Now the payoff. This lesson shows the three things DevOps scripts do constantly: run shell commands, read and write files, and call web APIs — then combines them into a small automation you could actually use.

🎯 Learning Objectives

By the end of this lesson you will:

• Run shell commands from Python with subprocess
• Read and write files safely
• Call a REST API and parse JSON
• Organize logic into reusable functions

Running Shell Commands

The subprocess module lets Python drive the same commands you’d type in Bash — while keeping Python’s logic and error handling:

import subprocess

result = subprocess.run(
  ["df", "-h", "/"],
  capture_output=True,
  text=True,
)
print(result.stdout)

Reading and Writing Files

The with block opens a file and closes it automatically, even if an error occurs:

# Write a config file
with open("report.txt", "w") as f:
  f.write("Disk check complete\n")

# Read it back
with open("report.txt") as f:
  contents = f.read()
print(contents)

Calling a Web API

Most DevOps work talks to APIs (cloud providers, monitoring, chatops). The requests library makes it simple:

pip3 install requests

import requests

resp = requests.get("https://api.github.com/repos/python/cpython")
resp.raise_for_status()        # fail loudly on HTTP errors

data = resp.json()             # parse JSON into a dict
print("Stars:", data["stargazers_count"])

Stars: 62000

Functions: Reusable Building Blocks

Wrap logic in functions so it can be named, reused, and tested:

def disk_usage(path="/"):
  result = subprocess.run(
      ["df", "-h", path],
      capture_output=True, text=True,
  )
  return result.stdout

print(disk_usage("/"))

Putting It Together: A Mini Health Check

import subprocess

def check(name, command):
  result = subprocess.run(command, capture_output=True, text=True)
  ok = result.returncode == 0
  print(f"[{'OK' if ok else 'FAIL'}] {name}")
  return ok

checks = {
  "disk":   ["df", "-h", "/"],
  "uptime": ["uptime"],
  "memory": ["free", "-h"],
}

results = [check(name, cmd) for name, cmd in checks.items()]
print(f"\n{sum(results)}/{len(results)} checks passed")

[OK] disk
[OK] uptime
[OK] memory

3/3 checks passed

This one script combines everything: functions, a dictionary, a loop, subprocess, and f-strings.

🧪 Hands-on Lab

def count_errors(path):
  with open(path) as f:
      return sum(1 for line in f if "ERROR" in line)

n = count_errors("app.log")
print(f"Found {n} errors")

🧠 Knowledge Check

💼 Interview Preparation

Summary

You can now run shell commands, work with files, call APIs, and structure scripts with functions — the real toolkit of Python automation. Next, we scale automation across many servers at once with Ansible… but first, the cloud that Ansible so often manages: AWS.