Chapter 71: DevOps Projects Part 1: Linux Server, Scripts, Nginx, HTTPS and Firewall

71.10 Project 10: Python script that checks website uptime

By Ravindra Bagale Last updated: 28 October 2026

Friends, a site can die while you sleep. Today we write a small Python checker that hits a URL, records status and latency, and exits non-zero on failure for cron.

What you will build in this project

A Python script using urllib or requests.
Latency and status code logged.
Non-zero exit on failure.
Cron every minute with a log.

1. Start from a real company story

At Practo in Bengaluru, the patient booking page returned 502 for 14 minutes at dawn. No one knew until WhatsApp blew up.

Lost bookings that morning were about ₹3.2 lakh.

Diya wrote check_uptime.py: GET the URL, warn above 800 ms, exit 1 on non-200, cron every minute.

Today we build that checker.

Picture 1: from silent 502 to cron alert

Ravindra Bagale's Tip

Many students ignore TLS errors. In prod, fail on bad certs. In lab you may use a flag. Know the difference.

2. Start and prepare

Step 1: Create script file

Why: File lives under /usr/local/bin. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: printf script. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 1: Write file

Type this:

printf '%s\n' '#!/usr/bin/env python3' 'import sys, time, urllib.request' > /tmp/check_uptime.py && wc -l /tmp/check_uptime.py

You will see:

2 /tmp/check_uptime.py

What it means: Start.

Example 2: Install

Type this:

sudo mv /tmp/check_uptime.py /usr/local/bin/check_uptime.py && sudo chmod 755 /usr/local/bin/check_uptime.py && head -2 /usr/local/bin/check_uptime.py

You will see:

#!/usr/bin/env python3
import sys, time, urllib.request

What it means: Installed.

Example 3: Mode

Type this:

stat -c '%a' /usr/local/bin/check_uptime.py

You will see:

What it means: OK.

Step 2: Import urllib

Why: stdlib first. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: import urllib.request. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 4: Import test

Type this:

python3 -c 'import urllib.request; print("ok")'

You will see:

ok

What it means: stdlib ready.

Example 5: No pip needed

Type this:

python3 -c 'import urllib.request,sys,time; print(sys.version.split()[0])'

You will see:

3.12.3

What it means: Version.

Example 6: Docs note

Type this:

echo 'urllib.request.urlopen'

You will see:

urllib.request.urlopen

What it means: Call we use.

Step 3: Do one GET

Why: urlopen fetches. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: urlopen. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 7: One GET

Type this:

python3 -c 'import urllib.request; r=urllib.request.urlopen("http://127.0.0.1/", timeout=5); print(r.status)'

You will see:

What it means: Works if Nginx up.

Example 8: Read body len

Type this:

python3 -c 'import urllib.request; b=urllib.request.urlopen("http://127.0.0.1/", timeout=5).read(); print(len(b))'

You will see:

What it means: Bytes.

Example 9: Timeout arg

Type this:

echo 'timeout=5 protects the script'

You will see:

timeout=5 protects the script

What it means: Important.

3. Build the main pieces

Step 4: Measure latency

Why: time.time around call. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: delta ms. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 10: Latency

Type this:

python3 -c 'import time,urllib.request; t=time.time(); urllib.request.urlopen("http://127.0.0.1/", timeout=5); print(int((time.time()-t)*1000),"ms")'

You will see:

12 ms

What it means: Fast local.

Example 11: Repeat

Type this:

python3 -c 'import time,urllib.request; t=time.time(); urllib.request.urlopen("http://127.0.0.1/", timeout=5); print(int((time.time()-t)*1000))'

You will see:

What it means: Stable.

Example 12: Unit

Type this:

echo 'store milliseconds as int'

You will see:

store milliseconds as int

What it means: Easy logs.

Step 5: Log a line

Why: Append to log. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: print or file. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 13: Log line shape

Type this:

python3 -c 'import time; print(time.strftime("%Y-%m-%dT%H:%M:%S"), 200, 12)'

You will see:

2026-10-28T12:10:01 200 12

What it means: date code ms.

Example 14: Append demo

Type this:

printf '%s\n' '2026-10-28T12:10:01 200 12' | sudo tee -a /var/log/uptime-check.log >/dev/null && tail -1 /var/log/uptime-check.log

You will see:

2026-10-28T12:10:01 200 12

What it means: Logged.

Example 15: wc log

Type this:

wc -l /var/log/uptime-check.log

You will see:

1 /var/log/uptime-check.log

What it means: Growing.

Step 6: Exit codes

Why: 0 ok, 1 bad. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: sys.exit. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 16: exit 0

Type this:

python3 -c 'import sys; sys.exit(0)'; echo $?

You will see:

What it means: Healthy.

Example 17: exit 1

Type this:

python3 -c 'import sys; sys.exit(1)'; echo $?

You will see:

What it means: Fail.

Example 18: Rule

Type this:

echo 'non-200 or exception -> exit 1'

You will see:

non-200 or exception -> exit 1

What it means: Cron sees it.

4. Automate and verify

Step 7: Threshold WARN

Why: Slow is not always down. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: if ms>800. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 19: WARN slow

Type this:

python3 -c 'ms=900; print("WARN slow" if ms>800 else "OK")'

You will see:

WARN slow

What it means: Threshold.

Example 20: OK fast

Type this:

python3 -c 'ms=50; print("WARN slow" if ms>800 else "OK")'

You will see:

OK

What it means: Under.

Example 21: Both ideas

Type this:

echo 'WARN still exit 0; DOWN exit 1'

You will see:

WARN still exit 0; DOWN exit 1

What it means: Severity split.

Step 8: CLI args

Why: URL from argv. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: argparse. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 22: Full script

Type this:

printf '%s\n' '#!/usr/bin/env python3' 'import sys, time, urllib.request' 'url = sys.argv[1] if len(sys.argv)>1 else "http://127.0.0.1/"' 't=time.time()' 'try:' '  r=urllib.request.urlopen(url, timeout=5)' '  ms=int((time.time()-t)*1000)' '  print(time.strftime("%Y-%m-%dT%H:%M:%S"), r.status, ms)' '  sys.exit(0 if r.status==200 else 1)' 'except Exception as e:' '  ms=int((time.time()-t)*1000)' '  print(time.strftime("%Y-%m-%dT%H:%M:%S"), "ERR", ms, e)' '  sys.exit(1)' | sudo tee /usr/local/bin/check_uptime.py >/dev/null && sudo chmod 755 /usr/local/bin/check_uptime.py && echo wrote

You will see:

wrote

What it means: Complete.

Example 23: Help run

Type this:

/usr/local/bin/check_uptime.py http://127.0.0.1/

You will see:

2026-10-28T12:11:00 200 11

What it means: Healthy line.

Example 24: argv

Type this:

/usr/local/bin/check_uptime.py http://127.0.0.1/docs/ || true

You will see:

2026-10-28T12:11:01 200 10

What it means: Path OK.

Step 9: Run healthy

Why: Expect 200. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: run. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 25: Healthy exit

Type this:

/usr/local/bin/check_uptime.py http://127.0.0.1/; echo exit:$?

You will see:

2026-10-28T12:11:02 200 10
exit:0

What it means: Pass.

Example 26: Log it

Type this:

/usr/local/bin/check_uptime.py http://127.0.0.1/ | sudo tee -a /var/log/uptime-check.log >/dev/null && tail -1 /var/log/uptime-check.log

You will see:

2026-10-28T12:11:03 200 9

What it means: Appended.

Example 27: grep 200

Type this:

grep ' 200 ' /var/log/uptime-check.log | wc -l

You will see:

What it means: History.

5. Harden and accept

Step 10: Force fail

Why: Stop Nginx briefly. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: systemctl stop. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 28: Force fail

Type this:

sudo systemctl stop nginx; /usr/local/bin/check_uptime.py http://127.0.0.1/; echo exit:$?; sudo systemctl start nginx

You will see:

2026-10-28T12:11:10 ERR 5 <urlopen error ...>
exit:1

What it means: Caught.

Example 29: Nginx back

Type this:

systemctl is-active nginx

You will see:

active

What it means: Restored.

Example 30: Healthy again

Type this:

/usr/local/bin/check_uptime.py http://127.0.0.1/; echo exit:$?

You will see:

2026-10-28T12:11:12 200 8
exit:0

What it means: OK.

Step 11: Cron install

Why: crontab or cron.d. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: cron.d. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 31: cron.d

Type this:

printf '%s\n' '* * * * * root /usr/local/bin/check_uptime.py http://127.0.0.1/ >> /var/log/uptime-check.log 2>&1' | sudo tee /etc/cron.d/uptime-check >/dev/null && cat /etc/cron.d/uptime-check

You will see:

* * * * * root /usr/local/bin/check_uptime.py http://127.0.0.1/ >> /var/log/uptime-check.log 2>&1

What it means: Every minute.

Example 32: Mode

Type this:

sudo chmod 644 /etc/cron.d/uptime-check && stat -c '%a' /etc/cron.d/uptime-check

You will see:

What it means: Required.

Example 33: grep

Type this:

grep check_uptime /etc/cron.d/uptime-check

You will see:

* * * * * root /usr/local/bin/check_uptime.py http://127.0.0.1/ >> /var/log/uptime-check.log 2>&1

What it means: Present.

Step 12: Acceptance checklist

Why: Diya signs off when these pass. We do this before the next step so mistakes stay small and easy to undo. Read the output carefully; the numbers and names tell you if the machine is healthy.

What: All green. Follow the numbered examples below. Type each command, read the exact output, and match the meaning line before you move on. That is how real teams learn a tool.

Example 34: script ok

Type this:

test -x /usr/local/bin/check_uptime.py && echo script_ok

You will see:

script_ok

What it means: Executable.

Example 35: cron ok

Type this:

test -f /etc/cron.d/uptime-check && echo cron_ok

You will see:

cron_ok

What it means: Scheduled.

Example 36: Sign-off

Type this:

echo 'Diya: PASS'

You will see:

Diya: PASS

What it means: Done.

Check yourself

Q1. Healthy HTTP code?

200

Q2. Exit code on failure?

Q3. Who wrote Practo checker?

Diya

Q4. Slow threshold Diya used?

800 ms

Q5. Library without pip?

urllib

Q6. Cron period?

Every minute

Friends, you built a Python uptime checker with cron. Did you understand? Let us try it once more. Point it at a practice URL and force a fail by stopping Nginx briefly.

Lab: try at home

Write check_uptime.py.
Run against http://127.0.0.1/.
Break the site and see exit 1.
Add cron every minute.
Keep a week of logs.

Recap

urllib can avoid extra deps.
Log status and latency.
exit 1 fails the cron loudly.
Thresholds catch slow pages.
Cron every minute is enough for many sites.

Lab

Finish all ten projects on one Ubuntu practice box.
Keep each project script under /usr/local/bin with mode 755.
Never store secrets in the script body.
After each project, write one paragraph: what broke and how you fixed it.
Reboot once and confirm cron, Nginx and fail2ban still run.

Practice task

Pick the disk alert and the uptime checker. Wire both to the same on-call practice mailbox and prove both fire.

Ravindra Bagale's Tip

Do the projects in order on one practice Ubuntu box. Keep a notebook of the commands that worked. Never use world-writable modes.

In short

Named users and SSH keys beat shared root passwords.
df -P and logrotate keep disks healthy.
Backups need verify, not only upload.
One Nginx can host many sites and balance many apps.
TLS and fail2ban are baseline, not optional.