Perl

Running a Perl script

Replacing #! with nix-shell

Perl scripts normally start something like this:

#!/usr/bin/env perl

In Nix, we often make isolated environments using nix-shell. You can do this in the #! (shabang) section directly in the script too. Here is an example from the manual — a Perl script that specifies that it requires Perl and the HTML::TokeParser::Simple and LWP packages:

#! /usr/bin/env nix-shell
#! nix-shell -i perl -p perl perlPackages.HTMLTokeParserSimple perlPackages.LWP

use HTML::TokeParser::Simple;

# Fetch nixos.org and print all hrefs.
my $p = HTML::TokeParser::Simple->new(url => 'http://nixos.org/');

while (my $token = $p->get_tag("a")) {
    my $href = $token->get_attr("href");
    print "$href\n" if $href;
}

Invoking nix-shell on command-line

If you run a perl script and encounter a dependency error like this: Can't locate DB_File.pm in @INC (you may need to install the DB_File module)

... use nix-shell to create a shell environment which includes the dependency. Here we searched NixOS packages and found an existing perl package which suits, like so.

$ nix-shell -p perl perl534Packages.DBFile --run ./myscript.pl

There is no /usr/bin/perl

By design, there is no /usr/bin/perl in Nix. So you may encounter messages like:

./myscript.pl: bad interpreter: /usr/bin/perl: no such file or directory Change the first line of the script to

#!/usr/bin/env -S perl

or start it with perl ./myscript.pl

Adding something from CPAN to nixpkgs

1. Enter a nix-shell that provides the necessary dependencies:

   $ nix-shell -p nix-generate-from-cpan
   

2. Use the nix-generate-from-cpan.pl script (see nixpkgs/maintainers/scripts/) to generate something appropriate.
   Example usage:

   $ nix-generate-from-cpan Devel::REPL
   

3. After reviewing the result from the previous step and making appropriate modifications, add it to pkgs/top-level/perl-packages.nix. Note that some things use buildPerlPackage while some use buildPerlModule. Also note the mostly-followed naming convention as well as the mostly-followed alphabetical ordering. There are plenty of examples in perl-packages.nix.
4. Build and test.

Adding something without a Makefile.PL

The perlPackages.buildPerlPackage assumes a Makefile.PL to exist as part of the configurePhase. If no such file is present in the project you are building, it is easiest to add a Makefile.PL into the source yourself.

The example below does so by adding it using the postPatch hook:

postPatch = ''
  cat <<EOF > Makefile.PL
    use v5.10;
    use ExtUtils::MakeMaker;
    WriteMakefile(
        NAME          => 'RevBank',
        VERSION_FROM  => 'revbank',
        ABSTRACT_FROM => 'lib/RevBank.pod',
        EXE_FILES     => [ 'revbank' ],
    );
  EOF
'';

The documentation for ExtUtils::MakeMaker can be found here.

Wrappers for installed programs

To make perl modules available to a program in your derivation:

1. Add makeWrapper to nativeBuildInputs
2. Add

   postFixup = ''
     wrapProgram $out/bin/something \
       --prefix PERL5LIB : "${with perlPackages; makePerlPath [ something ]}"
   '';
   

Also keep in mind that makePerlPath would not resolve transitive dependencies of Perl packages. Hence if you want to just reference top-level packages, then use makeFullPerlPath which would recursively resolve dependency graph for you.

See also

• Nixpkgs Manual - Perl section
• Overriding an existing Perl package in NixOS