How to insert "#" comment off whole line of the specific keyword by using perl?

Question

anted to comment off those line if found "dummy", how to write a perl to put # like below?

Answer 1

You can do this on a command line with something like this:

cat file | perl -nle 'if (/_dummy/) { print "#". $_ } else { print $_ }'

Or if you prefer a script file (assume the following is saved as commenter.pl):

#!/usr/bin/env perl

foreach my $line (<>) {
    if ($line =~ /dummy/) {
        print "#" . $line;
    } else {
        print $line;
    }
}

And then you can do cat file | ./commenter.pl or ./commenter.pl file

Answer 2

Well, I should also give you the one liner if you want to edit the file in place:

$ perl -pi -e 's/^(.*dummy.*)$/# $1/' <file>

The -i is for in-place editing, -p does a while (<>).

Although I enjoy perl, for something as simple as that in-place file replacement, most people would use sed instead:

$ sed -i.bak -E "s/^(.*dummy.*)$/# \1/" <file>

(works on Linux/BSD/Mac - on Linux only .bak can be omitted which won't save a backup obviously)

Answer 3

perl -pe's/^(?=.*dummy)/#/'

or

perl -pe'print "#" if /dummy/'

Specifying file to process to Perl one-liner