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The Java-based Bagger program has a built-in validation tool. To use it, click "Open Existing Bag" from the top menu, and select a bag in the window that appears. It may take some time for larger bags to open. Once the bag has been opened, click "Validate Bag" on the top menu to begin the process of validating the manifest files. Note that opening a bag allows you to edit the contents of its bag-info.txt file, so be careful not to make any changes!
You can also use the bag_validator Python script to validate a bag or set of bags. By default, the script will validate all bags in /dps/write_to_tape, but you can specify a specific bag to be validated with the -i option followed by a path to a bag, or a list of bags with the -f option followed by a path to a text file that lists the location of many bags. Adding the -r option will validate all bags in all subdirectories of a given folder.