Improve HAFS scripts to use prep_step to unset FORTxx

[BinLiu-NOAA]

Description

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From NCO SPA: Improve the HAFS scripts to use prep_step to unset FORTxx at the next upgrade.

Refer to WCOSS implementation standards, page 7 -

• prep_step unsets the FORT## variables used to pass unit assignments to Intel FORTRAN executables. Since there may be multiple FORTRAN programs running in a job, these variables must be reset before each program execution.

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• fixed by hafs-community/hafs/pull/<pr_number>

[BinLiu-NOAA]

According to the Implementation Standard, there are mainly two methods for Fortran programs to use FORT unit files. One is through the export FORTn variable, the other approach is through the symbolic link method. The following two items are adapted from the Implementation Standard:

• (An ifort extension) by omitting the FILE= option, in which case the file name must be set by exporting the value of the character FORTn variable, where n is the Fortran I/O unit number as set in the OPEN statement. For ifort, n is any positive integer fitting in a 4-byte variable. The production utility "prep_step" (clearing the values of all FORTn variables) must be called before each executable if this method is used.
• By omitting the FILE=var option, and not setting the FORTn variable, in which case the default file name “fort.n” will be used by the executable. This method is allowed only if this file is a symbolic link, eg: ln -sf $DATA/pgrbf01 fort.11.

In terms of the HAFS application/workflow, I believe we always use the second approach through symbolic links. And we do not export any FORTn variables, for which, my understanding is that prep_step utility is not required in this case.