capture_output.md

This Python script runs a shell command (ls -1), captures its output, and displays information about the process execution.

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Breakdown of the Code

import subprocess  # Import the subprocess module

# Run the 'ls -1' command and capture the output
res = subprocess.run(['ls', '-1'], stdout=subprocess.PIPE)

# Print the return code of the process
print('returncode:', res.returncode)

# Print the number of bytes in stdout and the decoded output
print(' {} bytes in stdout:\n{}'.format(len(res.stdout), res.stdout.decode('utf-8')))

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How It Works

1. import subprocess → Imports the subprocess module to execute shell commands from Python.
2. subprocess.run(['ls', '-1'], stdout=subprocess.PIPE)
   • Executes the ls -1 command in the shell.
   • ls -1 lists files in the current directory, one per line.
   • stdout=subprocess.PIPE captures the command’s output.
3. res.returncode → Gets the exit code of the process (0 indicates success).
4. len(res.stdout) → Finds the number of bytes in the output.
5. res.stdout.decode('utf-8') → Decodes the output from bytes to a readable string.

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Example Output

If the current directory contains file1.txt, file2.py, and folder1/, running the script would produce:

returncode: 0
 24 bytes in stdout:
file1.txt
file2.py
folder1

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How to Run the Script

1. Save the script as list_files.py.
2. Open a terminal and navigate to the script’s directory.
3. Run the script:

   python list_files.py

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Cross-Platform Compatibility

• Linux/macOS → ls works by default.
• Windows → ls is not a built-in command. Use:

  res = subprocess.run(['dir'], shell=True, stdout=subprocess.PIPE)

  Or use os.listdir() for a built-in alternative.

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Improved Version with Error Handling

To handle errors and ensure compatibility, use:

import subprocess

try:
    res = subprocess.run(['ls', '-1'], stdout=subprocess.PIPE, stderr=subprocess.PIPE, check=True)
    print('returncode:', res.returncode)
    print(' {} bytes in stdout:\n{}'.format(len(res.stdout), res.stdout.decode('utf-8')))
except subprocess.CalledProcessError as e:
    print('Error:', e)