File-not-found produces specific error

A raw ENOENT: no such file or directory, open 'path/to/file' stack trace tells the user the OS error code and the internal call stack — neither of which tells them what they should actually do. CWE-391 (Unchecked Error Condition) applies: the filesystem error is surfaced but not handled into a user-actionable message. When a CLI accepts user-specified file paths as arguments, it must own the responsibility of validating those paths before processing begins. Failing mid-processing — after partial output has already been written — is worse than failing fast at the boundary.

Validate file existence at argument parse time, before any processing starts:

import { existsSync } from 'fs'

function validateFile(filePath: string): void {
  if (!existsSync(filePath)) {
    console.error(`Error: File not found: ${filePath}`)
    console.error('Check the file path and try again.')
    process.exit(2)
  }
}

// call before any processing
validateFile(opts.input)

In click, use click.Path(exists=True) to get automatic validation with a clean error:

@cli.command()
@click.argument('file', type=click.Path(exists=True))
def lint(file):
    """Lint the specified FILE."""
    # click errors before reaching here: "Error: Path 'missing.js' does not exist."
    pass

The error message must name the specific path that was not found — generic "file not found" messages that omit the path force users to guess which argument was wrong.