PY-L7-36 · Scheduling: cron & Task Scheduler

Learning Goals

3 min

By the end of this lesson you can:

Read and write cron schedule syntax (the five fields).
Register a job with crontab (Linux/macOS) or Task Scheduler (Windows).
Write scripts that run correctly under a scheduler (absolute paths, env, logging).
Choose between schedule and the OS scheduler for a given need.

Warm-Up · Who Keeps Your Script Alive?

5 min

Last lesson's schedule loop only fires while that Python process runs. Close the terminal, reboot the machine, hit a crash — and your 8am report silently stops. Who's watching?

Today's big idea

The OS already has a rock-solid, always-running scheduler: cron on Linux/macOS, Task Scheduler on Windows. You register "run this command at this time" once, and the OS launches a fresh run of your script on schedule — surviving reboots and crashes. The shift in thinking: your script becomes a one-shot again (do the work, exit), and the OS handles the repetition.

New Concept · cron & Task Scheduler

14 min

cron syntax: five fields

┌─ minute (0-59)
│ ┌─ hour (0-23)
│ │ ┌─ day of month (1-31)
│ │ │ ┌─ month (1-12)
│ │ │ │ ┌─ day of week (0-6, Sun=0)
│ │ │ │ │
* * * * *  command-to-run

0 8 * * *      every day at 08:00
*/10 * * * *   every 10 minutes
0 2 * * 0      Sundays at 02:00
30 9 1 * *     09:30 on the 1st of each month

* = "every," */n = "every n," lists (1,15) and ranges (9-17) work too. The site crontab.guru is invaluable for reading/writing these.

Adding a cron job (Linux/macOS)

crontab -e            # opens your crontab in an editor

# add a line — note the ABSOLUTE paths:
0 8 * * * /usr/bin/python3 /home/aisha/report.py >> /home/aisha/report.log 2>&1

crontab -l            # list your jobs

Use the absolute path to python and your script — cron has a minimal environment, not your shell's.
>> logfile 2>&1 appends both stdout and stderr to a log — essential, since cron's output otherwise vanishes (or emails root).

Task Scheduler (Windows)

# via PowerShell, register a daily 8am task:
$action  = New-ScheduledTaskAction -Execute "python" `
             -Argument "C:\Users\aisha\report.py"
$trigger = New-ScheduledTaskTrigger -Daily -At 8am
Register-ScheduledTask -TaskName "DailyReport" `
             -Action $action -Trigger $trigger

Or use the Task Scheduler GUI: Create Task → Trigger (Daily 8am) → Action (Start a program: python, arguments: your script's full path), set "Start in" to the script's folder.

Writing scheduler-friendly scripts

⚠️ The classic gotchas (and fixes)

Working directory differs — relative paths break. Anchor with Path(__file__).resolve().parent (Lesson 8).
Minimal environment — your .env / PATH may be absent. Use absolute python, load .env explicitly, set vars in the cron line if needed.
No terminal — print goes nowhere useful. Log to a file (Lesson 14) with absolute paths.
Runs as a different user — permissions and home folder differ from your interactive session.

Worked Example · A Cron-Ready Script

12 min

Goal: take the report generator and make it bullet-proof under a scheduler — absolute paths, explicit env loading, file logging, and a clear exit code.

#!/usr/bin/env python3
"""report_cron.py — designed to be launched by cron / Task Scheduler."""
import sys, logging
from pathlib import Path
from logging.handlers import RotatingFileHandler
from datetime import datetime
from dotenv import load_dotenv

# 1) anchor everything to THIS file's location, not the cwd
HERE = Path(__file__).resolve().parent

# 2) load env explicitly from a known path
load_dotenv(HERE / ".env")

# 3) log to a FILE with an absolute path (cron has no terminal)
log = logging.getLogger("report")
log.setLevel(logging.INFO)
handler = RotatingFileHandler(HERE / "logs" / "report.log",
                              maxBytes=1_000_000, backupCount=5)
handler.setFormatter(logging.Formatter(
    "%(asctime)s %(levelname)s %(message)s"))
(HERE / "logs").mkdir(exist_ok=True)
log.addHandler(handler)

def main() -> int:
    log.info("=== run started %s ===", datetime.now().isoformat())
    try:
        data_file = HERE / "data" / "today.csv"      # absolute, not relative
        if not data_file.exists():
            log.error("no data file: %s", data_file)
            return 1
        # …generate the report (Lesson 22)…
        log.info("report generated successfully")
        return 0
    except Exception:
        log.exception("run failed")
        return 1

if __name__ == "__main__":
    sys.exit(main())        # exit code: 0 ok, non-zero failure

# crontab line that runs it daily at 8am, logging output too:
0 8 * * * /usr/bin/python3 /home/aisha/app/report_cron.py >> /home/aisha/app/cron.log 2>&1

# logs/report.log then accumulates:
2026-05-28 08:00:01 INFO === run started 2026-05-28T08:00:01 ===
2026-05-28 08:00:03 INFO report generated successfully

Read the code

Every scheduler gotcha is pre-empted: HERE anchors all paths to the script's own folder (so the cwd doesn't matter), load_dotenv(HERE / ".env") loads secrets from a known location, logging goes to a rotating file (the terminal is gone), and sys.exit(main()) returns a proper exit code the scheduler can act on. The script does its work once and exits — the OS handles the "every day." This is the difference between a script that works when you run it and one that works at 8am while you sleep.

Try It Yourself

13 min

01 🟢 Read cron lines

Translate these to English: 0 0 * * *, */15 9-17 * * 1-5, 0 0 1 1 *. Then write cron lines for "every Sunday at 23:00" and "every 5 minutes during business hours on weekdays." (Check with crontab.guru.)

02 🟡 Schedule a real job

Write a tiny script that appends the current timestamp to a log file (absolute path!), then schedule it to run every minute via cron or Task Scheduler. Watch the log grow, then remove the job.

Hint

# tick.py
from pathlib import Path
from datetime import datetime
HERE = Path(__file__).resolve().parent
with open(HERE / "tick.log", "a", encoding="utf-8") as f:
    f.write(datetime.now().isoformat() + "\n")

# crontab line:  * * * * * /usr/bin/python3 /full/path/tick.py

03 🔴 Diagnose a silent failure

Deliberately schedule a script that uses a relative path (e.g. open("data.csv")) and watch it fail silently under cron. Then fix it with Path(__file__).parent and confirm it works. Document what went wrong — this is the #1 real-world cron bug.

Mini-Challenge · The Install Helper

8 min

Write a Python helper that generates the correct scheduler command for the current OS: on Linux/macOS it prints the crontab line (with absolute python + script paths), on Windows it prints the PowerShell Register-ScheduledTask command. Use sys.executable and Path(__file__).resolve() so the paths are always right.

Show a sample solution

import sys
from pathlib import Path

def install_command(script: str, time_hhmm: str = "08:00") -> str:
    py = sys.executable                       # the exact python running this
    script_path = str(Path(script).resolve())
    hh, mm = time_hhmm.split(":")
    if sys.platform.startswith("win"):
        return (f'$a = New-ScheduledTaskAction -Execute "{py}" '
                f'-Argument "{script_path}"\n'
                f'$t = New-ScheduledTaskTrigger -Daily -At {time_hhmm}\n'
                f'Register-ScheduledTask -TaskName "MyJob" -Action $a -Trigger $t')
    else:
        log = str(Path(script_path).with_suffix(".log"))
        return f'{mm} {hh} * * * "{py}" "{script_path}" >> "{log}" 2>&1'

print("Add this to your scheduler:\n")
print(install_command("report_cron.py", "08:00"))

Non-negotiables: OS detection, absolute python (sys.executable) + script paths, correct cron/Task-Scheduler syntax.

Recap

3 min

For jobs that must survive reboots and crashes, register them with the OS scheduler — cron (five fields: min hour day month weekday) on Linux/macOS, Task Scheduler on Windows. Your script becomes a one-shot the OS launches on schedule. Make it scheduler-friendly: absolute paths (anchor to Path(__file__).parent), explicit .env loading, file logging (no terminal), and a proper sys.exit code. Use schedule for in-process/dev scheduling; use the OS scheduler for production reliability. The relative-path-under-cron failure is the classic bug — pre-empt it.

Vocabulary Card

cron: The Unix time-based job scheduler; jobs live in a crontab.
crontab: The file/list of cron jobs; edit with crontab -e.
Task Scheduler: Windows' built-in scheduler for running programs at set times.
cron environment: The minimal env cron jobs run in — why absolute paths matter.

Homework

4 min

Take one automation you've built and make it fully "cron-ready": absolute paths, explicit env loading, file logging, exit codes. Schedule it on your machine (every few minutes for testing), confirm it runs and logs correctly when launched by the scheduler (not by you), then change it to a sensible real cadence. Write down the exact scheduler line/command you used and one gotcha you hit.

Sample · what a complete submission shows

Script: report_cron.py — paths anchored to Path(__file__).parent,
        load_dotenv(HERE/".env"), RotatingFileHandler to HERE/logs,
        sys.exit(main()) returning 0/1.

Scheduler line (Linux):
  */5 * * * * /usr/bin/python3 /home/me/app/report_cron.py >> /home/me/app/cron.log 2>&1

Gotcha hit: first attempt used open("data/today.csv") (relative)
and produced an empty log + FileNotFoundError, because cron's
working directory was / not my project. Fixed with HERE / "data" /
"today.csv". Then switched cadence from */5 to '0 8 * * *'.

Non-negotiables: a cron-ready script, an actual scheduled run that logs to a file, the exact line used, and a documented gotcha.

┌─ minute (0-59) │ ┌─ hour (0-23) │ │ ┌─ day of month (1-31) │ │ │ ┌─ month (1-12) │ │ │ │ ┌─ day of week (0-6, Sun=0) │ │ │ │ │ * * * * * command-to-run 0 8 * * * every day at 08:00 */10 * * * * every 10 minutes 0 2 * * 0 Sundays at 02:00 30 9 1 * * 09:30 on the 1st of each month

crontab -e # opens your crontab in an editor # add a line — note the ABSOLUTE paths: 0 8 * * * /usr/bin/python3 /home/aisha/report.py >> /home/aisha/report.log 2>&1 crontab -l # list your jobs

# via PowerShell, register a daily 8am task: $action = New-ScheduledTaskAction -Execute "python" ` -Argument "C:\Users\aisha\report.py" $trigger = New-ScheduledTaskTrigger -Daily -At 8am Register-ScheduledTask -TaskName "DailyReport" ` -Action $action -Trigger $trigger

#!/usr/bin/env python3 """report_cron.py — designed to be launched by cron / Task Scheduler.""" import sys, logging from pathlib import Path from logging.handlers import RotatingFileHandler from datetime import datetime from dotenv import load_dotenv # 1) anchor everything to THIS file's location, not the cwd HERE = Path(__file__).resolve().parent # 2) load env explicitly from a known path load_dotenv(HERE / ".env") # 3) log to a FILE with an absolute path (cron has no terminal) log = logging.getLogger("report") log.setLevel(logging.INFO) handler = RotatingFileHandler(HERE / "logs" / "report.log", maxBytes=1_000_000, backupCount=5) handler.setFormatter(logging.Formatter( "%(asctime)s %(levelname)s %(message)s")) (HERE / "logs").mkdir(exist_ok=True) log.addHandler(handler) def main() -> int: log.info("=== run started %s ===", datetime.now().isoformat()) try: data_file = HERE / "data" / "today.csv" # absolute, not relative if not data_file.exists(): log.error("no data file: %s", data_file) return 1 # …generate the report (Lesson 22)… log.info("report generated successfully") return 0 except Exception: log.exception("run failed") return 1 if __name__ == "__main__": sys.exit(main()) # exit code: 0 ok, non-zero failure

# crontab line that runs it daily at 8am, logging output too: 0 8 * * * /usr/bin/python3 /home/aisha/app/report_cron.py >> /home/aisha/app/cron.log 2>&1 # logs/report.log then accumulates: 2026-05-28 08:00:01 INFO === run started 2026-05-28T08:00:01 === 2026-05-28 08:00:03 INFO report generated successfully

# tick.py from pathlib import Path from datetime import datetime HERE = Path(__file__).resolve().parent with open(HERE / "tick.log", "a", encoding="utf-8") as f: f.write(datetime.now().isoformat() + "\n") # crontab line: * * * * * /usr/bin/python3 /full/path/tick.py

import sys from pathlib import Path def install_command(script: str, time_hhmm: str = "08:00") -> str: py = sys.executable # the exact python running this script_path = str(Path(script).resolve()) hh, mm = time_hhmm.split(":") if sys.platform.startswith("win"): return (f'$a = New-ScheduledTaskAction -Execute "{py}" ' f'-Argument "{script_path}"\n' f'$t = New-ScheduledTaskTrigger -Daily -At {time_hhmm}\n' f'Register-ScheduledTask -TaskName "MyJob" -Action $a -Trigger $t') else: log = str(Path(script_path).with_suffix(".log")) return f'{mm} {hh} * * * "{py}" "{script_path}" >> "{log}" 2>&1' print("Add this to your scheduler:\n") print(install_command("report_cron.py", "08:00"))