Scheduling Scripts on Linux/Mac (Cron Jobs)

diagram why a script that works in your shell fails under cron mermaid

  flowchart LR
  A["works in your terminal"] --> B["cron runs it"]
  B --> C{"fails?"}
  C --> D["PATH is minimal -- 'python' may not resolve"]
  C --> E["cwd is HOME, not the project -- relative paths break"]
  C --> F["no virtualenv activated -- imports fail"]
  D --> G["use an absolute interpreter path: /path/venv/bin/python"]
  E --> H["cd in the command, or use absolute paths everywhere"]
  F --> G
  G --> I["redirect output to a log -- cron mail is easy to miss"]
  H --> I

Cron does not run your shell. It starts the job with a minimal environment, a different working directory and no virtualenv, so anything that relied on those is missing. Almost every 'it works when I run it manually' report comes from one of the three boxes on the right.

Cron basics

Cron runs commands on a schedule.

Common gotchas:

minimal PATH
different working directory

Pattern

use absolute paths
redirect outputs

Example crontab entry:

text

0 9 * * * /usr/bin/python3 /home/user/job.py >> /home/user/job.log 2>&1

Tip

Use logging in the script, not only stdout.

🧪 Try It Yourself

Exercise 1 – List Files with os.listdir

Exercise 2 – Join Paths with os.path.join

Exercise 3 – Write and Read a File

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