dema@o2-nixos  ~/tmp/zz  command nix-shell ~/nixos/nixpkgs -A netgen-mesher                                                                                                      ~/tmp/zz

[nix-shell:~/tmp/zz]$ unpackPhase
unpacking source archive /nix/store/8bgz6x9h210703hjsl6p28qsh65qzbg7-netgen-5.3.1.tar.gz
source root is tetgen1.5.0
setting SOURCE_DATE_EPOCH to timestamp 1383769596 of file tetgen1.5.0/example.poly

[nix-shell:~/tmp/zz]$ l
tetgen1.5.0/

[nix-shell:~/tmp/zz]$