A ENVIRONMENT VARIABLES AND TOKENS

A.1 Summary of Environment Variables Used

The following list summarises the environment variables used by the NDF library to control access to foreign format data. The symbols $<$FMT$>$ and $<$XTN$>$ are used to represent the names of foreign data formats and NDF extensions respectively. All environment variable names should be entirely in upper case.

NDF_FORMATS_IN

Comma-separated list of format names and associated file extensions, used when accessing pre-existing datasets. (Mandatory if pre-existing foreign datasets are to be accessed.)

NDF_FORMATS_OUT

Comma-separated list of format names and associated file extensions, used when accessing new datasets. (Mandatory if new foreign datasets are to be accessed other than by propagation of format information from a pre-existing dataset.)

NDF_FROM_$<$FMT$>$

Command to convert a foreign format dataset into an NDF. (Mandatory if datasets in that format are to be read.)

NDF_TO_$<$FMT$>$

Command to convert an NDF into a foreign format dataset. (Mandatory if datasets in that format are to be written or modified.)

NDF_DEL_$<$FMT$>$

Command to delete a foreign format dataset. (Optional.)

NDF_KEEP_$<$FMT$>$

Name of the NDF to be used to hold a native format copy of a foreign dataset in cases where the NDF is to be retained for future use. (Optional.)

NDF_TEMP_$<$FMT$>$

Name of the NDF to be used to hold a native format copy of a foreign dataset in cases where the NDF is temporary and will be deleted when the dataset is released. (Optional.)

NDF_XTN

Comma-separated list of NDF extensions to be recognised when accessing foreign datasets. (Optionally required if extension information is to be handled.)

NDF_XTN_$<$XTN$>$

Comma-separated list of NDF extensions to be recognised when accessing datasets in a particular foreign format. (Optionally overrides the value of NDF_XTN for that format.)

NDF_IMP_$<$XTN$>$

Command to import information into an NDF extension. (Optionally required to allow reading of extension information from foreign datasets.)

NDF_IMP_$<$XTN$>$_$<$FMT$>$

Command to import information into an NDF extension when accessing datasets in a particular foreign format. (Optionally overrides the value of NDF_IMP_$<$XTN$>$ for that format.)

NDF_EXP_$<$XTN$>$

Command to export information from an NDF extension. (Optionally required to allow writing or modification of extension information in foreign datasets.)

NDF_EXP_$<$XTN$>$_$<$FMT$>$

Command to export information from an NDF extension when accessing datasets in a particular foreign format. (Optionally overrides the value of NDF_EXP_$<$XTN$>$ for that format.)

A.2 Summary of Message Tokens Used

The following table summarises all the message tokens used by the NDF library in format conversion commands, etc:

Each environment variable which is used to control access to foreign format data, and which permits substitution of the above message tokens, is of the form:

where $<$CTX$>$ is the name of a “context” within which message token substitution takes place. The following table shows which tokens are defined for use within each context: