detect-input-type.sh

# How to detect whether input is from keyboard, a file, or another process.
# Useful for writing a script that can read from standard input, or prompt the
# user for input if there is none.

# Source: http://www.linuxquestions.org/questions/linux-software-2/bash-scripting-pipe-input-to-script-vs.-1-570945/

if readlink /proc/$$/fd/0 | grep -q "^pipe:"; then
    # Pipe input (echo abc | myscript)
elif file $( readlink /proc/$$/fd/0 ) | grep -q "character special"; then
    # Terminal input (keyboard)
else
    # File input (myscript < file.txt)
fi


# ALTERNATIVE:

# In Bash you can also use test -t to check for a terminal:

if [ -t 0 ]; then
    # Terminal input (keyboard) - interactive
else
    # File or pipe input - non-interactive
fi