Ro Backup
Ro is a multi-platform command-line application to automate backups of your Salesforce data.
Quick Start
Authenticate using OAuth, and back up all objects
$ ro -e login.salesforce.com
Authenticate with your Salesforce org using username and password, and back up all objects
$ ro -e login.salesforce.com -u username@org.example.com -p passwordToken
See Using Ro Backup for more examples and all of the available options.