Introduction

If you’ve ever set up a cron job on an Ubuntu VPS, you may have faced issues that left you scratching your head. One common problem is when the cron job attempts to execute a script without success, leading to zero-byte output files or incomplete operations. In this blog post, we explore a real-world scenario where a Ruby script fails to back up a MySQL database via a cron job, leaving users puzzled as to why their command works perfectly in the command line but not when scheduled.

Understanding the Problem

In the case discussed, the cron job was set up to execute a Ruby script that performs the following tasks:

1. MySQL Database Backup: Uses mysqldump to back up the database specified in database.yml.
2. File Compression: The backup file gets gzipped to save space.
3. File Transfer: The zipped file is sent to a remote server using SFTP.

Despite the script performing well when run directly from the command line, the cron job resulted in an empty file being generated. Below is a simplified version of the cron job command that was causing the issue.

PATH=/usr/bin
10 3 * * * ruby /home/deploy/bin/datadump.rb

Why Does This Happen?

The core of the problem lies in the environment in which the cron job executes. When a script or command runs through cron, it does so in a restricted environment that may not always mimic the user’s terminal session. This can result in unexpected behaviors, especially when:

• Working Directory: Cron jobs may not run in the expected working directory.
• Environment Variables: Certain environment variables may not be available when a script runs via cron.

Solution

Step 1: Check the Working Directory

When your cron job executes, it does not run in the context of your typical user environment. Often, it may not have the same home directory or working directory. One way to ensure your script behaves the same way is to explicitly define the working directory.

How to Specify the Working Directory:

1. You can use the cd command at the beginning of your cron job:

10 3 * * * cd /home/deploy/bin && ruby datadump.rb

Step 2: Use Absolute Paths

Another issue may arise if your script generates files using relative paths. The cron job’s execution environment might not have the necessary permissions for creating files in the default directories. To resolve this:

• Use absolute paths for file operations within your script instead of relative paths.

For example, revise file generation:

dump       = "/home/deploy/backups/myapp-#{Time.now.strftime(TIMESTAMP)}.sql.gz"

Step 3: Set Correct Permissions

Verify that the user running the cron job (in this case, deploy) has the necessary permissions for:

• The output directory where the backup files are created.
• Access to any other files or directories the script interacts with.

Step 4: Log Output for Debugging

Utilizing logging in your scripts can provide valuable insights into what’s happening when cron runs the job. Make sure to log:

• Messages indicating the start and completion of tasks.
• Any errors encountered during execution.

You can also redirect standard output and error to a log file within your cron job command:

10 3 * * * ruby /home/deploy/bin/datadump.rb >> /home/deploy/log/cron.log 2>&1

Step 5: Environment Variables

Lastly, make sure any required environment variables are set correctly. The cron job will not inherit all the variables from your shell, which can lead to failures in script execution.

Conclusion

By checking the working directory, using absolute paths, ensuring proper permissions, logging outputs for easier debugging, and reviewing environment variables, you can troubleshoot and resolve issues with non-functioning cron jobs effectively.

Facing technical problems with cron jobs can be frustrating, but with systematic troubleshooting, you can restore the reliability of scheduled tasks on your system.