Question

I’m running

perl -ple '$_=length' datafile

The datafile contains the following:

algorithm
student government
Fiumichino

The result is that it prints

9
18
10

What do the -p and -l options do? Also, what is $_?

Answer 1

$_ is the default input and pattern-searching variable. -p is a command line switch that puts an implicit while(<>) loop around your program, with a print statement at the end. The -l switch sets $/ and $\ to “\n” (newline) and chomps your input, or in layman’s terms, it handles newlines for you.

The diamond operator <> is “magic” in that it automatically chooses your input channel. If your script has arguments, it will interpret it as a file name, and open that file and read it. If not, it checks STDIN. In your case, it opens the file “datafile”.

What that oneliner does is read each line of datafile and sets $_ to the length of $_ (since length uses $_ if no argument is supplied), then prints that number.

You can deparse the one-liner and see what the code looks like:

$ perl -MO=Deparse -ple '$_=length' datafile
BEGIN { $/ = "\n"; $\ = "\n"; }
LINE: while (defined($_ = <ARGV>)) {
    chomp $_;
    $_ = length $_;
}
continue {
    die "-p destination: $!\n" unless print $_;
}
-e syntax OK