== An shell script idiom for choosing what file to operate on

This is one of those Bourne shell idioms where I show the code first
and explain the effect later:

>  #!/bin/sh
>  def=$1; shift
>  case "$#" in
>    0) if test -t 0; then
>          cat -- $def
>       else
>           cat
>       fi;;
>    *) cat -- "$@";;
>  esac

Call this _selfile_. You use it in things like log scanning scripts,
invoking it as:

> _selfile /var/log/something "$@" | ... scan stuff ..._

The _selfile_ script picks what to feed the rest of the script
in the following way:

# if you supplied any arguments, it takes them as filenames.
# if you're invoking it interactively, it uses a default file.
# if you're feeding something to its standard input, it just
  passes it on, so you can do things like '_zcat /var/log/Old/foo.*
  | scanner_'.

These three choices turn out to be pretty much what you want in
practice. You don't have to give any arguments if you want to scan
today's logs, while you still have the option of feeding it stuff
to digest.

The magic is '_test -t 0_', which is true only if standard input is a
terminal (real or pseudo); this is our somewhat crude way of telling
whether we're 'interactive' or non-interactive.

(Disclaimer: while you can use this trick on standard output to have
your script behave differently when it's in a pipe or otherwise having
its standard output redirected, you probably don't want to do that.
People get twitchy when commands produce different output like that, in
part because it makes it harder to build up shell scripts bit by bit.)