How to Redirect Output with Subprocess in Python?

Redirecting Output with Subprocess in Python

When executing shell commands in Python using the subprocess module, it's essential to know how to redirect output. One common scenario is redirecting the standard output of a command into a file. This article addresses this need by exploring Python's approach to achieve output redirection.

In the provided Python code snippet, the user attempts to use subprocess.call to redirect the output of the cat command to a file by splitting the command into arguments using shlex. However, the output is still displayed in the console.

To properly redirect the output in Python 3.5 and later, the stdout argument of subprocess.run can be used. This argument accepts an open file handle as its value, which is where the output will be directed. Here's an example:

# Use a list of args instead of a string
input_files = ['file1', 'file2', 'file3']
my_cmd = ['cat'] + input_files
with open('myfile', "w") as outfile:
    subprocess.run(my_cmd, stdout=outfile)

In this code, the command to be executed is split into a list of arguments, and the output is redirected to a file named 'myfile'. The with statement ensures that the file is properly opened, written to, and closed.

It's worth noting that using an external command like cat for this purpose is unnecessary. Python has built-in functions for reading and writing files, which can be used directly:

with open('myfile', "w") as outfile:
    outfile.write(open('file1').read())
    outfile.write(open('file2').read())
    outfile.write(open('file3').read())

By understanding how to properly redirect output with subprocess or utilizing Python's file handling capabilities, you can effectively manage the flow of information in your Python scripts.

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