// node mut.mjs <file> <from> <to>: replace exactly one occurrence, else exit 3.
import { readFileSync, writeFileSync } from 'node:fs';
const [f, a, b] = process.argv.slice(2);
const s = readFileSync(f, 'utf8'); const n = s.split(a).length - 1;
if (n !== 1) { console.error(`matches=${n}`); process.exit(3); }
writeFileSync(f, s.replace(a, () => b));