Improving CLIs with isatty

June 11, 2019

One thing I like to do to improve the command-line programs I maintain is to make them aware of whether they’re being run interactively. In this post I’ll show off an easy trick to make programs running interactively more usable.

This always used to trip me up when I was first learning to use the terminal:

❯ grep 'def foo'

I’d drop this into the command-line and what happens? It hangs… Is it because it’s taking a long time to search? Nope—I’ve forgetten to tell grep what files to search in!

When grep is given only a pattern to search for and no files to search in, it assumes we want to search for that pattern on stdin. This is great for shell scripts and one-liners at the command-line, but it’s super annoying when we’re just grepping interactively.

The thing is, it’s super easy to detect when the user might have made this mistake: if we’re defaulting to reading from stdin and the file corresponding to stdin represents a terminal (more specifically, a tty). And once we’ve detected it, we can print a helpful message.

Here’s how I did it when writing diff-locs, one of the command-line programs I’ve been working on lately:

Check if stdin is a tty in Haskell
fileIn <- case inputStyle of
  InputFromFile filename -> IO.openFile filename IO.ReadMode
  InputFromStdin         -> do
    isTTY <- hIsTerminalDevice IO.stdin
    when isTTY $ do
      errPutStrLn "Warning: reading from stdin, which is a tty."
    return IO.stdin

If we’ve been given a file explicitly, just open it. Otherwise, fall back to reading from stdin. But first, check if IO.stdin is a terminal device and when it is, print a warning.1 The complete file containing the snippet above is on GitHub.

I’ve implemented diff-locs as a standard Unix filter—it takes input on stdin and emits output on stdout. Normal usage looks something like this, where we pipe git diff into diff-locs:

❯ git diff | diff-locs

But if someone is just playing around at the terminal (maybe, trying to get the help output to show up), they might run diff-locs without args, and then be greeted with this message:

❯ diff-locs
Warning: reading from stdin, which is a tty.

This is much better than just sitting there appearing to hang!

isatty in other languages

The trick above works in pretty much every language that supports Unix programming. Under the hood, the Haskell snippet above is powered by the isatty function in the C standard library (man 3 isatty), which most other languages wrap in some way. For example, three other languages I’ve done this in recently:

Ruby
if STDIN.isatty?
  STDERR.puts 'Warning: reading from stdin, which is a tty.'
end
Bash
if [ -t 0 ]; then
  echo 'Warning: reading from stdin, which is a tty.' >&2
end
OCaml
if Unix.isatty Unix.stdin
then prerr_endline "Warning: reading from stdin, which is a tty."
else ()

And again, a quick search for isatty <language> should suffice for any language that supports Unix programming. It’s little things like this that add up and make certain command-line utilities delightful to use.


  1. We don’t really need to check whether the file we’re opening is a tty. If the user managed to pass in the name of a tty file, they probably know what they’re doing.

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