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read can be used to filter or manipulate data already retrieved into a file, see read section 5.4.

write is mainly used to save data from fieldsets, a temporary storage, into files, see section 5.2 Fieldsets.

Fieldsets

A fieldset is a temporary storage for fields, either retrieved from MARS or read from a UNIX file. Fieldsets can be seen as variables in a MARS request, and they can be referenced in subsequent MARS requests in the same execution. This is an example how to retrieve data into a fieldset named analysis:

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