Accept Input from Command Line in Bnlang

There are two layers for reading command-line arguments in Bnlang:

  • sys.arg(i) — the raw, low-level accessor. Good for one or two positional inputs.
  • cli module — a declarative parser for flags, options, and positional arguments. Good when you have anything more structured.

Raw Access with sys.arg

sys.argc() returns how many arguments the script received; sys.arg(i) returns the i-th one (0-indexed). The script path itself is not counted.

// file: greet.bnl
import "sys" as sys;

var name = "Guest";
if (sys.argc() > 0) { name = sys.arg(0); }

print("Hello,", name);

bnl greet.bnl Alice
# Output: Hello, Alice

Structured Parsing with the cli Library

For anything past one or two positional inputs, declare your CLI shape with the cli library. You get flags, options-with-values, positional arguments, and an auto-generated --help string.

// file: greet.bnl
import "cli" as cli;

var p = cli.parser("greet");
p.description = "Print a greeting.";
cli.flag    (p, {name: "shout",  short: "s", help: "Use upper case"});
cli.option  (p, {name: "lang",   short: "l", help: "en or bn", default: "en"});
cli.positional(p, {name: "name", help: "Who to greet"});

var args = cli.parse(p);

var msg = "Hello, ";
if (args.lang == "bn") { msg = "হ্যালো, "; }
msg = msg + args.name + "!";

if (args.shout) { msg = msg.to_upper(); }
print(msg);

bnl greet.bnl Alice
# Hello, Alice!

bnl greet.bnl -l bn আলিস
# হ্যালো, আলিস!

bnl greet.bnl -s Alice
# HELLO, ALICE!

Showing Help

cli.usage(p) returns the auto-generated help text. Define your own --help flag and check it after parsing:

cli.flag(p, {name: "help", short: "h", help: "Show usage"});
var args = cli.parse(p);
if (args.help) {
    print(cli.usage(p));
    return;
}

Best Practices

  • Reach for cli as soon as you have more than two inputs or any optional flags. It's much easier to maintain than handwritten parsing.
  • Provide sensible defaults so the script does something useful with no arguments.
  • Print a usage hint and exit with a non-zero code on bad input.