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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 and print all hrefs.
my $p = HTML::TokeParser::Simple->new(url => '');

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 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 ./

There is no /usr/bin/perl

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

./ 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 ./

Adding something from CPAN to nixpkgs

  1. Enter a nix-shell that provides the necessary dependencies:
    nix-shell -p perl perlPackages.CPANPLUS perlPackages.GetoptLongDescriptive perlPackages.LogLog4perl perlPackages.Readonly
  2. Use the script (see nixpkgs/maintainers/scripts/) to generate something appropriate.
    Example usage: 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 — use the source, Luke!
  4. Build and test.

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