Command-line arguments

This is a short tutorial for those who are writing their first OCaml program and are wondering how to read arguments that are passed on the command line.

argv

Like in the C programming language, the arguments that are passed to a given program are stored in an array. Following the tradition, this array is named argv. It is found in the Sys module of the standard library, therefore its full name is Sys.argv. The number of arguments including the name of the program itself is simply the length of the array. It is obtained using the Array.length function.

Simple example

The following program displays the arguments with their position in Sys.argv:

open Printf
  
let () =
  for i = 0 to Array.length Sys.argv - 1 do
    printf "[%i] %s\n" i Sys.argv.(i)
  done

If you save the program above as args.ml, and run ocaml args.ml arg1 arg2 arg3, here is what you get:

[0] args.ml
[1] arg1
[2] arg2
[3] arg3

Note that ocaml launched a subprocess that actually runs the program where argv is args.ml arg1 arg2 arg3. You can also compile your program using ocamlopt -o args args.ml, and then run ./args arg1 arg2 arg3 and you will get:

[0] ./args
[1] arg1
[2] arg2
[3] arg3

Tools for parsing command-line options

A few libraries exist that let you process command-line arguments without having to scan the Sys.argv array yourself:

• Arg is a module of the standard library.
• Getopt for OCaml is similar to GNU getopt.
• The Pa_arg library offers a convenient syntax for defining and reading options.