Piping into Shell scripts

[Dave Decot]

> >Is there any way of getting a Shell script (C or Bourne) to recognise
> >whether it is receiving input from a pipe?
> 
> tty(1) will report "not a tty" if it's input is not from a tty. 
> 
> So the following should work:
> 
> 	testdata="`tty`"
> 	if [ "$testdata" = "not a tty" ]; then
> 		echo "we are not running from a terminal.  therefore we must"
> 		echo "be running from a pipe or with stdin redirected from a" 
> 		echo "file of some sort (of course, it also could be closed)"
> 
> 		exit
> 	fi

This is not portable: many systems output different messages for this
same condition, and some systems write the message on the standard
*error* output.  For instance, "Not a typewriter" is a message used
on some systems.

Instead, use:

	if tty -s
	then
	    echo "Running from a terminal"
	else
	    echo "Not running from a terminal"
	fi

Dave Decot