Command Line Arguments in Fortran

Command-line arguments are a common way to parameterize execution of programs. For example, ./my_program arg1 arg2 uses arg1 and arg2 as arguments to the my_program executable.

program command_line_arguments
  use, intrinsic :: iso_fortran_env, only: output_unit
  implicit none
  
  integer :: num_args, i
  character(len=100), dimension(:), allocatable :: args
  
  ! Get the number of command-line arguments
  num_args = command_argument_count()
  
  ! Allocate an array to store the arguments
  allocate(args(0:num_args))
  
  ! Get the program name (equivalent to args[0] in some languages)
  call get_command_argument(0, args(0))
  
  ! Get the command-line arguments
  do i = 1, num_args
    call get_command_argument(i, args(i))
  end do
  
  ! Print all arguments including the program name
  write(output_unit,*) "All arguments (including program name):", args
  
  ! Print arguments without the program name
  write(output_unit,*) "Arguments without program name:", args(1:)
  
  ! Print a specific argument (if available)
  if (num_args >= 3) then
    write(output_unit,*) "Third argument:", args(3)
  end if
  
  ! Deallocate the array
  deallocate(args)
end program command_line_arguments

In Fortran, we use the intrinsic procedures command_argument_count() and get_command_argument() to access command-line arguments. The first argument (index 0) is typically the program name, similar to other languages.

To experiment with command-line arguments, it’s best to compile the program first:

$ gfortran command_line_arguments.f90 -o command_line_arguments
$ ./command_line_arguments a b c d
 All arguments (including program name):./command_line_arguments a b c d
 Arguments without program name:a b c d
 Third argument:c

In this example, we’ve demonstrated how to access all arguments, arguments without the program name, and how to access a specific argument by index.

Next, we’ll look at more advanced command-line processing with options parsing libraries available in Fortran.

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Command Line Arguments in Fortran