How to know if data is piped into my script

[Mahesh Asolkar]

I have the following script:

-----
#!/usr/bin/perl

use strict;
use warnings;

my %hist = ();
my @hist;

$hist{(split /\s+/)[3]}++
foreach (`$ENV{'SHELL'} -c history`);

print map {"$hist{$_} : $_\n"}
sort {$hist{$b} <=> $hist{$a}}
keys %hist;
-----

I want to change it such that it uses '`$ENV{'SHELL'} -c history`'
only if nothing is piped into the script. If I call the script like:

% myscript.pl

It should use '`$ENV{'SHELL'} -c history`'. However if it is called
like:

% history | myscript.pl

It should use the piped in data instead.

What would be the best approach?

Thanks,
Mahesh

 

[xhoster]

I have the following script:

-----
#!/usr/bin/perl

use strict;
use warnings;

my %hist = ();
my @hist;

$hist{(split /\s+/)[3]}++
foreach (`$ENV{'SHELL'} -c history`);

print map {"$hist{$_} : $_\n"}
sort {$hist{$b} <=> $hist{$a}}
keys %hist;
-----

Assuming it is safe to load all input into memory at once:

my @x=<>;
foreach (@x? @x : `$ENV{'SHELL'} -c history`) {

This will do what <> does, not just pipes. If you want only STDIN, and
only when STDIN is hooked up to a pipe, then maybe:

foreach (-p STDIN ? <STDIN> : `$ENV{'SHELL'} -c history`) {


Xho

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[Mahesh Asolkar]

unless (-t) {
    # STDIN is not opend to a tty but a file or pipe

}

Thanks Frank and Xho. -t and -p do exactly what I was looking for.

Xho, I had tried the first way you suggested too. It works in the case
of 'history | myscript.pl', but it blocks 'myscript.pl' waiting for <>
to return.

Here's the script with the changes (I picked -p):

---------
#!/usr/bin/perl

use strict;
use warnings;

my %hist = ();
my @hist;

$hist{(split /\s+/)[3]}++
foreach ((-p STDIN) ? <STDIN> : `$ENV{'SHELL'} -c history`);

print map {"$hist{$_} : $_\n"}
sort {$hist{$b} <=> $hist{$a}}
keys %hist;

 

[nntpman68]

Hi Makesh,


Small clarification.

-p checks whether input is a pipe
and "-p STDIN" would capture the case:

program | perlscript.pl


whereas -t CHECKS WHETHER input is a terminal.
so "unless -t STDIN" would also capture

perlscript.pl < filename


bye

N

unless (-t) {
# STDIN is not opend to a tty but a file or pipe

}

Thanks Frank and Xho. -t and -p do exactly what I was looking for.

Xho, I had tried the first way you suggested too. It works in the case
of 'history | myscript.pl', but it blocks 'myscript.pl' waiting for <>
to return.

Here's the script with the changes (I picked -p):

---------
#!/usr/bin/perl

use strict;
use warnings;

my %hist = ();
my @hist;

$hist{(split /\s+/)[3]}++
foreach ((-p STDIN) ? <STDIN> : `$ENV{'SHELL'} -c history`);

print map {"$hist{$_} : $_\n"}
sort {$hist{$b} <=> $hist{$a}}
keys %hist;