How do you compile the most basic fortran program (helloworld) using gfortran an cygwin?

Ok, step by step Cygwin Fortran(90) Hello ;-)

First in which package is the GNU Fortran compiler ? We ask the Cygwin server with cygcheck -p

$ cygcheck -p bin/gfortran Found 8 matches for bin/gfortran ... gcc-fortran-10.2.0-1 - gcc-fortran: GNU Compiler Collection (Fortran) gcc-fortran-7.4.0-1 - gcc-fortran: GNU Compiler Collection (Fortran) gcc-fortran-9.3.0-1 - gcc-fortran: GNU Compiler Collection (Fortran) gcc-fortran-9.3.0-2 - gcc-fortran: GNU Compiler Collection (Fortran) 

So after you install the latest gcc-fortran package we have

$ cygcheck -c gcc-fortran Cygwin Package Information Package Version Status gcc-fortran 10.2.0-1 OK $ cygcheck -l gcc-fortran |grep bin /usr/bin/f95 /usr/bin/gfortran.exe /usr/bin/x86_64-pc-cygwin-gfortran.exe 

so the compiler you are looking for is named gfortran

As you have written your Fortran Hello program we can check its format
You need to save it as Unix LF format, not Windows CRLF one. You can use d2u to convert if needed.

$ file hello.f90 hello.f90: ASCII text $ cat hello.f90 program hello implicit none write(*,*) 'Hello world!' end program hello 

and we can now compile and run it

$ gfortran hello.f90 -o hello -Wall $ ./hello.exe Hello world!