Bash stdin interactive vs piped

🧠 Detecting Piped Input vs Interactive Input in Bash (`-t 0` Explained)

When writing Bash scripts that can receive input either from the terminal or via pipes, it's crucial to know where that input is coming from.

This tutorial explains how to detect if your script's stdin is coming from: - an interactive terminal (e.g. user typing), - or redirected input (e.g. echo "..." | script.sh, cron, or .forward).

🧪 The Test: `-t 0`

[ -t 0 ]

-t checks whether the given file descriptor is open and refers to a terminal.
0 refers to stdin.

✅ Meaning:

Expression	Result
`-t 0`	stdin is a terminal (interactive shell)
`! -t 0`	stdin is piped or redirected (not interactive)
`-t 1`	stdout is a terminal
`-t 2`	stderr is a terminal

🎯 Example: Bash Script that Detects Input Source

#!/usr/bin/env bash

if [ ! -t 0 ]; then
  # stdin is being piped/redirected
  message="$(cat)"
else
  # interactive terminal
  message="$*"
fi

echo "Received: $message"

🔁 Real Use Case: Mail Piping in `.forward`

When using a .forward file to redirect local mail into a script:

# ~/.forward
|/home/username/bin/notify.sh

Your script will receive stdin, but not from a terminal. This check ensures your script handles it correctly:

if [ ! -t 0 ]; then
  message="$(cat)"
else
  message="$*"
fi

Without this, you may wrongly assume input is present, or attempt to read from a terminal that doesn’t exist (e.g. in cron or mail contexts).

📦 Summary

File Descriptor	Name	Use
`-t 0`	stdin	Checks if input is interactive
`-t 1`	stdout	Checks if output is to a terminal
`-t 2`	stderr	Checks if errors go to terminal
`! -t 0`	not stdin	Input is from pipe/cron/etc

Use ! -t 0 to reliably detect non-interactive (piped or redirected) input in scripts.

✅ TL;DR

Use this to make your script smart:

if [ ! -t 0 ]; then
  message="$(cat)"  # handle piped/cron/mail input
else
  message="$*"
fi

This makes your Bash scripts behave correctly in both interactive and automated environments.