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:
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!
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:
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.
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
But if someone is just playing around at the terminal (maybe, trying to
get the help output to show up), they might run
args, and then be greeted with this message:
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:
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.
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.↩