Detect if *nix shell script reading from pipe, readfile, or stdin

Copyright.md

Copyright 2007 "carlanderson" As retrieved from http://www.linuxquestions.org/questions/linux-software-2/bash-scripting-pipe-input-to-script-vs-%241-570945/#post2998640 on July 16, 2012

whereFromAllTheBits.sh

#!/bin/sh
# Copyright: see Copyright.md

if readlink /proc/$$/fd/0 | grep -q "^pipe:"; then
  echo "Pipe input:"
elif file $( readlink /proc/$$/fd/0 ) | grep -q "character special"; then
  echo "Standard input:"
else
  echo "File input:"
fi

cat # read from stdin and write to stdout

Why.md

Purpose

I wanted to investigate how to detect whether a shell script is being passed its data: via stdin, reading a file (<), or a pipe (|). This short script determines which and then echoes the data straight out to stdout.