An implementation of the unix "cat" program


  • Use the Deno runtime API to output the contents of a file to the console.
  • Deno.args accesses the command line arguments.
  • Deno.open is used to get a handle to a file.
  • Deno.copy is used to transfer data from the file to the output stream.
  • Files should be closed when you are finished with them
  • Modules can be run directly from remote URLs.


In this program each command-line argument is assumed to be a filename, the file is opened, and printed to stdout (e.g. the console).

 * cat.ts
for (let i = 0; i < Deno.args.length; i++) {
  const filename = Deno.args[i];
  const file = await Deno.open(filename);
  await Deno.copy(file, Deno.stdout);

To run the program:

deno run --allow-read https://deno.land/std@0.85.0/examples/cat.ts /etc/passwd