Perl command line - assume while loop around

Question

Can anyone explain the difference in output of the two perl (using cygwin) commands below:

$ echo abc | perl -n -e 'if ($_ =~ /a/) {print 1;}'

prints :

1

$ echo abc | perl -e 'if ($_ =~ /a/) {print 1;}'

The first prints '1' while second one outputs blank?

Thanks

mpapec · Accepted Answer · 2014-07-09 07:50:32Z

1

-n switch adds while loop around your code, so in your case $_ is populated from standard input. In second example there is no while loop thus $_ is leaved undefined.

Using Deparse you can ask perl to show how your code is parsed,

perl -MO=Deparse -n -e 'if ($_ =~ /a/) {print 1;}'

LINE: while (defined($_ = <ARGV>)) {
    if ($_ =~ /a/) {
        print 1;
    }
}

perl -MO=Deparse -e 'if ($_ =~ /a/) {print 1;}'

if ($_ =~ /a/) {
    print 1;
}

edited Jul 9, 2014 at 7:50

answered Jul 9, 2014 at 6:39

mpapec

50.7k8 gold badges71 silver badges133 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Rajeev Over a year ago

Thanks, that explains. Now I have question regarding use of perl command line option -MO=Deparse you have used. According to Perl --help this option means -[mM][-]module execute "use/no module..." before executing program. My question is, how do you know about the O and = characters in this option (-MO=Deparse). Is it documented?

mpapec Over a year ago

@Rajeev yes, -M does that.

Miller Over a year ago

@Rajeev O is the module name, and Deparse is the function that is requested to be imported. An alternative option would be -MList::Util=sum, which would important the sum function of List::Util

Collectives™ on Stack Overflow

Perl command line - assume while loop around

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related