.

By following these tips, you can use Python’s subprocess module and the ‘check_output’ function with shell=True more effectively and securely.