Using a variable in Fortran's SYSTEM subroutine

Question

How do I use a variable in the command executed in a system subroutine call? For example, if I want to create multiple directories like test_1_1,

Answer 1

character (len=8) :: test_name

do i=1, 3
   do j=1, 3
      write (test_name, '( "test_", I1, "_", I1 )' ) i, j
      call system ( "mkdir " // test_name )
   end do
end do

The format in my example will work as long as the numbers are single digits. If you want larger values you could use I2.2 (for up to two digits, with leading zero, if single digits), or I0, for whatever number of digits are needed.