54. Python's subprocess Module

1. Running a Simple Command

Running a simple system command like ls or dir using subprocess.run().

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import subprocess

# Run a system command (list files in current directory)
result = subprocess.run(['ls'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)

# Print output and errors
print('Output:', result.stdout.decode())
print('Errors:', result.stderr.decode())

This runs the ls command (or dir on Windows) and captures its output and error streams.

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2. Capturing Command Output

Capturing the output of a command in a variable.

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import subprocess

# Run a command and capture the output
result = subprocess.run(['echo', 'Hello, World!'], stdout=subprocess.PIPE)

# Print the captured output
print(result.stdout.decode())

This code runs the echo command and captures its output.

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3. Running a Command with Shell=True

Running a command as a string with the shell.

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import subprocess

# Run a command with shell=True
result = subprocess.run('ls -l', shell=True, stdout=subprocess.PIPE)

# Print the output
print(result.stdout.decode())

The shell=True argument allows running shell commands directly as a string.

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4. Handling Command Errors

Handling errors when running a system command.

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import subprocess

# Run a command that will likely fail (wrong command)
try:
    subprocess.run(['non_existent_command'], check=True)
except subprocess.CalledProcessError as e:
    print(f"Error occurred: {e}")

This runs a non-existent command and catches the error using CalledProcessError.

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5. Piping Output to Another Command

Piping the output of one command into another.

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import subprocess

# Run a command and pipe the output to another command
p1 = subprocess.Popen(['echo', 'Hello, World!'], stdout=subprocess.PIPE)
p2 = subprocess.Popen(['grep', 'Hello'], stdin=p1.stdout, stdout=subprocess.PIPE)

# Get the result of the second command
output = p2.communicate()[0]

# Print the output
print(output.decode())

This demonstrates piping output from one process to another using Popen objects.

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6. Running a Command with Input

Running a command that requires user input.

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import subprocess

# Run a command that accepts input (e.g., 'cat')
process = subprocess.Popen(['cat'], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
output, _ = process.communicate(input=b'Hello from Python\n')

# Print the output
print(output.decode())

This sends input to a command and captures the output.

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7. Timeout Handling

Running a command with a timeout.

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import subprocess

# Run a command with a timeout of 3 seconds
try:
    subprocess.run(['sleep', '5'], timeout=3)
except subprocess.TimeoutExpired:
    print("The command timed out!")

This snippet runs the sleep command for 5 seconds, but a timeout is set for 3 seconds, causing the command to be terminated.

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8. Running a Command in the Background

Running a process in the background.

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import subprocess

# Run a command in the background (non-blocking)
process = subprocess.Popen(['sleep', '10'])

# Print process ID (PID)
print(f"Process started with PID: {process.pid}")

This runs the sleep command in the background and prints its process ID.

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9. Getting Command Return Code

Getting the return code of a command to check for success or failure.

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import subprocess

# Run a command and check return code
result = subprocess.run(['ls'], stdout=subprocess.PIPE, stderr=subprocess.PIPE)

# Check the return code
if result.returncode == 0:
    print('Command executed successfully')
else:
    print(f'Command failed with return code: {result.returncode}')

This checks the return code to determine whether the command ran successfully.

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10. Using Subprocess with Environment Variables

Passing custom environment variables to a subprocess.

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import subprocess
import os

# Set custom environment variables
env = os.environ.copy()
env['MY_VAR'] = 'Hello'

# Run a command with the custom environment variable
result = subprocess.run(['echo', '$MY_VAR'], shell=True, env=env, stdout=subprocess.PIPE)

# Print the result
print(result.stdout.decode())

This example demonstrates how to pass custom environment variables to a subprocess.

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These snippets show how to interact with system processes, run commands, capture their output, and handle errors or custom conditions in Python using the subprocess module.