🎄 20/25. Using command-line options in Raku one-liners

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.

-e

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'

v6.c

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.

-n

This option repeats the code for each line of input data. This is quite handy when you want to process a file. For example, here’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:

100
10
26

There’s no difference whether you use shell’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'...'.

-p

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

After-party

Let’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 ‘blank’, 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!