This environment variable specifies the separator when writing list-directed output. It may contain any number of spaces and at most one comma. If you specify this on the command line, be sure to quote spaces, as in
$ GFORTRAN_LIST_SEPARATOR=' , ' ./a.out
when a.out is the compiled Fortran program that you want to run. Default is a single space.
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https://gcc.gnu.org/onlinedocs/gcc-7.2.0/gfortran/GFORTRAN_005fLIST_005fSEPARATOR.html