CODEWITHSUNDEEP
linuxperlunix
OregonTrail
OregonTrail

Reputation: 9049

How can I tell if no arguments have been given and nothing is coming in on stdin?

I'd like a Perl script that I'm writing to display a help message if it receives no input.

How can I tell if nothing is coming in on stdin?

Upvotes: 3

Views: 6475

Answers (5)

ikegami
ikegami

Reputation: 386501

It's possible to check if something has come in on STDIN (using a nonblocking read, select or ioctl FIONREAD), but it's impossible to check if something is coming in on stdin, since it hasn't happened yet.

Impossibility aside, you're asking to break convention. For example, take cat, perl, sort, etc, etc. If you provide no arguments, they'll happily wait on STDIN until you provide what they need.

The user knows to use man tool or tool --help to get info.

Upvotes: 0

Bill
Bill

Reputation: 924

If you are talking about input from STDIN, you can check if the variable is equal to the empty string (after removing the CR/LF). For example:

my $myInput = <STDIN>;
chomp($myInput);
if ($myInput eq "")
{
    print "Error! You didn't submit any data!\n";
}

However, as @thnee says, if you are checking for the arguments passed in via command line, you should use $#ARGV.

For example:

if ($#ARGV == -1)
{
    print "Error! No input arguments entered!\n";
    exit(-1);
}

References:

http://perldoc.perl.org/functions/chomp.html

http://perldoc.perl.org/perlvar.html

Upvotes: 1

riff
riff

Reputation: 129

You might also find this perldoc FAQ useful for checking whether there is any input from the keyboard:

http://perldoc.perl.org/perlfaq8.html#How-do-I-check-whether-input-is-ready-on-the-keyboard%3f

Upvotes: 0

tchrist
tchrist

Reputation: 80433

if (@ARGV == 0 && -t STDIN && -t STDERR) { 
    print STDERR "$0: WARNING: reading input from keyboard, type ^D for EOF, ^C to intr.\n";
}

Upvotes: 8

thnee
thnee

Reputation: 5927

$#ARGV contains the number of arguments.

http://www.devdaily.com/perl/perl-command-line-arguments-read-args

Upvotes: 0

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