Python Subprocess Stderr [In-Depth Tutorial]

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subprocess module to run external commands or processes and interact with them. To capture the standard error (stderr) output of a subprocess, you can use the subprocess.PIPE option for stderr in the subprocess.Popen constructor.

Here’s how you can do it:

import subprocess

# Command to run (replace with your desired command)
cmd = ["ls", "/nonexistent_directory"]

# Run the command and capture stdout and stderr
process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=True)

# Wait for the command to complete and capture the stdout and stderr outputs
stdout, stderr = process.communicate()

# Print the stdout and stderr outputs
print("Standard Output:")
print(stdout)

print("\nStandard Error:")
print(stderr)
Code language: Python (python)

In the above code:

  1. We import the subprocess module.
  2. Define the command you want to run as a list of strings (cmd). In this example, we’re running the ls command on a non-existent directory to generate an error message.
  3. We use subprocess.PIPE for both stdout and stderr to capture their respective outputs.
  4. We use universal_newlines=True to ensure that the captured outputs are treated as text (strings) rather than bytes.
  5. We use process.communicate() to wait for the subprocess to complete and capture both stdout and stderr.
  6. Finally, we print the captured stdout and stderr.

This way, you can run a subprocess and capture its stderr output for further processing or error handling in your Python script.

Python Subprocess Redirect Stderr To Stdout

In Python’s subprocess module, you can redirect the standard error (stderr) to standard output (stdout) by setting the stderr parameter to subprocess.STDOUT when creating the subprocess. This will cause both stdout and stderr to be combined and captured together. Here’s an example:

import subprocess

# Command to run (replace with your desired command)
cmd = ["ls", "/nonexistent_directory"]

# Run the command and redirect stderr to stdout
process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, universal_newlines=True)

# Wait for the command to complete and capture the combined stdout/stderr
combined_output = process.communicate()[0]

# Print the combined output
print(combined_output)
Code language: Python (python)

In this example:

  1. We define the command to run as a list of strings (cmd), where we’re running the ls command on a non-existent directory to generate an error message.
  2. We use subprocess.PIPE for stdout to capture its output, and subprocess.STDOUT for stderr to redirect it to stdout.
  3. We use universal_newlines=True to ensure that the captured output is treated as text (a string) rather than bytes.
  4. We use process.communicate() to wait for the subprocess to complete and capture the combined output (stdout and stderr). Since stderr is redirected to stdout, both are captured together.
  5. Finally, we print the combined output, which includes both stdout and stderr.

This approach is useful when you want to treat both stdout and stderr as a single stream of output.

Python Subprocess Stderr To Dev Null

To discard or redirect the standard error (stderr) of a subprocess to /dev/null (i.e., discard it completely) on Unix-like systems (including Linux and macOS), you can use the subprocess.DEVNULL constant as the value for the stderr parameter. Here’s an example:

import subprocess

# Command to run (replace with your desired command)
cmd = ["ls", "/nonexistent_directory"]

# Run the command and discard stderr by redirecting it to /dev/null
process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.DEVNULL, universal_newlines=True)

# Wait for the command to complete and capture the stdout
stdout = process.communicate()[0]

# Print the captured stdout
print(stdout)
Code language: Python (python)

In this example:

  1. We define the command to run as a list of strings (cmd), where we’re running the ls command on a non-existent directory to generate an error message.
  2. We use subprocess.PIPE for stdout to capture its output.
  3. We use subprocess.DEVNULL for stderr to discard it completely by redirecting it to /dev/null.
  4. We use universal_newlines=True to ensure that the captured output is treated as text (a string) rather than bytes.
  5. We use process.communicate() to wait for the subprocess to complete and capture the stdout.
  6. Finally, we print the captured stdout, while stderr is discarded.

This approach is useful when you want to run a command and ignore or discard any error messages it produces.

Get Exit Code And Stderr From Subprocess Call

You can get the exit code and standard error (stderr) from a subprocess call in Python using the subprocess module. To do this, you can use the subprocess.run() function, which provides a convenient way to run a command and capture its exit code, stdout, and stderr. Here’s an example:

import subprocess

# Command to run (replace with your desired command)
cmd = ["ls", "/nonexistent_directory"]

# Run the command and capture exit code, stdout, and stderr
result = subprocess.run(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, universal_newlines=True)

# Get the exit code
exit_code = result.returncode

# Get stdout and stderr
stdout = result.stdout
stderr = result.stderr

# Print the exit code, stdout, and stderr
print("Exit Code:", exit_code)
print("\nStandard Output:")
print(stdout)
print("\nStandard Error:")
print(stderr)
Code language: Python (python)

In this example:

  1. We define the command to run as a list of strings (cmd), where we’re running the ls command on a non-existent directory to generate an error message.
  2. We use subprocess.PIPE for both stdout and stderr to capture their respective outputs.
  3. We use universal_newlines=True to ensure that the captured outputs are treated as text (strings) rather than bytes.
  4. We run the command using subprocess.run() and capture the result object, which contains the exit code, stdout, and stderr.
  5. We retrieve the exit code using result.returncode.
  6. We retrieve the stdout and stderr using result.stdout and result.stderr, respectively.
  7. Finally, we print the exit code, stdout, and stderr.

This approach allows you to run a command, capture its exit code, and retrieve both stdout and stderr for further processing or error handling in your Python script.

Python Subprocess.check_output Stderr Usage

subprocess.check_output() function is used to run a command and capture its standard output (stdout). By default, it doesn’t capture the standard error (stderr) output. However, you can redirect the stderr output to stdout or capture it separately by using the stderr parameter. Here’s how you can use it:

  1. Redirect stderr to stdout:
import subprocess

# Command to run (replace with your desired command)
cmd = ["ls", "/nonexistent_directory"]

try:
    # Run the command and capture stdout and stderr
    output = subprocess.check_output(cmd, stderr=subprocess.STDOUT, universal_newlines=True)

    # Print the combined output (stdout and stderr)
    print("Combined Output:")
    print(output)
except subprocess.CalledProcessError as e:
    # If the command exits with a non-zero status code, you can capture stderr separately
    stderr = e.output
    print("Error Output:")
    print(stderr)
Code language: Python (python)

In this example, we use stderr=subprocess.STDOUT to redirect stderr to stdout, and we capture the combined output.

  1. Capture stderr separately:
import subprocess

# Command to run (replace with your desired command)
cmd = ["ls", "/nonexistent_directory"]

try:
    # Run the command and capture stdout
    output = subprocess.check_output(cmd, universal_newlines=True)

    # Print stdout
    print("Standard Output:")
    print(output)
except subprocess.CalledProcessError as e:
    # If the command exits with a non-zero status code, you can capture stderr separately
    stderr = e.stderr
    print("Error Output:")
    print(stderr)
Code language: Python (python)

In this example, we let subprocess.check_output() capture stdout as usual. If the command exits with a non-zero status code (indicating an error), we capture stderr separately using e.stderr.

Choose the approach that best fits your use case. Redirecting stderr to stdout is useful when you want to treat both outputs as a single stream. Capturing stderr separately allows you to handle stdout and stderr independently, which can be helpful for error reporting and diagnostics.

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  • Aniket Singh

    Aniket Singh holds a B.Tech in Computer Science & Engineering from Oriental University. He is a skilled programmer with a strong coding background, having hands-on experience in developing advanced projects, particularly in Python and the Django framework. Aniket has worked on various real-world industry projects and has a solid command of Python, Django, REST API, PostgreSQL, as well as proficiency in C and C++. He is eager to collaborate with experienced professionals to further enhance his skills.

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