In Python, the subprocess module provides a way to create new processes, interact with them, and retrieve their output. One useful function in this module is subprocess.check_output(), which allows you to capture the output of a command executed in a subprocess.

The check_output() Function

The check_output() function is a convenience method to run a command and capture its standard output as a byte string. It takes a single argument, which is the command to be executed, and returns the output as a byte string.

import subprocess

output = subprocess.check_output(["ls", "-l"])
print(output)

In the code above, subprocess.check_output(["ls", "-l"]) executes the ls -l command in a subprocess and captures its output. The output is then printed using the print() function.

Capturing Output as a String

By default, check_output() returns the output as a byte string. If you want to capture the output as a regular string, you can specify the encoding parameter.

import subprocess

output = subprocess.check_output(["ls", "-l"], encoding="utf-8")
print(output)

In this example, output will be a string representing the output of the ls -l command.

Handling Command Execution Errors

If the command executed by check_output() returns a non-zero exit status, a CalledProcessError exception is raised. You can catch this exception and handle the error accordingly.

import subprocess

try:
    output = subprocess.check_output(["invalid_command"])
except subprocess.CalledProcessError as e:
    print(f"Command execution failed with return code {e.returncode}.")

In this case, the invalid_command will raise a CalledProcessError, and the error message will be printed.

Conclusion

The subprocess.check_output() function provides a convenient way to execute commands in a subprocess and capture their output. It is a powerful tool for running external commands from within your Python scripts and automating various tasks.