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Perl

Running Perl programs on the shell

When executing a Perl script, it is possible you get an error such as ./myscript.pl: bad interpreter: /usr/bin/perl: no such file or directory. This happens when the script expects Perl to be installed at /usr/bin/perl, which is not the case when using Perl from nixpkgs. You can fix the script by changing the first line to:

#!/usr/bin/env perl

to take the Perl installation from the PATH environment variable, or invoke Perl directly with:

$ perl ./myscript.pl

When the script is using a Perl library that is not installed globally, you might get an error such as Can't locate DB_File.pm in @INC (you may need to install the DB_File module). In that case, you can use nix-shell to start an ad-hoc shell with that library installed, for instance:

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

If you are always using the script in places where nix-shell is available, you can embed the nix-shell invocation in the shebang like this:

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

Packaging Perl programs

Nixpkgs provides a function buildPerlPackage, a generic package builder function for any Perl package that has a standard Makefile.PL. It’s implemented in pkgs/development/perl-modules/generic.

Perl packages from CPAN are defined in pkgs/top-level/perl-packages.nix rather than pkgs/all-packages.nix. Most Perl packages are so straight-forward to build that they are defined here directly, rather than having a separate function for each package called from perl-packages.nix. However, more complicated packages should be put in a separate file, typically in pkgs/development/perl-modules. Here is an example of the former:

{
  ClassC3 = buildPerlPackage rec {
    pname = "Class-C3";
    version = "0.21";
    src = fetchurl {
      url = "mirror://cpan/authors/id/F/FL/FLORA/${pname}-${version}.tar.gz";
      hash = "sha256-/5GE5xHT0uYGOQxroqj6LMU7CtKn2s6vMVoSXxL4iK4=";
    };
  };
}

Note the use of mirror://cpan/, and the pname and version in the URL definition to ensure that the pname attribute is consistent with the source that we’re actually downloading. Perl packages are made available in all-packages.nix through the variable perlPackages. For instance, if you have a package that needs ClassC3, you would typically write

{
  foo = import ../path/to/foo.nix {
    inherit stdenv fetchurl /* ... */;
    inherit (perlPackages) ClassC3;
  };
}

in all-packages.nix. You can test building a Perl package as follows:

$ nix-build -A perlPackages.ClassC3

To install it with nix-env instead: nix-env -f. -iA perlPackages.ClassC3.

So what does buildPerlPackage do? It does the following:

  1. In the configure phase, it calls perl Makefile.PL to generate a Makefile. You can set the variable makeMakerFlags to pass flags to Makefile.PL
  2. It adds the contents of the PERL5LIB environment variable to .../bin/perl line of Perl scripts as -Idir flags. This ensures that a script can find its dependencies. (This can cause this shebang line to become too long for Darwin to handle; see the note below.)
  3. In the fixup phase, it writes the propagated build inputs (propagatedBuildInputs) to the file $out/nix-support/propagated-user-env-packages. nix-env recursively installs all packages listed in this file when you install a package that has it. This ensures that a Perl package can find its dependencies.

buildPerlPackage is built on top of stdenv, so everything can be customised in the usual way. For instance, the BerkeleyDB module has a preConfigure hook to generate a configuration file used by Makefile.PL:

{ buildPerlPackage, fetchurl, db }:

buildPerlPackage rec {
  pname = "BerkeleyDB";
  version = "0.36";

  src = fetchurl {
    url = "mirror://cpan/authors/id/P/PM/PMQS/${pname}-${version}.tar.gz";
    hash = "sha256-4Y+HGgGQqcOfdiKcFIyMrWBEccVNVAMDBWZlFTMorh8=";
  };

  preConfigure = ''
    echo "LIB = ${db.out}/lib" > config.in
    echo "INCLUDE = ${db.dev}/include" >> config.in
  '';
}

Dependencies on other Perl packages can be specified in the buildInputs and propagatedBuildInputs attributes. If something is exclusively a build-time dependency, use buildInputs; if it’s (also) a runtime dependency, use propagatedBuildInputs. For instance, this builds a Perl module that has runtime dependencies on a bunch of other modules:

{
  ClassC3Componentised = buildPerlPackage rec {
    pname = "Class-C3-Componentised";
    version = "1.0004";
    src = fetchurl {
      url = "mirror://cpan/authors/id/A/AS/ASH/${pname}-${version}.tar.gz";
      hash = "sha256-ASO9rV/FzJYZ0BH572Fxm2ZrFLMZLFATJng1NuU4FHc=";
    };
    propagatedBuildInputs = [
      ClassC3 ClassInspector TestException MROCompat
    ];
  };
}

On Darwin, if a script has too many -Idir flags in its first line (its “shebang line”), it will not run. This can be worked around by calling the shortenPerlShebang function from the postInstall phase:

{ lib, stdenv, buildPerlPackage, fetchurl, shortenPerlShebang }:

{
  ImageExifTool = buildPerlPackage {
    pname = "Image-ExifTool";
    version = "12.50";

    src = fetchurl {
      url = "https://exiftool.org/${pname}-${version}.tar.gz";
      hash = "sha256-vOhB/FwQMC8PPvdnjDvxRpU6jAZcC6GMQfc0AH4uwKg=";
    };

    nativeBuildInputs = lib.optional stdenv.isDarwin shortenPerlShebang;
    postInstall = lib.optionalString stdenv.isDarwin ''
      shortenPerlShebang $out/bin/exiftool
    '';
  };
}

This will remove the -I flags from the shebang line, rewrite them in the use lib form, and put them on the next line instead. This function can be given any number of Perl scripts as arguments; it will modify them in-place.

Generation from CPAN

Nix expressions for Perl packages can be generated (almost) automatically from CPAN. This is done by the program nix-generate-from-cpan, which can be installed as follows:

$ nix-env -f "<nixpkgs>" -iA nix-generate-from-cpan

Substitute <nixpkgs> by the path of a nixpkgs clone to use the latest version.

This program takes a Perl module name, looks it up on CPAN, fetches and unpacks the corresponding package, and prints a Nix expression on standard output. For example:

$ nix-generate-from-cpan XML::Simple
  XMLSimple = buildPerlPackage rec {
    pname = "XML-Simple";
    version = "2.22";
    src = fetchurl {
      url = "mirror://cpan/authors/id/G/GR/GRANTM/XML-Simple-2.22.tar.gz";
      hash = "sha256-uUUO8i6pZErl1q2ghtxDAPoQW+BQogMOvU79KMGY60k=";
    };
    propagatedBuildInputs = [ XMLNamespaceSupport XMLSAX XMLSAXExpat ];
    meta = {
      description = "API for simple XML files";
      license = with lib.licenses; [ artistic1 gpl1Plus ];
    };
  };

The output can be pasted into pkgs/top-level/perl-packages.nix or wherever else you need it.

Cross-compiling modules

Nixpkgs has experimental support for cross-compiling Perl modules. In many cases, it will just work out of the box, even for modules with native extensions. Sometimes, however, the Makefile.PL for a module may (indirectly) import a native module. In that case, you will need to make a stub for that module that will satisfy the Makefile.PL and install it into lib/perl5/site_perl/cross_perl/${perl.version}. See the postInstall for DBI for an example.