Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I'm trying to find a way of testing for the existence of standard input in a perl program : I've done a lot of searching but so far can't find the answer. I've seen some sample code from Randal where he made a perl grep program : 
#!/bin/perl -w
$names++, shift if $ARGV[0] eq "-l";
$search = shift;
$showname = @ARGV > 1;
@ARGV = "-" unless @ARGV;
@ARGV = grep { -T or $_ eq "-" } @ARGV;
exit 0 unless @ARGV;
while (<>) {
        next unless /$search/o;
        if ($names) {
                print "$ARGV\n";
                close ARGV;
        } else {
                print "$ARGV: " if $showname;
                print;
        }
}

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I'll call the above program "grep". The program works fine for either of these 2 methods : 
1.  grep pattern /tmp/file
2.  cat /tmp/file | grep pattern

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But when I run "grep" by itself, it waits for STDIN. I want the program to print out a "usage message" if the user just runs "grep". How would I do this ?

Replies are listed 'Best First'.
Re: Test for standard input
by zigdon (Deacon) on Oct 03, 2002 at 16:36 UTC

    testing STDIN with -t might help - see perlfaq - How do I know if I'm running interactively or not?.

    Sorry, I think I misunderstood the FAQ here - -t doesn't seem to work as I thought.

    update: After some more reading, I think it can't be done. it's easy to differentiate between these two 

    perl -ne 'print' /path/to/file
cat /path/to/file | perl -ne 'print'

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    by looking at $ARGV, but I'm not sure perl can even know the difference between these two 
    cat /path/to/file | perl -ne 'print'
perl -ne 'print'

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    -- Dan

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[d/l]
[select]
      I think you were on to something the first time. -t will check if the filehandle is hooked up to a tty, which is the case if you run it from the command line, without a pipe. So you can check with -t and check the arguments, and give an appropriate error message.

      Now, if the program is run through a fork/exec, or another non-interactive facility, you can't really know until the parent process closes its stdout. The parent might wait for 5 years before it decides to print something out, so any check based on testing for actual input would be wrong.

      Goldclaw

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        Thanks alot ! The -t flag worked. Here's the new code : 
        #!/bin/perl -w

$ARGS = @ARGV;
if (-t and $ARGS eq 0 ) {
print "No input found\n";
exit 1;
}

$names++, shift if $ARGV[0] eq "-l";
$search = shift;
$showname = @ARGV > 1;
@ARGV = "-" unless @ARGV;
@ARGV = grep { -T or $_ eq "-" } @ARGV;
exit 0 unless @ARGV;
while (<>) {
        next unless /$search/o;
        if ($names) {
                print "$ARGV\n";
                close ARGV;
        } else {
                print "$ARGV: " if $showname;
                print;
        }
}

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[reply]
[d/l]

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