iny — is INPUT available, yes?

A tiny, zero-libc utility to check if stdin has data without consuming it. Perfect for shell scripts and pipelines

by jaggz.h {who is at} gmail.com 2025-04-09

Why?

Sometimes you want to peek at stdin to see if there's any data — without slurping it up and leaving nothing for the next command.

Other tools check for 'readiness', but are not actually reporting data availability.

What's worse is that other tools (I'm talking to you, select), and their wrappers in other languages (like Perl'sIO::Select), are arguably a bit deceptive — they tell you a file descriptor is ready, but that could just mean EOF. And you have to read a bunch of their wonderful (true) manpages to see they don't have the capability to actually tell you whether DATA IS READY or not.

It turns out poll() can do it with its POLLIN flag.

This is where iny comes in:

• Uses poll() directly via syscall
• Does not consume input
• Exits 0 if data is ready to read
• Exits 1 if there's nothing (yet)
• I'm including a stripped down static, non-libc, version, for use on embedded systems, some containers, etc. It currently does not offer options to test anything other than stdin (fd=0) and 0 timeout.

Also — thanks to some ChatGPT-powered wizardry — there's an ultra-tiny version that: Requires no libc Is fully static Weighs in at just 8k (on my system)

(There's a dynamically linked version too if you prefer that.)

Issues

- Do not use with `<redirections`. See section: WHY NOT REDIRECTS
- I didn't make an installer. (I just build and symlink its binaries to my ~/bin)

Usage

Let cat eat only if there's input for it.

$ echo hello | { ./iny && cat || echo "No input available for cat"; }
hello

While we were sleeping...

$ sleep 5; ./iny && echo "Yup, you typed while we were sleeping."
lkdjflkdjf
Yup, you typed while we were sleeping.

Checking if input is there before doing a read:

$ echo abcd | { if ./iny; then read -n1 letter && echo "We got at least a letter: $letter"; else echo "Nothing was available when we checked."; fi; }
We got at least a letter: a

WHY NOT REDIRECTS:

This tool is intended to be used with pipes. When input is redirected from a regular file (e.g., using < /dev/null or < somefile) or even certain FIFOs, the operating system typically treats the file as always "ready" (i.e., POLLIN is set), regardless of whether any actual data is available. Do not rely on iny to indicate data availability when using such redirections. With -v verbose you will be able to see for yourself what the returned POLLIN and POLLHUP flags are (and that they are not reliable for redirects).

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That should inform users clearly about the limitations and prevent potential misuse.