Writing a line based filter that exits when no input - comp.unix.programmer

Message from discussion Writing a line based filter that exits when no input
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From: Barry Margolin <bar...@alum.mit.edu>
Newsgroups: comp.unix.programmer
Subject: Re: Writing a line based filter that exits when no input
Date: Sat, 04 Jul 2009 02:23:18 -0400
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In article <pan.2009.07.04.06.09...@tznvy.pbz>,
 Anand Hariharan <znvygb.nanaq.unevun...@tznvy.pbz> wrote:

> Am writing a line-based  filter a la sed or tr. What I  would like it to
> do, however, is if there is no input at start-up, it should present some
> usage information and exit.
> 
> E.g., 
> 
> $ MyLineFilter < TextFile # OK
> 
> $ Some Command | MyLineFilter # OK
> 
> $ MyLineFilter
> MyLineFilter: Did not find any text to process.  Exiting.
> 
> $ echo $?
> 1
> 
> $ MyLineFilter Command Line Args # Undecided what to do here ...
> 
> 
> Would appreciate some ideas on how to implement this.
> 
> Currently, am writing the usage information to stderr (unconditionally), 
> but the executable waits for input until it finds EOF.

In the third case, there IS input -- standard input is connected to the 
terminal, and the user is supposed to type the input.

If you don't want to take input from the terminal, you can use:

if test -t
then echo $0: Did not find any text to process. Exiting. >&2
     exit 1
fi

-- 
Barry Margolin, bar...@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
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