How to Schedule Python Scripts with Cron: A Beginner's Complete Guide

The best automation script is one that runs itself.

Cron is the Unix scheduler built into every macOS and Linux machine. No SaaS, no cloud subscriptions, no third-party scheduler — just a text file that tells your system when to run your code.

Here's everything you need to use it correctly.

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The crontab syntax

┌─ minute       (0-59)
│  ┌─ hour      (0-23)
│  │  ┌─ day    (1-31)
│  │  │  ┌─ month  (1-12)
│  │  │  │  ┌─ weekday (0=Sun, 1=Mon ... 6=Sat)
│  │  │  │  │
*  *  *  *  *  command

Common patterns:

# Every day at 10am
0 10 * * * python3 /path/to/script.py

# Every hour
0 * * * * python3 /path/to/script.py

# Every 15 minutes
*/15 * * * * python3 /path/to/script.py

# Every Monday at 9am
0 9 * * 1 python3 /path/to/script.py

# First of every month at midnight
0 0 1 * * python3 /path/to/script.py

Use crontab.guru to verify any expression before running it.

Step 1: Open your crontab

crontab -e   # opens in your default editor
crontab -l   # list current jobs
crontab -r   # remove all jobs (careful!)

If this is your first time, you'll be asked to choose an editor. Choose nano for simplicity.

Step 2: Write your script correctly

Before scheduling, your script needs two things:

1. Absolute paths — not relative:

# Wrong — cron runs from a different directory
with open("state.json") as f:    # FileNotFoundError at runtime
    data = json.load(f)

# Right — always resolve relative to the script itself
import os
BASE_DIR = os.path.dirname(os.path.abspath(__file__))
STATE_FILE = os.path.join(BASE_DIR, "state.json")

with open(STATE_FILE) as f:
    data = json.load(f)

2. A shebang line so cron knows which Python to use:

#!/usr/bin/env python3
"""
publish_queue.py — publishes one article per day.
Cron: 0 10 * * * /usr/bin/python3 /Users/me/scripts/publish_queue.py
"""
import os, json, requests
# ...

Step 3: Log everything

Cron jobs run silently. Without logging, you won't know if your script ran, crashed, or succeeded.

#!/usr/bin/env python3
import logging
import os
from datetime import datetime

# Log next to the script file
BASE_DIR = os.path.dirname(os.path.abspath(__file__))
LOG_FILE = os.path.join(BASE_DIR, "publish.log")

logging.basicConfig(
    filename=LOG_FILE,
    level=logging.INFO,
    format="%(asctime)s %(levelname)s %(message)s",
    datefmt="%Y-%m-%d %H:%M:%S",
)
log = logging.getLogger(__name__)

def main():
    log.info("Script started")
    try:
        # ... your actual work ...
        log.info("Published: My Article Title")
    except Exception as e:
        log.error(f"Failed: {e}", exc_info=True)
        raise

if __name__ == "__main__":
    main()

Your crontab entry should also capture stdout/stderr:

0 10 * * * /usr/bin/python3 /Users/me/scripts/publish_queue.py >> /Users/me/scripts/publish.log 2>&1

The >> file 2>&1 part appends both stdout and stderr to your log file.

Step 4: Environment variables

Cron runs with a minimal environment — none of your shell aliases, PATH entries, or exported variables are available.

The problem:

import os
token = os.environ["DEVTO_TOKEN"]  # KeyError — cron doesn't have this

Solution 1: Use a .env file

# Load environment from a file at the top of your script
def load_env(path: str) -> None:
    """Load KEY=value pairs from a file into os.environ."""
    with open(path) as f:
        for line in f:
            line = line.strip()
            if line and not line.startswith("#") and "=" in line:
                key, _, value = line.partition("=")
                os.environ[key.strip()] = value.strip().strip('"').strip("'")

BASE_DIR = os.path.dirname(os.path.abspath(__file__))
load_env(os.path.join(BASE_DIR, ".env"))

token = os.environ["DEVTO_TOKEN"]  # works

Your .env file:

DEVTO_TOKEN=abc123yourtoken
GUMROAD_TOKEN=xyz789yourtoken

Solution 2: Set variables directly in crontab

# At the top of your crontab file
DEVTO_TOKEN=abc123yourtoken
GUMROAD_TOKEN=xyz789yourtoken

0 10 * * * python3 /path/to/publish_queue.py >> /path/to/publish.log 2>&1

Step 5: Fix the PATH problem

Cron's PATH is usually just /usr/bin:/bin. Commands like python3 may not be found if you installed Python via Homebrew or pyenv.

The fix: use the full path to python3.

# Find your python3 path in terminal:
which python3
# /usr/local/bin/python3  (Homebrew)
# /opt/homebrew/bin/python3  (Apple Silicon Homebrew)
# /usr/bin/python3  (system Python)

# Then use that full path in crontab:
0 10 * * * /opt/homebrew/bin/python3 /Users/me/scripts/publish.py >> /Users/me/scripts/pub.log 2>&1

Or set PATH explicitly at the top of your crontab:

PATH=/opt/homebrew/bin:/usr/local/bin:/usr/bin:/bin

0 10 * * * python3 /Users/me/scripts/publish.py >> /Users/me/scripts/pub.log 2>&1

Step 6: Virtual environments

If your script uses third-party packages (requests, pillow, etc.), you need to point cron at your venv's Python:

# Create and activate venv (one time)
python3 -m venv /Users/me/scripts/.venv
source /Users/me/scripts/.venv/bin/activate
pip install requests pillow

# In crontab — use the venv python directly
0 10 * * * /Users/me/scripts/.venv/bin/python /Users/me/scripts/publish.py >> /Users/me/scripts/pub.log 2>&1

No need to activate the venv in crontab — using the venv's python directly is equivalent.

Complete working example

The two cron jobs that power the publishing pipeline:

#!/usr/bin/env python3
"""
auto_publish_queue.py
Reads publish_queue.json and publishes the next pending article to Dev.to.

Cron: 0 10 * * * /path/to/.venv/bin/python /path/to/auto_publish_queue.py
"""
import os, json, re, logging, requests
from datetime import date

BASE_DIR = os.path.dirname(os.path.abspath(__file__))
QUEUE_FILE = os.path.join(BASE_DIR, "publish_queue.json")
LOG_FILE = os.path.join(BASE_DIR, "queue.log")

logging.basicConfig(
    filename=LOG_FILE,
    level=logging.INFO,
    format="%(asctime)s %(message)s",
)
log = logging.getLogger(__name__)

TOKEN = os.environ.get("DEVTO_TOKEN", "")

def load_queue():
    with open(QUEUE_FILE) as f:
        return json.load(f)

def save_queue(q):
    with open(QUEUE_FILE, "w") as f:
        json.dump(q, f, indent=2)

def publish(filepath):
    with open(os.path.join(BASE_DIR, filepath)) as f:
        content = f.read()
    match = re.match(r'^---\n(.*?)\n---\n', content, re.DOTALL)
    fm = {}
    for line in match.group(1).splitlines():
        if line.startswith('tags: '):
            fm['tags'] = [t.strip() for t in line[6:].split(',')]
        elif ': ' in line:
            k, _, v = line.partition(': ')
            fm[k.strip()] = v.strip().strip('"')
    headers = {"api-key": TOKEN, "Content-Type": "application/json"}
    resp = requests.post("https://dev.to/api/articles", headers=headers, json={
        "article": {
            "title": fm["title"],
            "body_markdown": content,
            "published": True,
            "tags": fm.get("tags", []),
            "description": fm.get("description", ""),
        }
    })
    resp.raise_for_status()
    return resp.json()

def main():
    q = load_queue()
    if not q["pending"]:
        log.info("Queue empty — nothing to publish")
        return

    item = q["pending"][0]
    log.info(f"Publishing: {item['title']}")

    result = publish(item["filename"])
    url = f"https://dev.to{result['path']}"
    log.info(f"Published: {url}")

    q["pending"].pop(0)
    q["published"].append({
        "filename": item["filename"],
        "title": item["title"],
        "date": str(date.today()),
        "url": url,
        "id": result["id"],
    })
    save_queue(q)

if __name__ == "__main__":
    main()

Crontab setup:

# Edit with: crontab -e
DEVTO_TOKEN=your_token_here

# 9am: ping RSS aggregators
0 9 * * * /path/to/.venv/bin/python /path/to/daily_ping.py >> /path/to/ping.log 2>&1

# 10am: publish next article from queue
0 10 * * * /path/to/.venv/bin/python /path/to/auto_publish_queue.py >> /path/to/queue.log 2>&1

Debugging checklist

When a cron job doesn't run or fails silently:

# 1. Check if cron is running (macOS)
sudo launchctl list | grep cron

# 2. Check system cron log (macOS)
log show --predicate 'process == "cron"' --last 1h

# 3. Check your log file
tail -f /path/to/your/script.log

# 4. Test the exact command cron will run
/path/to/python3 /path/to/script.py >> /tmp/test.log 2>&1
cat /tmp/test.log

# 5. Check permissions
ls -la /path/to/script.py  # must be readable
chmod +x /path/to/script.py

Common errors:

Error	Cause	Fix
Script never runs	Cron daemon not running	`sudo launchctl load /System/Library/LaunchDaemons/com.vix.cron.plist`
`python3: not found`	PATH too minimal	Use full path: `/usr/bin/python3` or `/opt/homebrew/bin/python3`
`ModuleNotFoundError`	Wrong Python (no venv)	Use venv's Python: `/path/.venv/bin/python`
`FileNotFoundError`	Relative path in script	Use `os.path.abspath(__file__)` to build absolute paths
`KeyError` on env var	Shell env not loaded	Set vars at top of crontab or load from `.env` file

My two cron jobs publish one article per day and ping RSS aggregators — same pattern, different scripts: germy5.gumroad.com/l/xhxkzz — pay what you want, min $9.99.

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