2.10.2.1 Using PrologScript

A Prolog source file can be used directly as a Unix program using the Unix #! magic start. The same mechanism is useful for specifying additional parameters for running a Prolog file on Windows. The Unix #! magic is allowed because if the first letter of a Prolog file is #, the first line is treated as a comment.^{10The #-sign can be the legal start of a normal Prolog clause. In the unlikely case this is required, leave the first line blank or add a header comment.} To create a Prolog script, make the first line start like this:

#!/path/to/swipl <options> -s

Prolog recognises this starting sequence and causes the interpreter to receive the following argument list:

/path/to/swipl <options> -s <script> -- <ScriptArguments>

Instead of -s, the user may use -f to stop Prolog from looking for a personal initialisation file.

Here is a simple script doing expression evaluation:

#!/usr/bin/swipl -q -t main -f

eval :-
        current_prolog_flag(argv, Argv),
        append(_, [--|Args], Argv),
        concat_atom(Args, ' ', SingleArg),
        term_to_atom(Term, SingleArg),
        Val is Term,
        format('~w~n', [Val]).

main :-
        catch(eval, E, (print_message(error, E), fail)),
        halt.
main :-
        halt(1).

And here are two example runs:

% eval 1+2
3
% eval foo
ERROR: Arithmetic: `foo/0' is not a function
%

The Windows version supports the #! construct too, but here it serves a rather different role. The Windows shell already allows the user to start Prolog source files directly through the Windows file-type association. However, Windows makes it rather complicated to provide additional parameters for opening an individual Prolog file. If the file starts with #!, the first line is analysed to obtain additional command line arguments. The example below runs the system in `quiet' mode.

#!/usr/bin/swipl -q -s

....

Note the use of /usr/bin/swipl, which specifies the interpreter. This argument is ignored in the Windows version, but must be present to ensure best cross-platform compatibility.