Python - Introduction to Shell Scripting and System Administration

Introduction to Shell Scripting and System Administration in Python

1. Introduction

Python is a versatile, high-level scripting language that excels in system automation, administration, and shell scripting. Unlike traditional shell scripts, Python brings power, readability, and a comprehensive standard library—making it a great choice for automating routine tasks, managing systems, and interacting with the file system and shell environment. In this document, we will explore how to use Python for shell scripting and system administration tasks.

2. Why Python for Shell Scripting?

2.1 Readability and Maintainability

Python syntax is clear and concise, which makes scripts easier to write, review, and maintain compared to Bash or batch scripts.

2.2 Cross-platform Support

Python runs on Linux, Windows, and macOS—scripts written on one OS often work on others with minimal modifications.

2.3 Rich Standard Library

Modules like os, sys, subprocess, and pathlib provide powerful tools without external dependencies.

2.4 Scalability

As script complexity grows, Python scales seamlessly—supporting modularity, error handling, and testing.

3. Writing a Basic Shell Script in Python

3.1 Shebang and Permissions

#!/usr/bin/env python3
import os

def greet():
    print("Hello from Python shell script!")

if __name__ == "__main__":
    greet()

Save as greet.py, then:

chmod +x greet.py
./greet.py

3.2 Parsing Arguments with argparse

#!/usr/bin/env python3
import argparse

parser = argparse.ArgumentParser(description="Greet someone.")
parser.add_argument("name", help="Name of the person")
parser.add_argument("-t", "--times", type=int, default=1, help="Number of times to greet")
args = parser.parse_args()

for i in range(args.times):
    print(f"Hello, {args.name}!")

4. Working with the File System

4.1 Listing Files in a Directory

import os

directory = "/var/log"
for filename in os.listdir(directory):
    print(filename)

4.2 File and Directory Operations with pathlib

from pathlib import Path

path = Path("/tmp/myfolder")
path.mkdir(exist_ok=True)

file = path / "log.txt"
file.write_text("Log entry")

for child in path.iterdir():
    print(child.name, child.stat().st_size)

4.3 Recursive File Search

from pathlib import Path

path = Path(".")
for file in path.rglob("*.py"):
    print(file)

4.4 File Metadata

import os
import time

stat = os.stat("greet.py")
print("Size:", stat.st_size)
print("Last modified:", time.ctime(stat.st_mtime))

5. Running Shell Commands

5.1 Using subprocess.run()

import subprocess

result = subprocess.run(["ls", "-l", "/tmp"], capture_output=True, text=True)
print(result.stdout)

5.2 Handling Errors

try:
    subprocess.run(["ls", "/nonexistent"], check=True)
except subprocess.CalledProcessError as err:
    print("Command failed with code", err.returncode)

5.3 Capturing and Processing Output

result = subprocess.run(["ps", "aux"], capture_output=True, text=True)
lines = result.stdout.splitlines()
for line in lines[:5]:
    print(line)

6. Process and System Information

6.1 Using psutil for Advanced Info

import psutil

for proc in psutil.process_iter(['pid', 'name', 'memory_info']):
    print(proc.info)

6.2 Memory and CPU Usage

print("CPU usage:", psutil.cpu_percent(interval=1))
print("Memory:", psutil.virtual_memory())

6.3 Disk and Network Stats

print("Disk usage:", psutil.disk_usage("/"))
print("Network IO:", psutil.net_io_counters())

7. Automating Backups and File Rotation

7.1 Zipping a Directory

import zipfile
from pathlib import Path

def backup_dir(src, dest_zip):
    with zipfile.ZipFile(dest_zip, 'w', zipfile.ZIP_DEFLATED) as zf:
        for file in Path(src).rglob("*"):
            zf.write(file, file.relative_to(src))

backup_dir("/home/user/data", "/home/user/data_backup.zip")

7.2 Rotating Log Files

import os
import time

log_dir = "/var/log/myapp"
now = time.time()
for file in os.listdir(log_dir):
    path = os.path.join(log_dir, file)
    if os.stat(path).st_mtime < now - 7 * 86400:
        os.remove(path)

8. Scheduling Tasks

8.1 Using cron for Python Scripts

# Edit crontab
crontab -e

# Add line to run daily
0 2 * * * /usr/bin/env python3 /home/user/scripts/backup.py

8.2 Using schedule Module

import schedule
import time

def backup():
    print("Running backup...")

schedule.every().day.at("02:00").do(backup)

while True:
    schedule.run_pending()
    time.sleep(60)

9. Monitoring and Alerting

9.1 Checking Disk Space

import shutil

total, used, free = shutil.disk_usage("/")
if free / total < 0.1:
    print("Warning: Low disk space!")

9.2 Sending Email Alerts

import smtplib
from email.mime.text import MIMEText

def send_alert(subject, body):
    msg = MIMEText(body)
    msg["Subject"] = subject
    msg["From"] = "admin@example.com"
    msg["To"] = "you@example.com"
    with smtplib.SMTP("smtp.example.com") as s:
        s.send_message(msg)

send_alert("Low disk", "Disk free space below 10%")

10. Managing Users and Permissions

10.1 Listing Users

import pwd

for u in pwd.getpwall():
    print(u.pw_name, u.pw_uid, u.pw_gecos)

10.2 Checking File Permissions

import os
import stat

mode = os.stat("/etc/passwd").st_mode
print("Readable by others:", bool(mode & stat.S_IROTH))

10.3 Running Commands as sudo

import subprocess

subprocess.run(["sudo", "systemctl", "restart", "apache2"])

11. Working with Configuration Files

11.1 INI Files with configparser

import configparser

config = configparser.ConfigParser()
config.read("app.ini")

db_host = config["database"]["host"]