nix: Add commands to query contents of NARs / binary caches

For example,

  $ NIX_REMOTE=file:///my-cache nix ls-store -lR /nix/store/f4kbgl8shhyy76rkk3nbxr0lz8d2ip7q-binutils-2.23.1
  dr-xr-xr-x                    0 ./bin
  -r-xr-xr-x                30748 ./bin/addr2line
  -r-xr-xr-x                66973 ./bin/ar
  ...

Similarly, "nix ls-nar" lists the contents of a NAR file, "nix
cat-nar" extracts a file from a NAR file, and "nix cat-store" extract
a file from a Nix store.

1 files changed, 74 insertions, 0 deletions

diff --git a/src/nix/cat.cc b/src/nix/cat.cc
new file mode 100644
index 000000000000..2405a8cb44ef
--- /dev/null
+++ b/src/nix/cat.cc
@@ -0,0 +1,74 @@
+#include "command.hh"
+#include "store-api.hh"
+#include "fs-accessor.hh"
+#include "nar-accessor.hh"
+
+using namespace nix;
+
+struct MixCat : virtual Args
+{
+    std::string path;
+
+    void cat(ref<FSAccessor> accessor)
+    {
+        auto st = accessor->stat(path);
+        if (st.type == FSAccessor::Type::tMissing)
+            throw Error(format("path ‘%1%’ does not exist") % path);
+        if (st.type != FSAccessor::Type::tRegular)
+            throw Error(format("path ‘%1%’ is not a regular file") % path);
+
+        std::cout << accessor->readFile(path);
+    }
+};
+
+struct CmdCatStore : StoreCommand, MixCat
+{
+    CmdCatStore()
+    {
+        expectArg("path", &path);
+    }
+
+    std::string name() override
+    {
+        return "cat-store";
+    }
+
+    std::string description() override
+    {
+        return "print the contents of a store file on stdout";
+    }
+
+    void run(ref<Store> store) override
+    {
+        cat(store->getFSAccessor());
+    }
+};
+
+struct CmdCatNar : StoreCommand, MixCat
+{
+    Path narPath;
+
+    CmdCatNar()
+    {
+        expectArg("nar", &narPath);
+        expectArg("path", &path);
+    }
+
+    std::string name() override
+    {
+        return "cat-nar";
+    }
+
+    std::string description() override
+    {
+        return "print the contents of a file inside a NAR file";
+    }
+
+    void run(ref<Store> store) override
+    {
+        cat(makeNarAccessor(make_ref<std::string>(readFile(narPath))));
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdCatStore>());
+static RegisterCommand r2(make_ref<CmdCatNar>());