๐ŸŽ„ 20/25. Using command-line options in Perl 6 one-liners

๐ŸŽ„ 20/25. Using command-line options in Raku one-liners

N. B. Perl 6 has been renamed to Raku. Click to read more.

Welcome to Day 20 of the Perl 6 One-Liner Advent Calendar! So far, we created about 25 different one-liners, but never talked about the command-line options that the Rakudo Perl 6 compiler offers to us.


The first option to know when working with (Rakudo) Perl 6 is -e. It takes a string with your Perl 6 one-liner and executes it immediately.

For example, print the version of the current Perl 6 specification:

$ perl6 -e'$*PERL.version.say'

Be careful not to use the Perl 5.10+ styled capital -E, which does the same as -e but also activates features such as say. In Perl 6, the option is always lowercase.


This option repeats the code for each line of input data. This is quite handy when you want to process a file. For example, hereโ€™s a one-liner that adds up the values in a row and prints the sum:

$ perl6 -ne'say [+] .split(" ")' data.txt 

If the data.txt file contains the following:

10 20 30 40
1 2 3 4
5 6 7 8

then the result of the one-liner is:


Thereโ€™s no difference whether you use shellโ€™s input redirection or not; the following line also works:

$ perl6 -ne'say [+] .split(" ")' < data.txt 

Make sure you place the e option the last in the list (so, not perl6 -en'...') or split the options: perl6 -n -e'...'.


This option is similar to -n, but prints the topic variable after each iteration.

The following one-liner reverses the lines in the file and prints them to the console:

$ perl6 -npe'.=flip' data.txt

For the same input file, the result will look like this:

04 03 02 01
4 3 2 1
8 7 6 5

Notice that you have to update the $_ variable, so you type .=flip. If you only have .flip, you will reverse the string, but the result will not be used and the original line will be printed.

An equivalent program with .flip and with no -p will look like this:

$ perl6 -ne'.flip.say' data.txt


Letโ€™s go through a few one-liners from the Perl One-Liners book and create one-liners in Perl 6.

Double-space a file

$ perl6 -npe's/$/\n/' text.txt 

Remove all blank lines

$ perl6 -ne'.say if .chars' text.txt 

Depending on how you define โ€˜blankโ€™, you may want another one-liner that skips the lines containing whitespaces:

$ perl6 -ne'.say if /\S/' text.txt 

Number all lines in a file

$ perl6 -ne'say ++$ ~ ". " ~ $_' text.txt

This code, probably, requires a comment. The $ variable is a state variable and it can be used without declaration.

Convert all text to uppercase

$ perl6 -npe'.=uc' text.txt

Strip whitespace from the beginning and end of each line

$ perl6 -npe'.=trim' text.txt

Print the first line of a file

$ perl6 -ne'.say ; exit' text.txt 

Print the first 10 lines of a file

$ perl6 -npe'exit if $++ == 10' text.txt 

This time, a postfix ++ was applied to $.

I hope that was a useful journey today. See you tomorrow!

13 thoughts on “๐ŸŽ„ 20/25. Using command-line options in Perl 6 one-liners”

      1. on rakudo command line or in windows command line i copied the line and it said :

        > perl6 -ne’say [+] .split(” “)’ data.txt

        SORRY! Error while compiling:
        Two terms in row
        perl6 -ne’say [+] .split(” “)’ data.txt
        expecting any of:
        infix stopper
        statement end
        statement modifier
        statement modifier loop

        All other one line worked very fine…

      2. i also tried it in the windows 7 command line it says:

        C:\Users\Me\Desktop>perl6 -neโ€™say [+] .split(โ€ โ€œ)โ€™ data.txt
        SORRY! Error while compiling -e
        Unable to parse expression in high curly single quotes couldn’t find final (corresponding starter was at line 1) at -e:1
        expecting any of:
        high curly single quotes

  1. Hi, really nice post, can you give many many more examples in your next post please? people could use it as an indepth tutorial!

    also please cover more advanced topics like functional stuffs, grammers in detail!

    thank you!

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