Processes and Subprocesses
Processes and subprocesses are essential components of any operating system. A process is simply a program in execution, while a subprocess is a program that is started by another program.
In this article, we will explore the concept of processes and subprocesses in detail and learn about the Python subprocess module.
Processes and the Operating System
Processes are responsible for many tasks in the operating system, from running the user’s programs to managing system resources. The CPU is the heart of the computer and manages the processes in the system.
The operating system schedules the different processes, giving them control of the CPU at different times. This is known as multitasking.
Process Lifetime
A process is created when a program is loaded into the computer’s memory and is assigned a unique Process ID (PID). The PID is used to manage the process in the process table.
If a process creates another process, it becomes the parent process, and the new process is called a child process.
Active Processes on Your System
Tracking running processes is essential when troubleshooting problems on a computer. In Linux, the ps
command can list all the running processes on the system with their respective PIDs. In Windows, the Task Manager can be used to view the list of running processes.
The Python subprocess Module
Python provides several modules to run external programs. The subprocess
module is a powerful tool to execute shell commands from within the Python environment.
The module makes it easy to use the output of the command in subsequent parts of the program.
Basic Usage of the Python subprocess Module
The subprocess
module provides several functions to execute external commands. The run()
function is used to run a command and capture its output.
The Popen()
function enables more control over the execution of the command, including the ability to send input to the command’s standard input stream and redirecting the command’s standard output stream to a file.
Timer Example
The Timer Example is a straightforward program that uses argparse
to parse command-line arguments and the sleep()
function from the time
module to introduce a delay. The program then displays a simple animation using the print
function.
This example shows how subprocess
can be used to create cross-platform programs.
Use of subprocess
to Run Any App
One of the most significant advantages of using the subprocess
module is being able to run any application from the command line, regardless of the platform. This ability is essential when writing cross-platform GUI programs.
In conclusion, processes and subprocesses are critical components of any operating system. The Python subprocess
module provides a flexible and powerful way to run external programs and manage their output.
Understanding the basics of these concepts is vital for programmers to write efficient and robust code.
3) The CompletedProcess
Object
The subprocess
module in Python returns a CompletedProcess
object after successfully executing a command. This object encapsulates the process’s status, including the return code, standard output, and standard error.
Additionally, this object also provides an easy way to retrieve the command’s output and status code.
Subprocess Exceptions
The subprocess
module provides several exceptions that can be raised during execution. The CalledProcessError
exception is raised when the command returns a non-zero return code, indicating a failure.
The TimeoutExpired
exception is raised when the command fails to execute within the specified timeout period. Lastly, the FileNotFoundError
exception is raised when the specified command cannot be found.
An Example of Exception Handling
Exception handling is an essential programming concept that provides a way to gracefully handle errors and unexpected events during program execution. In the context of subprocess
, exception handling is necessary to handle errors that occur during the execution of a command.
Consider the following example:
import subprocess
try:
completed_process = subprocess.run(['command', 'arg1', 'arg2'], check=True, capture_output=True)
# Access the output and status code of the command
print(completed_process.stdout)
print(completed_process.returncode)
except subprocess.CalledProcessError as e:
print(f'Error executing command: {e}')
In this example, the try
block attempts to execute a command using subprocess.run()
. If the command returns a non-zero status code, the except
block catches the exception and prints an error message.
4) Interacting With the Shell and Text-Based Programs With subprocess
The shell is the traditional interface for interacting with the operating system in UNIX-based systems. In the context of subprocess
, the shell can be used to execute command-line programs with arguments and options in a familiar environment.
The subprocess
module provides a way to invoke a shell to run command-line programs.
Use Cases for the Shell and subprocess
The shell is commonly used to execute multiple commands together in a single session. Also, many command-line programs require additional environment variables and settings that may be easier to set up in the shell.
In addition, subprocess
can be used to invoke any program in a text-based environment, including the console, terminal, or command prompt.
Basic Usage of subprocess
With UNIX-Based Shells
In UNIX-based systems such as Linux and macOS, the shell is typically either Bash or sh. The subprocess
module can be used to invoke a shell session and run commands within it.
Consider the following example that uses pipes to extract the first ten lines of a file:
import subprocess
process = subprocess.Popen(['cat', 'file.txt'], stdout=subprocess.PIPE)
process2 = subprocess.Popen(['head', '-10'], stdin=process.stdout)
output = process2.communicate()[0]
print(output)
In this example, the cat
command is used to read the contents of a file. The output of the cat
command is passed to the head
command using a pipe.
The head
command extracts the first 10 lines of the file and sends it to stdout. The communicate()
function returns the output of the command as a byte string.
Basic Usage of subprocess
With Windows Shells
In Windows, the shell is not the standard command prompt but rather PowerShell. The subprocess
module can be used to run any command using PowerShell.
Additionally, there are several third-party tools such as Sysinternals that can be invoked using subprocess
. Consider the following example that uses subprocess
to get a list of running processes on a Windows system using the tasklist
command:
import subprocess
process = subprocess.Popen(['powershell.exe', 'tasklist'], stdout=subprocess.PIPE)
output = process.communicate()[0]
print(output)
In this example, the tasklist
command is executed using the PowerShell executable. The output of the command is returned as a byte string using the communicate()
function.
In conclusion, subprocess
is a powerful and versatile module in Python that provides an easy way to run external programs and manage their output. Whether you are working on UNIX-based or Windows-based systems, subprocess
allows you to execute text-based programs and interact with the shell in a familiar environment.
The CompletedProcess
object and exception handling are essential concepts that provide a way to handle errors and unexpected events during program execution.
5) Communication With Processes
The subprocess
module provides several methods to communicate with running processes. One of the most fundamental concepts in subprocess
communication is the standard I/O streams.
The Standard I/O Streams
The standard I/O streams are the standard input, output, and error streams of a process. The standard input (stdin) provides a way to send data to the process, while the standard output (stdout) and standard error (stderr) streams provide a way to receive output and error messages from the process.
The Magic Number Generator Example
Consider the following example that generates a random number between 0 and 100 using the external program bc
:
import subprocess
process = subprocess.Popen(['bc', '-l'], stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE)
out, err = process.communicate(b'import randomnfor i in range(1):n print int(100 * random.random())n')
print(int(out))
In this example, the bc
command is used to execute the script that generates a random number between 0 and 100. The communicate()
function is used to pass the script to the process’s standard input and the process’s standard output.
The Decoding of Standard Streams
The standard streams are binary streams, and it is necessary to decode the byte strings into a readable format. The subprocess
module provides methods for decoding and encoding the byte strings using the specified encoding.
6) Pipes and the Shell
The Unix philosophy encourages the use of pipes, a mechanism for inter-process communication. Pipes allow the output of one process to be used as the input of another process seamlessly.
The subprocess
module provides several mechanisms to create pipes between processes.
Pipes
In the context of subprocess
, a pipe is a unidirectional channel of communication between two processes, allowing the output of one process to be used as the input of another process.
The Pipes of subprocess
The subprocess
module provides several options for creating pipes. The stdout
argument can be set to subprocess.PIPE
to create a pipe for the standard output stream, and similarly, the stdin
argument can be set to subprocess.PIPE
to create a pipe for the standard input stream.
The communicate()
method can then be used to pass input to the process and receive output from the process.
Pipe Simulation with run()
Consider the following example that simulates a Unix pipeline with subprocess
:
import subprocess
process1 = subprocess.Popen(['cat', 'file.txt'], stdout=subprocess.PIPE)
process2 = subprocess.Popen(['grep', 'pattern'], stdin=process1.stdout, stdout=subprocess.PIPE)
output = process2.communicate()[0]
print(output.decode('utf-8'))
In this example, the cat
command is used to read the contents of a file, and the output is passed to the grep
command, which searches for a pattern in the output. The output of the grep
command is returned as a byte string using the communicate()
function.
In conclusion, communication with processes is a fundamental concept in subprocess
programming. Understanding the standard I/O streams, pipes, and the interaction between processes is key to writing efficient and robust code.
Subprocess
provides a flexible and powerful way to execute shell commands and run external programs from within the Python environment. With a solid understanding of these concepts, programmers can build complex text-based applications with ease.
7) Practical Ideas
The subprocess
module in Python is a powerful tool that allows programmers to execute shell commands and run external programs from within the Python environment. In this section, we will explore some practical ideas and use cases for using subprocess
.
Creating a New Project: An Example
When creating a new project, it is essential to have a template and a standard directory structure to follow. The subprocess
module can be used to create the directory structure and initialize any necessary files.
Consider the following example that creates a new project with a standard directory structure:
import subprocess
import os
main_dir = 'myproject'
# Create main directory and sub-directories
os.mkdir(main_dir)
os.mkdir(os.path.join(main_dir, 'bin'))
os.mkdir(os.path.join(main_dir, 'data'))
os.mkdir(os.path.join(main_dir, 'docs'))
os.mkdir(os.path.join(main_dir, 'tests'))
# Create necessary files
subprocess.call(['touch', os.path.join(main_dir, 'README.md')])
In this example, the os
module is used to create the directory structure, while the touch
command is invoked using subprocess
to create the README.md
file.
Changing Extended Attributes
Extended attributes are additional metadata associated with a file in the file system. Some attributes can be modified using the chattr
command in Linux and macOS systems.
The subprocess
module can be used to modify these extended attributes programmatically. Consider the following example that sets the immutable flag on a file in a Linux system:
import subprocess
file_name = 'example.txt'
subprocess.call(['sudo', 'chattr', '+i', file_name])
In this example, the chattr
command is used to set the immutable flag on the file example.txt
. The sudo
command is used to obtain elevated privileges necessary to modify the file.
Python Modules Associated With subprocess
Several Python modules are associated with subprocess
that extend its functionality and add more flexibility. The psutil
module provides an easy way to interface with process information, including the current CPU usage, memory, and process status.
The shlex
module can be used to parse command-line arguments and options. The select
module provides a way to monitor file descriptors for I/O activity.
The signal
module allows a Python process to interact with other processes using signals.
8) The Popen
Class
The Popen
class is the main class of the subprocess
module. It represents a running process and provides methods to interact with the process’s standard input, output, and error streams.
Consider the following example that uses Popen
to run a simple script:
import subprocess
process = subprocess.Popen(['python', '-c', 'print("Hello from subprocess!")'], stdout=subprocess.PIPE)
output, error = process.communicate()
print(output.decode('utf-8'))
In this example, the Popen
class is used to run a simple Python script that prints a message to the standard output stream. The communicate()
method is used to retrieve the script’s output.
In conclusion, the subprocess
module is a powerful tool that provides an easy way to run external programs and manage their output from within the Python environment. Understanding the basics of the module, including the standard I/O streams, pipes, and the Popen
class, is essential to building efficient and robust applications.
Subprocess
is a versatile module that has a wide range of use cases, from creating new projects to modifying extended attributes and interfacing with other Python modules. With practice and advanced knowledge of the module, developers can create complex and powerful text-based applications that interact with the operating system at a fundamental level.
In conclusion, the subprocess
module in Python is an essential tool that allows programmers to execute shell commands and run external programs from within the Python environment. This article covered the fundamentals of processes, subprocesses, the CompletedProcess
object, communication with processes, and pipes and the shell.
We also discussed practical ideas such as creating a new project, changing extended attributes, and Python modules associated with subprocess
. Understanding the basics of the subprocess
module is critical to building efficient and robust text-based applications that interact with the operating system at a fundamental level.
Subprocess
provides a versatile and powerful way to execute shell commands and run external programs from within the Python environment. With practice and advanced knowledge of the module, developers can create complex and powerful text-based applications that communicate with other processes.