Cron and virtualenv

Getting cron to work within your virtualenv

When setting up a cron job within a virtualenv, you need to ensure the appropriate Python interpreter and libraries are being accessed. Here's a step-by-step guide to understanding and implementing this interaction.

Use the right Python interpreter

Instead of sourcing the activate script, directly reference the virtualenv’s Python executable in the cron job:

* * * * * /path/to/virtualenv/bin/python /path/to/script.py  # Like borrowing sugar from a specific neighbour's house!
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Direct referencing provides a more predictable environment, allowing your cron job to execute flawlessly.

Configuring the required environment variables

Cron jobs run with a limited environment, which can cause issues with path and variable recognition. Here are some ways to tackle these:

1. You can prepend the PATH with your virtualenv's bin directory:

   PATH=/path/to/virtualenv/bin:$PATH
   * * * * * python /path/to/script.py >> /path/to/logfile.log 2>&1  # Like bookmarking your favourite site
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2. You can also use the env command for a more familiar environment:

   * * * * * /usr/bin/env bash -c '/path/to/virtualenv/bin/python /path/to/script.py'  # env is the GPS for your command!
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Navigating Django projects

If you're running a Django-specific task, ensure that the PYTHONPATH includes your project and that the correct directory is in use:

* * * * * cd /path/to/django/project && /path/to/virtualenv/bin/python manage.py your_command # Like going home before starting a movie!
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The && ensures the directory is changed before the manage.py command is run.

Your logbook: Logging and troubleshooting

If your cron job talks, but there's nobody around to hear it, did it make a sound? Capture its output:

* * * * * /path/to/virtualenv/bin/python /path/to/script.py >> /path/to/logfile.log 2>&1  # WHY do we always forget a letter in paswords?
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Capturing the output resolves the no-terminal issue with cron jobs. Also, don't forget to check /var/log/syslog for cron messages and set up email notifications for errors.

Command trial: Testing before relying

Before you trust the cron to run your job, test run your commands manually from your shell to ensure their correctness. Tiny errors can stop your command straight in its tracks! Watch out!

Avada Kedavra! Er...I mean, Activate

If you prefer a magic wand for environment activation, use the activate_this.py script from your Python script:

activate_this = '/path/to/virtualenv/bin/activate_this.py'
with open(activate_this) as file_:
    exec(file_.read(), dict(__file__=activate_this))  # Who needs magic when you have Python!
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Best practices to follow

Keep these tips in your arsenal for better code:

• Update the manage.py shebang with your virtualenv's Python executable path.
• Set up email notifications for job failures by aliasing root.
• If logs aren't your thing, redirect job output to '/dev/null'.