os - Operating System Interfaces

The os module provides a portable way to interact with the operating system, including file operations, process management, and environment variables.

Module Import

import os

File and Directory Operations

Current Directory

import os

# Get current working directory
cwd = os.getcwd()
print(cwd)  # '/home/user/project'

# Change directory
os.chdir('/tmp')

# Create directory
os.mkdir('new_folder')
os.makedirs('path/to/nested/folder')  # Create nested directories

# Remove directory
os.rmdir('empty_folder')  # Only removes empty directories
os.removedirs('path/to/folder')  # Remove nested empty directories

Listing Directory Contents

import os

# List all files and directories
contents = os.listdir('.')
print(contents)  # ['file1.txt', 'folder1', 'file2.py']

# List with full paths
for item in os.listdir('/path/to/dir'):
    full_path = os.path.join('/path/to/dir', item)
    print(full_path)

# Filter for specific files
python_files = [f for f in os.listdir('.') if f.endswith('.py')]

os.scandir() - Efficient Directory Iteration

import os

# More efficient than listdir for detailed info
with os.scandir('.') as entries:
    for entry in entries:
        if entry.is_file():
            print(f"File: {entry.name} ({entry.stat().st_size} bytes)")
        elif entry.is_dir():
            print(f"Directory: {entry.name}")

os.walk() - Recursive Directory Traversal

import os

# Walk through directory tree
for dirpath, dirnames, filenames in os.walk('.'):
    print(f"Current directory: {dirpath}")
    print(f"Subdirectories: {dirnames}")
    print(f"Files: {filenames}")
    print()

# Find all Python files recursively
for dirpath, _, filenames in os.walk('.'):
    for filename in filenames:
        if filename.endswith('.py'):
            full_path = os.path.join(dirpath, filename)
            print(full_path)

File Operations

Creating and Removing Files

import os

# Remove file
os.remove('file.txt')
os.unlink('file.txt')  # Alias for remove

# Rename/move file
os.rename('old_name.txt', 'new_name.txt')
os.replace('source.txt', 'dest.txt')  # Atomic replacement

File Information

import os
from datetime import datetime

# Get file stats
stats = os.stat('file.txt')
print(f"Size: {stats.st_size} bytes")
print(f"Modified: {datetime.fromtimestamp(stats.st_mtime)}")
print(f"Created: {datetime.fromtimestamp(stats.st_ctime)}")
print(f"Accessed: {datetime.fromtimestamp(stats.st_atime)}")

# Check if path exists
if os.path.exists('file.txt'):
    print("File exists")

# Check if it's a file or directory
if os.path.isfile('file.txt'):
    print("It's a file")

if os.path.isdir('folder'):
    print("It's a directory")

if os.path.islink('symlink'):
    print("It's a symbolic link")

Path Manipulation (os.path)

Joining Paths

import os

# Join path components (cross-platform)
path = os.path.join('folder', 'subfolder', 'file.txt')
print(path)  # 'folder/subfolder/file.txt' on Unix
             # 'folder\\subfolder\\file.txt' on Windows

# Build absolute path
abs_path = os.path.join(os.getcwd(), 'data', 'file.txt')

Path Components

import os

path = '/home/user/documents/file.txt'

# Split into directory and filename
dir_name, file_name = os.path.split(path)
print(dir_name)  # '/home/user/documents'
print(file_name)  # 'file.txt'

# Get directory name
dir_name = os.path.dirname(path)
print(dir_name)  # '/home/user/documents'

# Get filename
file_name = os.path.basename(path)
print(file_name)  # 'file.txt'

# Split extension
name, ext = os.path.splitext('file.txt')
print(name)  # 'file'
print(ext)  # '.txt'

Path Properties

import os

path = '/home/user/file.txt'

# Get absolute path
abs_path = os.path.abspath(path)

# Resolve relative path
real_path = os.path.realpath(path)  # Follows symlinks

# Expand user home directory
expanded = os.path.expanduser('~/documents')
print(expanded)  # '/home/user/documents'

# Check if path is absolute
if os.path.isabs(path):
    print("Absolute path")

Environment Variables

os.environ

Dictionary of environment variables.

import os

# Get environment variable
path = os.environ.get('PATH')
home = os.environ.get('HOME')
user = os.environ.get('USER', 'unknown')  # Default value

# Set environment variable
os.environ['MY_VAR'] = 'value'

# Delete environment variable
del os.environ['MY_VAR']

# Iterate over all variables
for key, value in os.environ.items():
    print(f"{key} = {value}")

os.getenv() and os.putenv()

import os

# Get environment variable
value = os.getenv('PATH')
value = os.getenv('CUSTOM_VAR', 'default')  # With default

# Set environment variable (prefer os.environ)
os.putenv('MY_VAR', 'value')

Process Management

Current Process

import os

# Get process ID
pid = os.getpid()
print(f"Current PID: {pid}")

# Get parent process ID
ppid = os.getppid()
print(f"Parent PID: {ppid}")

Running Commands

import os

# Execute command (deprecated, use subprocess instead)
os.system('ls -la')

# Get command output (use subprocess.run instead)
# output = os.popen('ls').read()

# Better approach:
import subprocess
result = subprocess.run(['ls', '-la'], capture_output=True, text=True)
print(result.stdout)

File Descriptors

Low-Level File Operations

import os

# Open file (returns file descriptor)
fd = os.open('file.txt', os.O_RDWR | os.O_CREAT)

# Read from file descriptor
data = os.read(fd, 1024)  # Read up to 1024 bytes

# Write to file descriptor
os.write(fd, b'Hello, World!')

# Close file descriptor
os.close(fd)

File Descriptor Operations

import os

# Duplicate file descriptor
fd2 = os.dup(fd)

# Change file position
os.lseek(fd, 0, os.SEEK_SET)  # Beginning
os.lseek(fd, 0, os.SEEK_END)  # End
os.lseek(fd, 10, os.SEEK_CUR)  # Current + 10

Permissions and Ownership

File Permissions

import os
import stat

# Change file permissions
os.chmod('file.txt', 0o644)  # rw-r--r--
os.chmod('script.sh', 0o755)  # rwxr-xr-x

# Using stat constants
os.chmod('file.txt', stat.S_IRUSR | stat.S_IWUSR | stat.S_IRGRP | stat.S_IROTH)

# Check permissions
stats = os.stat('file.txt')
mode = stats.st_mode
if mode & stat.S_IXUSR:
    print("File is executable by owner")

Ownership (Unix only)

import os

# Change owner (requires privileges)
os.chown('file.txt', uid, gid)

Platform-Specific

Path Separators

import os

print(os.sep)  # '/' on Unix, '\\' on Windows
print(os.pathsep)  # ':' on Unix, ';' on Windows
print(os.linesep)  # '\n' on Unix, '\r\n' on Windows

# Platform name
print(os.name)  # 'posix', 'nt', 'java'

Special Paths

import os

print(os.devnull)  # '/dev/null' on Unix, 'nul' on Windows
print(os.curdir)  # '.'
print(os.pardir)  # '..'

Practical Examples

Find All Files with Extension

import os

def find_files(directory, extension):
    """Find all files with given extension recursively"""
    matches = []
    for root, dirs, files in os.walk(directory):
        for file in files:
            if file.endswith(extension):
                matches.append(os.path.join(root, file))
    return matches

# Usage
python_files = find_files('.', '.py')
for file in python_files:
    print(file)

Get Directory Size

import os

def get_directory_size(directory):
    """Calculate total size of directory in bytes"""
    total_size = 0
    for dirpath, dirnames, filenames in os.walk(directory):
        for filename in filenames:
            filepath = os.path.join(dirpath, filename)
            if os.path.isfile(filepath):
                total_size += os.path.getsize(filepath)
    return total_size

# Usage
size = get_directory_size('.')
print(f"Directory size: {size / (1024**2):.2f} MB")

Create Backup with Timestamp

import os
from datetime import datetime
import shutil

def backup_file(filepath):
    """Create timestamped backup of file"""
    if not os.path.exists(filepath):
        return None
    
    timestamp = datetime.now().strftime('%Y%m%d_%H%M%S')
    name, ext = os.path.splitext(filepath)
    backup_path = f"{name}_{timestamp}{ext}"
    
    shutil.copy2(filepath, backup_path)
    return backup_path

# Usage
backup = backup_file('important.txt')
print(f"Backup created: {backup}")

Safe File Operations

import os
import tempfile

def safe_write(filepath, content):
    """Write to file atomically using temp file"""
    # Get directory and create temp file in same location
    directory = os.path.dirname(filepath) or '.'
    
    # Write to temporary file
    fd, temp_path = tempfile.mkstemp(dir=directory, text=True)
    try:
        with os.fdopen(fd, 'w') as f:
            f.write(content)
        
        # Atomic replacement
        os.replace(temp_path, filepath)
    except:
        # Clean up temp file on error
        if os.path.exists(temp_path):
            os.remove(temp_path)
        raise

# Usage
safe_write('config.txt', 'new configuration')

Best Practices

Use pathlib for modern path handling: For new code, consider using pathlib.Path instead of os.path:

from pathlib import Path

path = Path('folder') / 'subfolder' / 'file.txt'
if path.exists():
    content = path.read_text()

Be careful with os.remove(): It permanently deletes files without moving them to trash. Always verify the path before removing.

if os.path.exists(filepath) and os.path.isfile(filepath):
    os.remove(filepath)

Use os.path.join() for cross-platform paths: Always use os.path.join() instead of string concatenation with / or \\.

# Good
path = os.path.join('folder', 'file.txt')

# Bad
path = 'folder' + '/' + 'file.txt'

Migration to pathlib

Many os and os.path functions have pathlib equivalents:

import os
from pathlib import Path

# os module → pathlib
os.getcwd()              # Path.cwd()
os.path.exists(path)     # Path(path).exists()
os.path.isfile(path)     # Path(path).is_file()
os.path.isdir(path)      # Path(path).is_dir()
os.listdir(path)         # list(Path(path).iterdir())
os.mkdir(path)           # Path(path).mkdir()
os.rename(old, new)      # Path(old).rename(new)
os.remove(path)          # Path(path).unlink()
os.rmdir(path)           # Path(path).rmdir()

pathlib

Object-oriented filesystem paths

shutil

High-level file operations

glob

Unix-style pathname pattern expansion

tempfile

Temporary files and directories

Documentation Index

​Module Import

​File and Directory Operations

​Current Directory

​Listing Directory Contents

​os.scandir() - Efficient Directory Iteration

​os.walk() - Recursive Directory Traversal

​File Operations

​Creating and Removing Files

​File Information

​Path Manipulation (os.path)

​Joining Paths

​Path Components

​Path Properties

​Environment Variables

​os.environ

​os.getenv() and os.putenv()

​Process Management

​Current Process

​Running Commands

​File Descriptors

​Low-Level File Operations

​File Descriptor Operations

​Permissions and Ownership

​File Permissions

​Ownership (Unix only)

​Platform-Specific

​Path Separators

​Special Paths

​Practical Examples

​Find All Files with Extension

​Get Directory Size

​Create Backup with Timestamp

​Safe File Operations

​Best Practices

​Migration to pathlib

​Related Modules

pathlib

shutil

glob

tempfile

Module Import

File and Directory Operations

Current Directory

Listing Directory Contents

os.scandir() - Efficient Directory Iteration

os.walk() - Recursive Directory Traversal

File Operations

Creating and Removing Files

File Information

Path Manipulation (os.path)

Joining Paths

Path Components

Path Properties

Environment Variables

os.environ

os.getenv() and os.putenv()

Process Management

Current Process

Running Commands

File Descriptors

Low-Level File Operations

File Descriptor Operations

Permissions and Ownership

File Permissions

Ownership (Unix only)

Platform-Specific

Path Separators

Special Paths

Practical Examples

Find All Files with Extension

Get Directory Size

Create Backup with Timestamp

Safe File Operations

Best Practices

Migration to pathlib

Related Modules