Perl One LinersHi all! I am starting yet another article series here. Remember my two articles on Awk One-Liners Explained and Sed One-Liners Explained? They have received more than 150,000 views total now and they attract several thousand new visitors every week. Inspired by their success, I am going to create my own perl1line.txt file and explain every single oneliner in it. I hope it becomes as popular as awk1line.txt and sed1line.txt.

A tiny intro to those unfamiliar with the awk1line.txt and sed1line.txt files – they contain around 80 useful awk and sed one-liners for doing various text file manipulations. Some examples are double spacing files, numbering lines, reversing text, and replacing certain words. They were all compiled by Eric Pement.

The article will be divided in seven or more parts. In parts 1 - 6 I'll create the one-liners and explain them, and in the last part I will release the perl1line.txt file. I found that splitting the article in many parts is much easier to get it written and that's what I will do.

Here is the general plan:

These one-liners will make heavy use of Perl special variables. Luckily, a few years ago I compiled all the special vars in a single file and called it Perl special variable cheat-sheet. Even tho it's mostly copied out of perldoc perlvar, it's still handy to have in front of you. I suggest that you print it so that you always had them in front of you and you could memorize them faster.

Awesome news: I have written an e-book based on this article series. Check it out:

Perl book

Other than that, I can't wait to start writing the article, so here I go:

File Spacing

1. Double space a file.

perl -pe '$\="\n"'

This one-liner double spaces a file. There are three things to explain in this one-liner. The "-p" and "-e" command line options, and the "$\" variable.

First let's start with the "-e" option. The "-e" option can be used to enter a Perl program directly in the command line. Typically you don't want to create source files for every small program. By using "-e" you can handily specify the program to execute on the command line.

Next the "-p" switch. Specifying "-p" to a Perl program causes it to assume the following loop around your program:

while (<>) {
    # your program goes here
} continue {
    print or die &#34;-p failed: $!\n&#34;;
}

This construct loops over all the input, executes your code and prints the value of "$". This way you can effectively modify all or some lines of input. The "$" variable can be explained as an anonymous variable that gets filled with the good stuff.

The "$\" variable is similar to ORS in Awk. It gets appended after every "print" operation. Without any arguments "print" prints the contents of "$_" (the good stuff).

In this one-liner the code specified by "-e" is '$\="\n"', thus the whole program looks like this:

while (<>) {
    $\ = &#34;\n&#34;;
} continue {
    print or die &#34;-p failed: $!\n&#34;;
}

What happens is that after reading each line, "$" gets filled with it (including the existing line's newline), the "$\" gets set to a newline itself and "print" is called. As I already mentioned, without any arguments "print" prints the contents of "$" and "$\" gets appended at the end. The result is that each line gets printed unmodified and it's followed by the "$\" which has been set to newline. The input has been double-spaced.

There is actually no need to set "$\" to newline on each line. It was just the shortest possible one-liner that double-spaced the file. Here are several others that do the same:

perl -pe 'BEGIN { $\="\n" }'

This one sets the "$\" to newline just once before Perl does anything (BEGIN block gets executed before everything else).

perl -pe '$_ .= &#34;\n&#34;'

This one-liner is equivalent to:

while (<>) {
    $_ = $_ . &#34;\n&#34;
} continue {
    print or die &#34;-p failed: $!\n&#34;;
}

It appends another new-line at the end of each line, then prints it out.

The cleanest and coolest way to do it is probably use the substitution "s///" operator:

perl -pe 's/$/\n/'

It replaces the regular expression "$" that matches at the end of line with a newline, effectively adding a newline at the end.

2. Double space a file, except the blank lines.

perl -pe '$_ .= "\n" unless /^$/'

This one-liner double spaces all lines that are not completely empty. It does it by appending a newline character at the end of each line that is not blank. The "unless" means "if not". And "unless /^$/" means "if not 'beginning is end of line'". The condition "beginning is end of line" is true only for lines that contain the newline character.

Here is how it looks when expanded:

while (<>) {
    if ($_ !~ /^$/) {
        $_ .= &#34;\n&#34;
    }
} continue {
    print or die &#34;-p failed: $!\n&#34;;
}

A better test that takes spaces and tabs on the line into account is this one:

perl -pe '$_ .= "\n" if /\S/'

Here the line is matched against "\S". "\S" is a regular expression that is the inverse of "\s". The inverse of "\s" is any non-whitespace character. The result is that every line that has at least one non-whitespace character (tab, vertical tab, space, etc) gets double spaced.

3. Triple space a file.

perl -pe '$\="\n\n"'

Or

perl -pe '$_.="\n\n"'

They are the same as one-liner #1, except that two new-lines get appended after each line.

4. N-space a file.

perl -pe '$_.="\n"x7'

This one-liner uses inserts 7 new lines after each line. Notice how I used '"\n" x 7' to repeat the newline char 7 times. The "x" operator repeats the thing on the left N times.

For example:

perl -e 'print "foo"x5'

would print "foofoofoofoofoo".

5. Add a blank line before every line.

perl -pe 's//\n/'

This one-liner uses the "s/pattern/replacement/" operator. It substitutes the first pattern (regular expression) in the "$_" variable with the replacement. In this one-liner the pattern is empty, meaning it matches any position between chars (and in this case it's the position before first char) and replaces it with "\n". The effect is that a newline char gets inserted before the line.

6. Remove all blank lines.

perl -ne 'print unless /^$/'

This one-liner uses "-n" flag. The "-n" flag causes Perl to assume to following loop around your program:

LINE:
while (<>) {
    # your program goes here
}

What happens here is that each line gets read by the diamond "<>" operator and is placed in the dolar underscore special variable "$_". At this moment you are free to do with it whatever you want. You specify that in your main program text.

In this one-liner the main program is "print unless /^$/", it gets inserted in the while loop above and the whole Perl program becomes:

LINE:
while (<>) {
    print unless /^$/
}

Unraveling it further:

LINE:
while (<>) {
    print $_ unless $_ =~ /^$/
}

This one-liner prints all non-blank lines.

A few other ways to do the same:

perl -lne 'print if length'

This one uses the "-l" command line argument. The "-l" automatically chomps the input line (basically gets rid of newline at the end). Next the line is tested for its length. If there are any chars left, it evals to true and the line gets printed.

perl -ne 'print if /\S/'

This one-liner behaves differently than the "print unless /^$/" one-liner. The "print unless /^$/" one-liner prints lines with spaces and tabs, this one doesn't.

7. Remove all consecutive blank lines, leaving just one.

perl -00 -pe ''

Ok, this is really tricky, ain't it? First of all it does not have any code, the -e is empty. Next it has a silly -00 command line option. This command line option turns paragraph slurp mode on. A paragraph is text between two newlines. All the other newlines get ignored. The paragraph gets put in "$_" and the "-p" option prints it out.

Later I found a shorter version of this one-liner:

perl -00pe0

8. Compress/expand all blank lines into N consecutive ones.

perl -00 -pe '$_.="\n"x4'

This one-liner combines the previous one and one-liner #4. It slurps lines paragraph wise, then appends (N-1) new-line. This one-liner expands (or compresses) all new-lines to 5 ("\n" x 4 prints four, and there was one at the end of paragraph itself, so 5).

Perl one-liners explained e-book

I've now written the "Perl One-Liners Explained" e-book based on this article series. I went through all the one-liners, improved explanations, fixed mistakes and typos, added a bunch of new one-liners, added an introduction to Perl one-liners and a new chapter on Perl's special variables. Please take a look:

Perl book

Have fun and until next time

Have fun with these file spacing one-liners. The next part is going to be about line numbering and calculations one-liners and I'll publish it in the next week or two.