functionexpand_vars

function expand_vars(path) result(out)
    character(len=*), intent(in) :: path    !! the text
    character(len=:), allocatable :: out, name, value
    character(len=*), parameter :: NAME_CHARS = 'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_'
    integer :: i, j, n, stat
    logical :: braced

    out = ''
    i = 1
    do while (i <= len(path))
        if (path(i:i) /= '$') then
            out = out//path(i:i)
            i = i + 1
            cycle
        end if
        braced = .false.
        if (i < len(path)) braced = path(i + 1:i + 1) == '{'
        if (braced) then
            j = index(path(i + 2:), '}')
            if (j > 0) j = i + 1 + j
        else
            j = verify(path(i + 1:), NAME_CHARS)
            j = merge(len(path), i + j - 1, j == 0)
        end if
        if (j == 0 .or. j == i) then
            out = out//'$'
            i = i + 1
            cycle
        end if
        if (braced) then
            name = path(i + 2:j - 1)
        else
            name = path(i + 1:j)
        end if
        call get_environment_variable(name, length=n, status=stat)
        if (stat == 0) then
            allocate (character(len=n) :: value)
            call get_environment_variable(name, value)
            out = out//value
            deallocate (value)
        else
            out = out//path(i:j)
        end if
        i = j + 1
    end do
end function expand_vars