Symlink an old version of a shared library in nix-shell

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1

Hi all,

I’m trying to write a nix-shell to run a binary, that is using an old unpackaged version of libpcap.
The binary is accepting the symlinked current version of libpcap, so I wanted to setup my nix-shell with just symlinking the shared lib.

shell.nix

with import <nixpkgs> {};

mkShell {
  libpcapPatched = pkgs.libpcap.overrideAttrs (oldAttrs: {
    postInstall = oldAttrs.postInstall + ''
      echo symlinking old libpcap.so.0.8 
      mkdir -p $out/lib
      ln $out/lib/libpcap.so $out/lib/libpcap.so.0.8
      echo $out
      echo symlinking old libpcap.so.0.8 ... done.
      '';}
  );

  shellHook = ''
    echo "Entering shell"
    Something like this --> export LD_LIBRARY_PATH=${libpcapPatched.out}/lib:$LD_LIBRARY_PATH
    fish
  '';
}

If I enter this shell with nix-shell, the postInstall hook outputs a nix store path (via the line echo $out), that includes the symlinked shared library.
If I add this path by hand to the LD_LIBRARY_PATH environment variable, the binary can run.

How can I access the $out variable of the override libpcap in the shellHook?

Thanks!

2

I figured it out by further studying the nix language. Here is my shell.nix:

shell.nix

with import <nixpkgs> {};

let 
 libpcapPatched = pkgs.libpcap.overrideAttrs (oldAttrs: {
    postInstall = oldAttrs.postInstall + ''
      echo symlinking old libpcap.so.0.8 
      mkdir -p $out/lib
      ln $out/lib/libpcap.so $out/lib/libpcap.so.0.8
      echo $out
      echo symlinking old libpcap.so.0.8 ... done.
      '';}
  );
in

pkgs.mkShell {
  LD_LIBRARY_PATH = lib.makeLibraryPath [ libpcapPatched ];

  shellHook = ''
    echo "Entering shell"
    fish
  '';
}